During the last 10 years atmospheric science has been quite well supported by the EU as well as by Germany (BMBF). In particular, kinetic and mechanistic studies in the laboratory, during the first stage mainly in the gas phase but in the second stage also in the aqueous phase and heterogeneous phase (multi-phase) received significant support. During the coming years the support for atmospheric science seems to be reasonable, however, other areas like soil and water pollution and biodiversity is also attracting a larger proportion of the available resources. Laboratory studies of atmospheric chemical systems will certainly receive less support in the future by the EU as well as by Germany (BMBF). At present it is still an open question how much support can be provided by the National Science Foundation (DFG) in Germany. In France laboratory studies are regularly supported by CNRS, however, on a low level. In the UK a new programme has been submitted to NERC, the outcome remains open. European Chemical Industry represented by CEFIC does not see it as a priority to support atmospheric science; ecotoxicological studies have higher priorities.

At present integrated research projects combining laboratory work, field studies and modelling might attract further support. The knowledge on reaction kinetics and product yields of atmospheric processes seems to be sufficiently developed, at least as viewed outside from of chemistry circles.

In the troposphere, volatile organic compounds (VOCs) emitted from anthropogenic and biogenic sources can undergo photolysis and react with hydroxyl (OH) radicals, nitrate (NO₃) radicals and ozone (O₃). Over the past two decades, much progress has been made in elucidating the kinetics, products and mechanisms of the gas-phase reactions of VOCs with OH and NO₃ radicals and O₃. However, these are still areas of uncertainty associated with the atmospheric chemistry of VOCs. The present status of the atmospheric chemistry of VOCs and of the inorganic reactions occurring in the troposphere will be briefly presented, and the current areas of uncertainty discussed.

Air quality models with atmospheric chemistry and transport are being used to test the scientific understanding of air pollution and for applications such as the planing of air pollution control strategies and the forecasting of severe air pollution episodes. The chemical mechanism used in the model affects the accuracy of the model’s calculated concentrations of air pollutants. Many air quality models are using outdated chemistry mechanisms based on data that is over 10 years out of date.

The Regional Atmospheric Chemistry Mechanism (RACM) was developed to fill the need for a mechanism based on more recent data. The RACM mechanism is a highly revised version of the RADM2 mechanism. The changes to the inorganic chemistry were relatively minor but there were substantial changes for many organic compounds. These revisions included improvements to the mechanisms for the oxidation of alkanes by HO radical, the ozonolysis of alkenes, the reaction of alkenes with NO₃ radical and aromatic chemistry. The reactions of peroxy radicals with NO, NO₂ and NO₃ were revised. The RACM mechanism has a new condensed mechanism for the oxidation of isoprene and terpenes.

The purpose of this lecture will be to discuss recent developments in gas-phase mechanisms and their application to studies of the ozone formation reactivity of organic compounds, nitrogen transport and the formation of secondary aerosols.

A detailed mechanism for the gas-phase atmospheric reactions of volatile organic compounds (VOCs) and oxides of nitrogen (NOx) in urban and regional atmospheres is discussed. This mechanism, designated SAPRC-99, represents a complete update of the SAPRC-90 mechanism of Carter (1990), and incorporates recent reactivity data from a wide variety of VOCs. The mechanism has assignments for ~400 types of VOCs, and can be used to estimate reactivities for ~550 VOC categories. A condensed version was developed for use in regional models. A unique feature of this mechanism is the use of a computerized system to estimate and generate complete reaction schemes for most non-aromatic hydrocarbons and oxygenates in the presence of NOx, from which condensed mechanisms for the model can be derived. The mechanism was evaluated against the results of almost 1700 environmental chamber experiments carried out at the University of California at Riverside, including experiments to test ozone reactivity predictions for over 80 types of VOCs. The mechanism was used to update the various ozone reactivity scales developed by Carter (1994a), including the widely used Maximum Incremental Reactivity (MIR) scale. However, the reactivity estimates for many VOC classes are uncertain, which must be taken into account when using these data for regulatory applications. To aid this, uncertainty classifications have been assigned to all VOCs, and upper limit MIRs for VOCs with uncertain mechanisms are presented.

A Master Chemical Mechanism containing over 2400 chemical species and over 7100 chemical reactions is employed here to describe the atmospheric degradation of 123 organic compounds and the associated regional scale ozone and PAN formation under conditions appropriate to the polluted boundary layer over north-west Europe. Photochemical ozone and PAN creation potentials (POCP and PPCP) are derived for each organic compound from their propensities to form ozone and PAN relative to ethylene and propylene, respectively. When we have compared the POCPs calculated under the European conditions appropriate to multi-day regional scale photochemical episodes against the literature MIR values determined for intense USA urban single-day conditions, the scatter plot showed distinct curvilinear behaviour. We interpreted this behaviour as indicating that the multi-day episode allowed less reactive organic compounds to build up in concentrations and hence to produce heightened contributions to ozone formation. This present study confirms that this is indeed the explanation and shows that reactivity scales for alkanes, alcohols, esters and ethers should take both single day and multi-day conditions into account.

In recent years, a master chemical mechanism (MCM) has been developed to describe the atmospheric degradation of around 120 VOC in the atmosphere. The mechanism is explicit, and as such, does not suffer from the limitations imposed by a lumped mechanism or one that utilises surrogate species.

In this paper, we describe the evaluation of the MCM both through smog chamber and field experiments. The chamber experiments have been carried out in the European Photochemical Reactor (EUROPHORE) at Valencia. We show results, which indicate that the MCM chemistry describes well the degradation of n-butane, toluene and alpha-pinene under smog chamber conditions.

The field experiments have been carried out at the Mace Head Atmospheric Research Station, a remote coastal site on the west coast of Ireland, and also at the Cape Grim Baseline Atmospheric Pollution Station on the north-western tip of Tasmania. As well as measurements of key radical species such as OH, HO₂, RO₂ and NO₃, there were also measurements of the long-lived species that determine the radical concentrations (CO, CH₄, NMHC, NOx, O₃ etc.). It is essential that concentrations of these longer-lived species are determined both simultaneously and collocated with those of the radicals, in order interpret the radical chemistry.

To construct tailored atmospheric models to be used to predict radical concentrations at each location, we use measurements of NMHC, CO and CH₄ to construct a reactivity index with OH. Those species found to be most important for OH loss are then extracted from the MCM along with a comprehensive inorganic scheme, and used to construct a campaign-tailored mechanism. We then simulate the concentration of radical species and compare with in situ measurements. Some of these comparisons will be shown, and there will also be a discussion of the understanding gained through using a detailed mechanism such as the MCM.

The atmospheric oxidation mechanisms of isoprene and important monoterpenes, which are still subject to major uncertainties, have been investigated in laboratory and smog chamber studies. The smog chamber runs have been carried out in the outdoor simulation chamber EUPHORE in Valencia, Spain. A three-component VOC (Volatile Organic Compounds) mixture (base mix), consisting of n-butane, ethene, and toluene, was chosen as a reference case, in relation to which changes in the NO_x transformation and ozone formation, caused by adding relatively small amounts of a fourth VOC of interest, were observed.

In this contribution we report on experiments with the biogenic VOCs isoprene, a-pinene, and limonene as additives to the base mix. The initial carbon and NO_x concentrations of 2 ppm-C and 200 ppb, respectively, as well as the proportions of the base mix VOCs were kept constant in all experiments. Besides the reacting VOCs, ozone, NO, and NO₂, also the concentrations of a number of reaction products, including HNO₃, PAN and a range of organic carbonyls, could be measured time-resolved.

Deviations in ozone formation and VOC and NO_x transformation between the different runs will be discussed in terms of reaction mechanisms. The data sets have also been used for the validation of chemical mechanisms. Comparisons of experimental data with results from model runs will be presented.

Unsubstituted alkoxy and carbonyl Radicals, RCH₂O and RCO, R = linear and branched) are prepared by stationary photolysis of appropriate precursors in a temperature controlled photo-reactor from stainless steel (v = 12 L). Branching ratios of the two common atmospheric loss processes for both types of radicals, unimolecular transformation vs. O₂ addition, are determined from the FT-IR absorption analysis of stable products. The effect of branching of R on the thermochemistry and kinetics of RCH₂O and RCO and their fate in the atmosphere are discussed. Characteristic properties of a newly developed reaction chamber from quartz (v = 200 L, temperature controlled, UV and IR absorption light paths, photolysis lamps) are briefly presented.

Hydroxycarbonyls are formed as products of the gas-phase atmospheric reactions of volatile organic compounds (VOCs) with OH and NO₃ radicals and O₃. In particular, ?-hydroxycarbonyls are formed from the reactions of the OH radical with $C₄ n-alkanes (and other >C₄ alkanes) and ?-hydroxycarbonyls are formed under laboratory conditions from the reactions of OH radicals with alkenes at low NO concentrations (such that the organic peroxy radicals react with organic peroxy radicals). To date, rate constants for the atmospherically important reactions of hydroxycarbonyls are only available for glycolaldehyde [HOCH₂CHO], hydroxyacetone [HOCH₂C(O)CH₃] and 4-hydroxy-4-methyl-2-pentanone. In this work, we have measured rate constants, or upper limits thereof, for the reactions of OH and NO₃ radicals and O₃ with the hydroxycarbonyls 1-hydroxy-2-butanone, 3-hydroxy-2-butanone, 4-hydroxy-3-hexanone, 3-hydroxy-3-methyl-2-butanone, 1-hydroxy-3-butanone, and 1-hydroxy-2-methyl-3-butanone. In addition, we have investigated the products of the reactions of the OH radical with 3-hydroxy-2-butanone and 4-hydroxy-3-hexanone, and investigated the formation of ?-hydroxycarbonyls from the reactions of a number of alkenes with the OH radical in the presence and absence of NO.

Chlorine atoms may be generated by reactions of sea salt particles transported inland with air masses. Chlorine atom precursors have been measured at coastal sites and in the Arctic at polar sunrise (Keene et al., 1993; Pszenny et al., 1993; Impey et al., 1997a, b; Spicer et al., 1998). A potential approach to investigate chlorine atom production in the troposphere is to identify and measure unique chlorine-containing products of the reactions of Cl with organics, such as 1,3-butadiene, in air. 1,3-Butadiene was classified as a hazardous air pollutant under 1990 Clean Air Act and is emitted from motor vehicles. If any unique chlorine-containing products can be determined from the reaction of Cl with 1,3-butadiene, they could serve as "markers" for chlorine atom chemistry in urban coastal areas where there are both sources of Cl atoms and 1,3-butadiene. We present studies of mechanisms and products of this reaction in 1 atm air and room temperature.

Product studies of the Cl-butadiene reaction were carried out by GC-MS and FTIR. 4-Chlorocrotonaldehyde (CCA) has been identified as a unique chlorine-containing product from the reaction of 1,3-butadiene with chlorine atoms in the presence or absence of NO in air. The yield of CCA is (35 ± 7)% (2s ) independent of the presence of NO. The yield of chloromethyl vinyl ketone (CMVK), another chlorine-containing compound from this reaction, is (18 ± 3)% (2s ) in the absence of NO. In the presence of NO, the upper limit of the yield of CMVK is < 9%. CCA could serve as a marker for Cl chemistry in coastal urban areas.

Impey, G.A., P.B. Shepson, D.R. Hastie and L.A. Barrie, Measurement technique for the determination of photolyzable chlorine and bromine in the atmosphere, J. Geophys. Res., 102, 15999-16004, 1997a.

Impey, G.A., P.B. Shepson, D.R. Hastie, L.A. Barrie and K. Anlauf, Measurements of photolyzable chlorine and bromine during the Polar Sunrise Experiment 1995, J. Geophys. Res., 102, 16005-16010, 1997b.

Keene, W.C., J.R. Maben, A.A.P. Pszenny, and J.N. Galloway, Measurement technique for inorganic chlorine gases in the marine boundary layer, Environ. Sci. Technol., 27, 866-874, 1993.

Pszenny, A.A.P., W.C. Keene, D.J. Jacob, S. Fan, J.R. Maben, M.P. Zetwo, M. Springer-Young and J.N. Galloway, Evidence of inorganic chlorine gases other than hydrogen chloride in marine surface air, Geophys. Res. Lett., 20, 699-702, 1993.

Spicer, C.W., E.G. Chapman, B.J. Finlayson-Pitts, R.A. Plastridge, J.M. Hubbe, J.D. Fast and C.M. Berkowitz, Unexpectedly high concentrations of molecular chlorine in coastal air, Nature, 394, 353-356, 1998.

Environmental chamber studies involve numerous chemical compounds with a variety of physical properties that make it difficult to identify all of them with a single analytical technique. Mass spectrometry is unique in its ability to detect most compounds, and is a valuable component for environmental chamber studies because of this characteristic. We show here that atmospheric-pressure-ionization mass spectrometry (API-MS) in the negative ion mode is a highly sensitive and selective technique ideal for measuring halogen compounds such as HOCl, Cl₂, and Br₂ both in laboratory systems and in air. This presentation will focus on the quantitative analysis of HOCl with API-MS.

HOCl may be an important intermediate in mid-latitude marine boundary layer chemistry and at high latitudes in spring-time when surface-level O₃ depletion occurs. However, techniques do not currently exist to measure HOCl in the troposphere. API-MS was used in the negative ion mode for this study. HOCl can be measured as the (HOCl· O₂)^- adduct in air. The intensity of this adduct is sensitive to the presence of acids and to the amount of water vapor, complicating its use for quantitative measurements. However, the addition of bromoform vapor to the corona discharge region forms bromide ions which attach to HOCl and allow its detection via the (HOCl· Br)^- adduct. The ion-molecule chemistry associated with the use of air or bromoform as the ionizing agent is discussed, with particular emphasis on the effects of relative humidity and gas phase acid concentrations. HOCl was quantified using its reaction with HCl to form Cl₂ for which calibrations could be easily performed. Detection limits are ~ 4 ppb using air in the ion source and ~ 0.4 ppb using bromoform. The atmospheric implications of this work are discussed.

Dimethylsulfide (DMS) is the major natural source of sulfur to the atmosphere with a source strength of between 12-54 Tg S yr^-1. The chemistry of DMS has been postulated to play a pivotal role in regulating the Earth’s radiation budget through a phytoplankton-DMS-clould albedo-climate feedback cycle. Although the atmospheric chemistry of DMS has been the subject of intense research an in-depth understanding of many facets of its chemistry still remain elusive.

Within the EU DOMAC project and also the German AFS project different aspects of the oxidation of DMS are being investigated. We report here combined results from both of these projects on the products of the OH-radical initiated oxidation of DMS as a function of temperature and O₂ partial pressure. Trends in the observed products SO₂, OCS, dimethylsulfoxide (DMSO: CH₃SOCH₃) methane sulfinic acid (MSIA: CH₃S(O)OH) and methane sulfonic acid (MSA: CH₃SO₃OH) and their relevance for the oxidation mechanism will be discussed. Much higher yields of MISA (up to 10%) have been observed than in previous product studies. The higher yields are attributed to improvements in the sampling method for MSIA achieved within the AFS project; it is thought that previous filter sampling methods have led to the destruction of MISA and thus very low apparent yields. At present it is not clear if MSIA is a direct product of the DMS oxidation or whether it is formed in the secondary oxidation of DMSO, the latter alternative appears to be the most probable. In a recent absolute product study of the OH + DMSO reaction (Urbanski et al. J. Phys. Chem. 102 (1998) 10522) a near 100% yield has been measured for the CH₃ radical which is the expected co-product if the DMSO oxidation leads to MSIA formation. Preliminary attempts to detect MSIA in the OH-radical initiated oxidation of DMSO will also be reported.

The results show quite conclusively that the addition pathway in OH + DMS leads to the formation of dimethylsulfoxide (DMSO) and that this pathway will dominate at low temperature. Yields of DMSO for the addition pathway as a function of temperature will be reported and their repercussions for other product channels in the CH₃S(OH)CH₃ + O₂ reaction will be discussed. First aerosol measurements on the oxidation of DMS will be discussed in the light of the present results.

Excerpts from Introduction

The formation of ozone and other oxidants in urban and rural areas remains a persistent problem that affects both the public health and economic vigor of many areas around the globe.  Oxidant formation results from photo-chemistry of organic compounds in the pres-ence of nitrogen oxides.  The organic component begins mainly as non-methane hydrocarbons (NMHC, containing only carbon and hydrogen) from biogenic and anthropogenic sources.  These compounds are progressively oxidized to CO and CO2 over peri-ods of hours to weeks.  The variety of primary hydrocarbons and their oxidation products is large.  While separate techniques exist to measure groups of compounds (e.g., C2-C8 hydrocarbons with some oxygenates, or alcohols, or formaldehyde or organic nitrates, etc.) no tech-niques to assess the total loading of non-methane organic carbon (TNMOC) have been widely applied in the atmo-sphere.  The goal of this work is to determine the relationship between the total non-methane organic compounds and the sum of the speciated volatile organic compounds (VOC’s) measured by standard techniques.  The primary sci-entific motivations are to define the airborne quantity of reactive organic carbon, find how close this quantity is to the standard measurements of VOC’s, and address the question of what happens to the multifunctional products of the photo-oxidation reactions of emitted hydrocarbons; do they stay in the gas phase or are they removed to the aerosol phase or surfaces?

There are several reasons to expect the TNMOC measurement to result in a number that is larger than the sum of the routinely measured VOC’s.  Loss of oxygenated and semi-volatile compounds in sampling and on columns is well known, although the extent of the losses is very difficult to quantify.  Modeling results indicate that while the initial oxidation step of a hydrocarbon may take place within hours, if our understanding of the relevant processes is correct, complete removal of the partially oxidized fragments should take weeks. Finally, in controlled smog chamber photooxidations, a standard gas chromatograph with flame ionization detection (GC/FID) measurement can account for much less than 100% of the reacted parent hydrocarbon.

Here we describe development of a new instrument to measure the airborne total non-methane organic carbon concentration (TNMOC), and the ratio of this value to the sum of speciated VOC’s measured by standard gas chromatography (GC-FID).  The approach is to make an in situ measurement that minimizes sample contact, cryo-trapping whole air samples with minimal trapping of CO2, CO and CH4.  Samples are processed by an oxidation catalyst to generate CO2 that is measured as TNMOC.  Alternatively, a standard speciated VOC’s measurement is made with the same instrument.  The TNMOC instrument described here has several improvements compared to previous approaches.  Samples introduced into our instrument are exposed to only an inlet tube, valve and short length of transfer tubing, and then immediately analyzed.  Water management is not needed.  Speciated VOC’s are measured with the same instrument, avoiding problems created by comparing different sample inlets and calibrations.

Understanding complex chemical systems, such as the chemistry of the polluted urban atmosphere, requires a mathematical model describing the physics and chemistry of the assemblage. Despite many years of laboratory experiments that have improved our understanding immensely, it is not possible to measure every possible reaction that should be considered in the model. Thus, estimation techniques based on laboratory understanding have been used extensively. Methodologies such as Benson's "Thermochemical Kinetics" and additivity and structure activity relationships (SAR) have been widely applied. Often however insufficient experimental measurements exist with which to begin extrapolation to larger molecules.

Recent advances in computational quantum mechanics have made it possible to compute potential energy surfaces for reactions that have not been measured. It would then seem possible to compute gas phase rate constants from judicious use of transition state theory (TST) and/or the microcanonical version applied to unimolecular reactions and their reverse known as RRKM theory.

We have set out to test this posit by computing the structural properties of reactants and transition states needed to apply the above theory. We have computed these properties for the reaction X + ethane = HX + ethyl for X = H, O, OH, NH₂, CH₃, and Cl. The thermochemistry is well-known for these reactions and the rate constants have been measured as functions of temperature for all of them. We have employed the following quantum chemistry methods: B3LYP, MP2 and QCISD and used the gaussian basis sets: 6-311G(d,p) and 6-311++G(3df,2dp). We first examined the computed values for DH for the reaction. We found that even the purportedly best calculation, QCISD(T) with the larger basis set, missed the experimental values by about 3 kcal/mole in several cases (O, OH and NH₂). As has been noted by others, the structures and frequencies for the stable molecules and the transition states were similar for all levels of calculation. Thus we used the B3LYP structures and frequencies in conjunction with a TST code, written by the late Alan Rodgers, that compares experimental rate constants as functions of temperature with computed values, accepting the B3LYP structural information for reactants and transition states, while searching for the best value of the activation barrier (DH¹₀). (Tunneling is accounted for iteratively using Eckart corrections.) We have found that using the ab initio structures we cannot fit the Arrhenius curvature measured for either OH or NH₂ with ethane. We have also looked at the O-atom reaction, but there is some question about the low temperature data.

There remains little question in our minds that the approach is sound. The questions to be resolved center around the meaning and accuracy of frequencies computed for the modes that represent internal rotation in the transition state. For instance, in the HO-Ethane reaction the motion corresponding to HO rotating about the newly forming bond that we may denote as HO---H---C₂H₅, is computed to be 41 cm^-1. We can fit the Arrhenius curvature with a value of 110cm^-1. Is this within the realm of uncertainty in these kinds of frequencies?

Hydroxyacetone (HA) is mainly produced in the atmosphere from oxidation of hydrocarbons of the type, CH₃(R)C=CH₂. Tuazon and Atkinson (1990) reported HA yield of 41% from the OH-initiated oxidation of methacrolein in the presence of NO_x. Since methacrolein is a major product of isoprene oxidation (Carter and Atkinson, 1996), isoprene, a key biogenic hydrocarbon, is therefore expected to be an important source for HA. Consequently, knowledge of ambient concentration of HA would provide information needed to examine the applicability of isoprene reaction mechanisms developed in laboratory and to assess the contribution of isoprene to photooxidant production.

The commonly used GC-FID technique involving cryo-focusing is unsuitable for HA owing to HA’s thermal instability. When subjected to a temperature of 100 C for only a few seconds, HA was found to disappear completely. Since HA is highly soluble in water (it’s Henry’s law constant being ~2 x 10⁴ M atm^-1 at 20 °C, Zhou and Lee, unpublished data), we developed a wet chemical technique similar in principle to the one we reported earlier (Lee and Zhou, 1993), namely, based on derivatization following liquid scrubbing. To increase the sensitivity, we adopted a fluorescence detection scheme based on o-phthaldialdehyde (OPA) chemistry. The technique was deployed in the field during two measurement periods at a NARSTO site located on Long Island (LI), New York. We report the principle and the operation of this technique and the results obtained from these field studies.

Fast capillary gas chromatography has been coupled to a luminol based chemiluminescent detection system for the rapid monitoring of nitrogen dioxide and peroxyacyl nitrates. A first generation instrument has been previously described recently in the literature (J.S. Gaffney, R.M. Bornick, Y.-H. Chen, and N.A. Marley, "Capillary Gas Chromatographic Analysis of Nitrogen Dioxide and PANs with Luminol Chemiluminescent Detection." Atmospheric Environment, 32, 1145-1154 (1998). This system is capable of monitoring nitrogen dioxide and PANs (to the C4 species) with one minute time resolution. This is an improvement of a factor of five over GC/ECD methods. Applications in aircraft research has been published electronically and will appear shortly in Environmental Science and Technology (J.S. Gaffney, N.A. Marley, H. D. Steele, P.J. Drayton, and J.M. Hubbe, "Aircraft Measurements of Nitrogen Dioxide and Peroxyacyl Nitrates Using Luminol Chemiluminescence With Fast Capillary Gas Chromatography." Environ. Sci. Technol., in press (1999)). An improved version of this instrument has been designed and built which makes use of a Hammatsu photon counting system. Detection limits of this instrumentation are at the low tens of ppt. Range of the instrument can be adjusted by sampling volumes and detection counting times. This system will be described in detail and an example of the application of this approach to measurement of nitrate radical production by concurrent measurement of ozone will be presented, along with the use of the instrument to evaluate chamber surfaces for nitrogen dioxide and PAN losses.

Aldehydes are major secondary products from the atmospheric oxidation of most of the volatile organic compounds. Their night-time reactivity toward NO3 could lead (Cantrell et al., 1986) to the production of related peroxyacylnitrate (PANs). However, the kinetic database for the reactions NO3 + RCHO is still inconsistent and the need of absolute measurement have been pointed out (Wayne et al., 1991).

To elucidate the full mechanism of the degradation of formaldehyde, acetaldehyde and propionaldéhyde, simulation chamber experiments have been conducted together with numerical simulation. Both long path in-situ FTIR and UV-visible DOAS measurements have been used in order to perform absolute studies by monitoring the concentrations of both VOCs and NO3.

The kinetics and the mechanism of the initial attack of the aldehyde by NO3 have been verified, the important reaction (Canosa-Mas et al., 1996) between NO3 and peroxyacyl radicals have been studied for the first time under atmospheric conditions, the extent of the heterogeneous reaction of peroxyacyl radicals have been quantified and the impact of the secondary chemistry leading to OH radicals production on relative rate measurements have been reviewed.
Canosa-Mas, C. E., M. D. King, R. Lopez, C. J. Percival, R. P. Wayne, D. E. Shallcross, J. A. Pyle and V. Daële; Is the reaction between CH3C(O)O2 and NO3 important in the night-time troposphere ?, Journal of the chemical society - Faraday Transaction 92 (1996) 2211-2222.

Cantrell, C. A., J. A. Davidson, K. L. Busarow and J. G. Calvert; The CH3CHO - NO3 reaction and possible nighttime PAN generation, Journal of the geophysical research 91 (1986) 5347-5353.

Wayne, R. P., I. Barnes, P. Biggs, J. P. Burrows, C. E. Canosas-Mas, J. Hjorth, B. Le, G., G. K. Moortgat, D. Perner, G. Poulet and G. Restelli, Sidebottom,H.; The Nitrate radical : Physics, chemistry and the atmosphere., Atmospheric Environment 25 A (1991) 1-206.

(View manuscript in PDF format)

Although PM2.5 can be directly introduced into the atmosphere through primary emissions, the mass concentration of PM2.5 is also affected by secondary processes such as nucleation or condensation of nonvolatile and semivolatile compounds on pre-existing PM2.5. To address these issues, a laboratory research program was developed at EPA to investigate the key chemical processes that determine the contributions of secondary processes to the overall mass concentrations of PM2.5.  The program consists of experiments to (1) measure  the secondary organic aerosol (SOA) yields of atmospherically relevant hydrocarbons under ambient concentration and relative humidities; (2) determine the chemical composition of the multi-functional compounds in the SOA chamber samples and compare those findings with field study results; (3) measure the partitioning coefficients of semivolatile compounds in SOA; and (4) evaluate methods for collecting semivolatile SOA.

As part of the program a series of chamber experiments was carried out to determine to what extent photochemical oxidation products of aromatic hydrocarbons contribute to SOA aerosol formation through uptake on pre-existing inorganic aerosols in the absence of liquid water films (1).  The irradiation experiments were conducted with toluene, p-xylene, and 1,3,5-trimethylbenzene in the presence of NOX and ammonium sulfate aerosol, with propylene added to enhance the production of radicals in the system.  The mass concentration of the organic fraction was obtained by multiplying the measured organic carbon concentration by 2.0, a correction factor that takes into account the presence of hydrogen, nitrogen, and oxygen atoms in the organic species.  In addition, mass concentrations of ammonium, nitrate, and sulfate as well as total gravimetric mass concentrations were measured.  The reconstructed mass concentrations were in reasonable agreement with the gravimetrically determined values.  The largest secondary organic aerosol yield of 1.59 +/- 0.40% was found for toluene at an organic aerosol concentration of 8.2 micro g m-3, followed by 1.09 +/- 0.27% for p-xylene at 6.4 micro g m-3, and 0.41 +/- 0.10% for 1,3,5-trimethylbenzene at 2.0 micro g m-3.  In general, these results agreed with those reported by Odum et al.,(2) and support the gas-aerosol partitioning theory developed by Pankow (3).  The presence of organics in the aerosol did not affect significantly the hygroscopic properties of the aerosol for relative humidities between 35% and 80%.

Atmospheric research in the School of Chemistry (Leeds) involves several groups working in closely related areas, studying a wide variety of topics of relevance to atmospheric processes.  The Tropospheric Chemistry Modelling group is involved with all aspects of model construction and application.  Part of this work has been a collaborative project, funded by the UK Department of the Environment, Transport and the Regions (DETR), to develop and apply predictive models to the formation of tropospheric ozone on a range of different geographical scales (i.e. global, regional and national).  The insight gained in this manner can aid in the formulation of policy with regards to the air quality and ambient levels of ozone in the United Kingdom.  The master chemical mechanism (MCM) underpins much of the current ozone modelling undertaken on the behalf of the DETR.
The main intention of the web site is to provide a flexible, easily utilised platform for the MCM that is readily accessed by the whole research community, and to promote its collaborative development and evaluation.  This paper details updates and developments that have occurred since the launch of the MCMv1.0 web site (Saunders et al. 1997).  The current MCMv2.0 consists of around 10500 reactions and 3500 species and the web site is located at: http://chem.leeds.ac.uk/Atmospheric/MCM/mcmproj.html

An overview will be given of experiments on the OH initiated degradation of aromatic hydrocarbons that were performed in the European Photoreactor (EUPHORE). EUPHORE is a large volume out-door reaction chamber located in Valencia/Spain. Various analytical techniques were used to monitor parameters like the concentrations of reactants and products, the temperature, the solar light intensity and particles as a function of time.

The data presented will primarily focus on toluene. In addition, photolysis experiments on selected dicarbonyl products of the OH initiated oxidation of aromatic hydrocarbons were performed.

The results obtained will be discussed with regard to their implications for degradation mechanisms of aromatic hydrocarbons. Comparisons will be made with existing chemical degradation schemes that are employed in computer modeling studies.

Many different types of oxygenated organic compounds are currently being investigated for their suitability as alternative solvents or as fuel additives. The main stimulus for the interest in such compounds in Europe is existing or pending legislation demanding the regulation of the reactivity of the anthropogenic VOC emissions to the atmosphere, in particular from solvents and motorised vehicles. Obviously to assess the environmental acceptability of alternative oxygenated solvents or fuel additives a comprehensive understanding of their gas-phase oxidation mechanism is required, preferably prior to deployment of the compounds.

This presentation will deal with work within the EU INFORMATEX project, part of which dealt with investigating the atmospheric photo-oxidation mechanisms of ether compounds used as fuel additives and the atmospheric chemistry of acetals, potential diesel additives.

The INFORMATEX project concentrated on investigating the atmospheric photo-oxidation mechanisms of ether compounds already in use for gasoline reformulation, i.e. MTBE, ETBE and TAME. A major aim of the project was the investigation of the photo-oxidant formation with real exhaust samples obtained from a test engine run with differently reformulated fuels. Detailed chemical mechanisms have been developed within the project to simulate the oxidation processes occurring upon irradiation of the exhaust from the differently blended fuels. In addition, the data obtained within the project has been used for scenario calculations with a three-dimensional photochemical meso-scale model taking Geneva and Athens as examples. The results from the project will be summarised.

Acetals, are compounds with the diether type structure ROCH₂OR, and are possible alternative solvents with potentially low atmospheric reactivity. Recent tests have also shown that addition of small quantities of methylal (CH₃OCH₂OCH₃) and butylal (C₄H₉OCH₂OC₄H₉) to diesel fuel are very effective in reducing the engine particle emissions and in the case of butylal also the NO_x emissions. Investigations on the atmospheric chemistry of a series of acetals performed in the laboratory and also in the large outdoor EUPHORE simulation chamber in Valencia in Spain will reported. Apart from details on the kinetics and mechanisms of their gas-phase atmospheric oxidation, studies will be presented on the ozone forming potential of the compounds and their primary oxidation products. This information is being used to assess the possible major atmospheric advantages and/or disadvantages of the widespread use of this type of compound on local and larger scales.

In recent years the refomulation of gasoline by adding oxygenated VOCs (Volatile Organic Compounds) has become more common. There are several advantages of using oxygenated organic compounds such as ethers and alcohols as fuel additives. These compounds enhance the octane level, increase the efficiency of combustion, and reduce the emission of atmospheric pollutants, e.g. hydrocarbons, CO and particles. In addition, oxygenated VOCs are used as alternative solvents.

The mechanistic knowledge about the tropospheric degradation of a large number of oxygenated hydrocarbons is rather poor. Accordingly, the present work was attended to the development of OH-initiated degradation mechanisms of selected oxygenated VOCs, namely dimethoxymethane (DMM), dimethoxyethane (DMET), 1,3-dioxolane and 1,4-dioxane.

The reaction schemes developed were checked against suitable experimental photoreactor data. Sensitivity analyses of the chemical models provide a focus on key reactions of the mechanisms investigated.

For the compound class of acetals with the general structure RO-CH₂-OR the established structure activity relationship technique of Kwok and Atkinson [1] has been modified. The new approach yields much better agreement between theoretical and experimental rate coefficients for the reactions of acetals with OH than the former technique. Also the branching ratios of the OH attack to the different H atoms of the target molecules are now described much better than by the original method.

A number of laboratory studies have been performed on the hydroxyl radical initiated oxidation of ethers resulting in chemical mechanisms for their tropospheric degradation. The main objective of this work was to validate the proposed mechanisms for diethyl ether and diisopropyl ether through photooxidation experiments carried out in the presence and absence of NO_x at the outdoor European Photoreactor, EUPHORE. For reactions carried out under high NO_x conditions, the oxidation products and their yields were similar to those reported in laboratory-based investigations. For reactions carried out in the absence of NO_x, a slight difference in the product yields was observed for diethyl ether, whilst for diisopropyl ether a completely different product distribution was obtained. These findings are explained by chemical mechanisms that describe the hydroxyl radical initiated oxidation of the ethers in the presence and absence of NO_x.

Kinetics and mechanism of OH and ozone reactions with ethyl vinyl ether, methyl acrylate and methyl methacrylate have been investigated. The rate coefficients of the OH reaction with these compounds have been measured using the pulsed photolysis laser induced fluorescence method. The kinetics of ozone reaction and the oxidation mechanisms initiated by OH and ozone in air have been studied using the European photoreactor (EUPHORE). A teflon bag (100 L) irradiated by UV lamps has also been used for the studies the OH-initiated oxidation studies. The obtained results will be presented.

The distribution and fate of volatile organochlorine compounds has recently attracted considerable attention. In particular, it has been suggested that phytotoxic chloroacetic acids found in a number of environmental compartments arise from the atmospheric oxidation of chlorinate industrial solvents such as trichloroethene, tetrachloroethene and 1,1,1-trichloroethane. A number of investigations have been previously carried out to elucidate the OH radcial and Cl atom initiated oxidation of these compounds which suggest that chloroacetyl chlorides may be formed in these reactions under certain experimental conditions. The main objective of this work was to validate the proposed mechanisms through potoxidation studies carried out in the presence and absence of NO_x at the European Photoreactor EUPHORE. The contributions of trichloroethene tetrachloroethene, and 1,11-trichloroethane to the formation of chloroacetic acids in the atmosphere have been estimated and compared to the observed levels in the various geographical regions.

It has been recognized for more than 30 years that the walls of “smog chambers” provided sites for heterogeneous reactions that influenced the outcome of the homogeneous gas phase reactions. In the early 1980’s several efforts were made to elucidate the origins of these effects, with most attention focused on the heterogeneous formation of nitrous acid (HONO) which was subsequently released into the gas phase. While these efforts effectively demonstrated that most likely HONO was being formed, actual mechanisms and process rates for this phenomena have not been developed for chambers. Instead a small set of un-real or parameterized reactions have been used to approximate the HONO formation based upon “characterization experiments.” These experiments have included the simplest type of experiments, i.e., NOx/CO, NOx/CH4, and NOx/n-butane experiments. The basic conceptual model has been that the gas phase chemistries of these simple systems are so well understood, that any failure of a reaction mechanism simulation to repro-duce the observations from these experiments was due to chamber wall processes or chamber-dependent initial conditions. These chamber-dependent conditions included unintended creation of initial HONO due to injection of the initial NOx from high concentration sources. Such chamber-dependent reactions and inputs have often been expressed as “Auxiliary Mechanisms,” one for each chamber to be simulated. When testing or evaluating a reaction mechanism in a given chamber, a chamber-dependent mechanism is combined with a “Principle (or Core) Mechanism,” which is asserted to be chamber-independent. Obviously mistakes or misrepresentations in the auxiliary mechanism can introduce compensating errors in the principle or core mechanism.

Chemical reactions occurring on the walls of environmental chambers provide a large fraction of the radical species which initiate gas phase smog chemistry, but the nature and magnitude of these processes have not received focused attention. For example, the empirical models used in conjunction with the UNC and Riverside smog chambers contain different reaction schemes and product distributions for the adsorption and reaction of NO2 on surfaces. In addition, several parameters are often adjusted empirically when environmental chamber data is analyzed. The current understanding of wall chemistry and its parameterization in models are briefly reviewed here.

Determination of the fundamental processes involved in wall reactions is needed for prediction of heterogeneous radical generation in different chambers under a variety of conditions. Techniques available for quantitative study of these heterogeneous processes include Knudsen cell, flow tube, and aerosol methods. The use of these methods in our laboratory is demonstrated, and their applicability to the study of environmental chamber wall processes is discussed.

Heterogeneous hydrolysis of N₂O₅ on aqueous aerosol surfaces is an important atmospheric source of HNO₃. Reaction probabilities g_N2O5 of N2O5 on several aqueous inorganic substrates, like ammonium sulfates, which compose the tropospheric aerosol, have been reported in the literature. These g_N2O5 are typically of the order of several times 10^-2 and vary relatively little for a wide range of substrates, from pure water over deliquescent aerosols to metastable aerosols. The relative insensitivity of g_N2O5with respect to the changing water activity has lead to the proposal of an ionic reaction mechanism in the aqueous phase with reaction (R2f) is as the rate limiting step:

We measured g_N2O5on aqueous sodium nitrate, sulfate, and bisulfate aerosols, in the large aerosol chamber at the FZ-Juelich at different relative humidities and room temperature. At a relative humidity of 50% corresponding to a NO3- molality of 27 we observed a g_N2O5 of 0.0018, which is an order of magnitude smaller than g_N2O5 on water, ammonium and sodium sulfate aerosol at similar ionic strength. g_N2O5 increases by an order of magnitude to 0.023 when the relative humidity is raised to 90% which corresponds to a NO3- molality of 4. These findings can be explained by the increasing importance of the recombination reaction (R2b) with decreasing relative humidity and correspondingly increasing NO3- molality.

The effect of NO3- can be rationalized by a steady state consideration for N2O5(aq) and NO2+ within a thin aerosol surface shell. The observation of a specific nitrate effect is a direct experimental indication for the ionic uptake mechanism (R1)-(R3).

References: A. Wahner et al., J. Geophys. Res. (1998)

Results from recent photochemical and kinetic laboratory and modelling studies of the formation and reactivity of aqueous phase free radicals such as OH, NO3, and Cl/Cl2- will be presented. Laser-based methods have been applied for the specific generation and time-resolved detection of the above transient species in systematic studies. Photolytic radical generation in solution, radical phase transfer, radical interconversion reactions and the influence of organic compounds on the chemistry within the aqueous tropospheric phase will be discussed. Results indicate that solution reactions of the above radicals may significantly influence the net effects of chemistry within droplets and aerosols dispersed in air.

A multiphase box model coupling an advanced aqueous phase mechanism (CAPRAM 2.4) to RADM2/RACM is applied to quantify effects of multiphase conversions. It will be discussed how aqueous phase processes alter the oxidation capacity of the tropospheric gas phase by uptake and release of trace gases and radicals. Current restrictions of models will be outlined.

One major obstacle that will limit the extent to which environmental chambers can generate useful mechanism evaluation data under low NOx conditions is offgasing of NOx from the walls of environmental chambers. Evidence for NOx offgasing comes from the formation of O3 in “pure air” irradiations, which cannot occur to any significant extent in the absence of NOx, the observation of PAN formation in acetaldehyde - air irradiations, and other data (see Carter and Lurmann, 1991, and references therein.)  In addition, NOx offgasing in the form of HONO is considered to be the most likely explanation for the “unknown chamber radical source”, which also significantly complicates mechanism evaluation using environmental chamber data (Carter et al, 1982, 1995a,b)

In this paper, we give a brief summary of preliminary analysis of previous and new data obtained concerning NOx offgasing from environmental chamber surfaces. The emphasis will be on NOx offgasing from indoor chambers constructed of FEP Teflon film, since this is currently the preferred material for constructing chamber walls because of its relative inertness and good light transmission properties. However, data from a chamber with a different type of surface will also be presented, for comparison.

The investigation of chemical processes in ‚smog chambers‘ or photoreactors is a quite old tool in atmospheric chemistry research. In fact the factors determining urban ozone formation were empirically known from smog chamber studies long before the photochemical theories involving free radicals (i.e. OH) were formulated and experimentally verified.

Recent advances in measurement techniques made it possible to directly determine concentrations of a variety free radicals during photoreactor experiments. For instance in the European Photoreactor (EUPHORE) in Valencia successful, direct detection of several free radical species at concentrations comparable to ambient levels was demonstrated:

• Nitrate radicals (NO₃) by Differential Optical absorption spectroscopy (DOAS)
• Hydroxyl radicals (OH) by laser induced fluorescence (LIF)
• Sum of peroxy radicals (RO₂) by chemical amplification

Compared to indirect determination from e.g. relative rate experiments direct detection of free radicals allows better characterisation of chemical mechanisms, for instance in the degradation of aromatic hydrocarbons. In addition absolute reaction rate constants can be measured, this has been done for NO₃ + VOC reactions. Another example is the determination of the equilibrium constant NO₃ + NO₂ Û N₂O₅.

The various techniques employed for the measurement of free radicals in the EUPHORE photoreactor are described, some applications, and future plans are discussed.

Peroxy radicals (HO₂ and RO₂) are key species in the photochemical formation of ozone, peroxides and organic nitrates in the troposphere. They arise from the oxidation of volatile organic compounds (VOC) and CO by hydroxyl radicals (OH) and also by photolysis of carbonyls, e.g HCHO. Measurements of HO₂ and RO₂ allow a check of the representation of the radical chemistry in chemical models. We present measurements of HO₂ and RO₂ made by Matrix-Isolation followed by electron spin resonance (MIESR) and also by using a chemical amplifier (CA). OH measurements were performed by Laser-induced fluorescence (LIF). We studied the oxidation of i-butane in the presence of NO_x and HCHO in the European Photoreactor EUPHORE, where O₃, HCHO and i-butane were measured using FTIR. Nitrates were measured using GC-ECD. The radical yield from the photolysis of HCHO was determined in additional experiments. Comparison of the measurements with model calculations shows that the split between HO₂ and RO₂ can only be explained when assuming a rate coefficient of 4 ´ 10^-12 cm³s^-1 for the reaction of t-C₄H₉O₂ with NO, as reported by Peeters et al., 1992. Future work will include alkenes and aromatics.

J. Peeters, J. Vertommen, and I. Langhans, Ber. Bunsenges. Phys. Chem. 96 (1992) 431-436.

Oxidized nitrogenous species play a critical role in the formation of ozone in the atmosphere. For both smog chambers and ambient air a knowledge of the mass balance of such species is useful for the development and validation of methods to model ozone formation. The term NOy has been used to describe the total reactive nitrogen oxides, or odd nitrogen, while (NOy)i has been used as an operational definition as the response of a chemiluminescent NO analyzer after the sample is treated with a converter that reduces more highly oxidized species to NO. These definitions, though slightly different should be quite similar based on our knowledge of the individual oxidized nitrogenous species typically found in ambient air. It is generally agreed that (NOy)i consists primarily of nitric oxide (NO), nitrogen dioxide (NO2), peroxyacetyl nitrates (PAcNs), nitric acid (HNO3), particulate nitrate, and nitrous acid (HONO) roughly in that order of importance. Commercial converters have been shown to readily reduce these gaseous species while they have no efficiency for N2O or organic nitro compounds, which are not considered photochemically reactive. The reduction efficiency for particulate nitrate varies widely with converter design and cation. The objective of this paper is to describe some of the interferences, biases, and idiosyncrasies that we and others have observed when measuring (NOy)i. It should be noted that he sub ppb sensitivity of the new generation of commercial NO analyzers have made these measurement artifacts more noticeable.

(NOy)i converters are typically placed outside while the detector assembly remains in an air-conditioned environment. This and not using a particulate filter is done to insure that the sampling lines do not adsorb nitric acid. Most converters commercial NO-NOx analyzers use molybdenum-based heated converters. Although it has been reported that these converters reduce little ammonia, we have found converter efficiencies that varied from 5 to 25% for this species. The efficiency did not correlate with the converter temperature. In addition to a long lag time for nitric acid, we have observed a memory effect after sampling high (NOy )i concentrations, that resulted in an elevated zero that required hours to return to baseline. We have also observed a converter hysteresis in which converter have developed variable efficiencies dependent on prior usage. Converters consisting of heated gold tubing have been reported in a number of research applications, particularly in rural areas. Either CO or H2 is added to the sample stream to facilitate the reduction of NOy. While these converters work efficiently, they tend to require frequent cleaning. These converters have been shown to have the potential for significant efficiency for ammonia and cyanides and to be dependent on water, ozone concentration and previous sampling history. Other converters have been constructed with ferrous sulfate at ambient temperature and vitrous carbon at high temperature.

Studies where the major individual NOy species are measured and added together result in a summation that is typically significantly less than the measured (NOy)i. This missing species has been dubbed compound X and the significance of this missing species appears to be location and method dependent. Based on our findings on ambient and laboratory-generated polluted air, it may well be ammonia.

Conclusions

• There are considerable conflicting reports with respect to converter efficiencies for NOy compounds and interfering compounds.
• The use of presently used converters result in significant biases and interferences
• Better converters need to be developed
• Until better converters are developed frequent zero checks (perhaps every hour or two) are needed as well as routine assessment of converter efficiency for both (NOy)i species and ammonia.

Aerosol smog chamber studies are performed to elucidate the chemical and physical processes that occur in the atmosphere leading to the formation of fine particulate matter. In order to obtain this goal, the number concentration, size distribution and chemical composition of secondary aerosol produced in an environmental chamber must be monitored as it evolves with time. An ideal instrument would be able to obtain both the chemical and physical properties of the aerosol at time scales comparable to the evolution of the aerosol in the chamber at infinite resolution. A road map on the available instrumentation and the desired measurements for an environmental chamber is laid out. Critical environmental control parameters are also discussed as are different techniques to ensure that the reactions and secondary aerosol formation in the environmental chamber can occur at time scales and size ranges that can be easily detected by current instrumentation. Also mentioned are some of the new techniques being implemented at Caltech to increase the accuracy and precision of measurements made in our chamber as well as improvements in the time and spatial resolution of our measurements.

As part of a research program focusing on studies of the chemistry of gas-to-particle conversion, we have recently developed a new instrument for particle chemical analysis. This instrument, which we refer to as a thermal desorption particle beam mass spectrometer (TDPBMS), can be used for real-time, quantitative analysis of the components of organic particles, at least within the ~0.02-0.5 micrometer size range. We have also developed a temperature-programmed TDPBMS technique to aid in compound identification. Here we describe the operation of the TDPBMS and present results from our recent application of TDPBMS to studies of the chemistry of secondary aerosol formation, in which we have analyzed the composition of aerosol particles formed in environmental chamber reactions of 1-tetradecene and ozone in the presence of alcohols, carboxylic acids, and water vapor.

A kinetic mechanism was used to describe the gas-phase reactions of a-pinene with ozone. This reaction scheme produces low vapor pressure reaction products that distribute between gas and particle phases. Partitioning was treated as an equilibrium between the rate of particle uptake and rate of particle loss of semivolatile terpene reaction products. Given estimated liquid vapor pressures and activation energies of desorption, it was possible to calculate gas-particle equilibrium constants and aerosol desorption rate constants at different temperatures. Gas and aerosol phase reactions were linked together in one chemical mechanism and a chemical kinetics solver was used to predict reactant and product concentrations over time. Aerosol formation from the model was then compared with aerosol production observed from outdoor chamber experiments. Approximately 10-40% of the reacted a-pinene carbon appeared in the aerosol phase. Models vs. experimental aerosol yields illustrate that reasonable predictions of secondary aerosol formation are possible from both dark ozone and light/NOx a-pinene systems.

Global biogenic emissions from natural and agricultural plants of non-methane hydrocarbons are an order of magnitude higher than anthropogenic emissions of organic carbon. Biogenic hydrocarbons, composed mainly of isoprene and monoterpenes, react quickly with atmospheric oxidants to form ozone and atmospheric aerosols. The secondary organic aerosol yields of 14 biogenic precursors are investigated here. Individual contributions of the nitrate radical, ozone and the hydroxyl radical to SOA formation are deconvoluted. Additionally, the products of a-pinene and D3-carene dark ozonolysis reactions are presented here. For more detail on biogenic SOA yields or procedures for analysis of the chemical composition of SOA products, please refer to the following papers:

Results of studies of aerosol formation carried out in a large outdoor smog chamber (EUPHORE, Valencia, Spain) as well as in smaller chamber facilities in Ispra, Italy, will be presented. Analysis of the available data, obtained by photo-oxidation experiments, where mixtures of terpenes and NO_x are exposed to sunlight, as well as by ozonolysis of terpenes in purified air, suggest that ozonolysis reactions give higher particle yields than OH radical reactions. However, more information about the influence of [NO_x] on aerosol formation is needed. Chemical analyses of particles formed by oxidation of terpenes have been performed and a number of components (particularly organic acids) have been identified. Also the formation of products with low volatility by the further degradation of some main primary products of the oxidation of terpenes, has been investigated.

Small aldehydes are formed during the photochemical oxidation of many VOC’s, olefins and terpenes in urban as well as in rural areas. Photolysis and reaction with the OH radical are the most important initiation reactions for the removal of these compounds conducting to the formation of peroxy radicals and in the case of photolitic decomposition, either to stable molecules or free radicals.

The photolysis frequencies for various small aldehydes were measured in the EUPHORE Smog Chambers by photolysing the aldehydes with natural sunlight. The actinic flux during the experiment was measured with a spectroradiometer. The decay of aldehydes and formation of products were analysed by FTIR spectroscopy, gas chromatography and HPLC. The major products are explained in the case of acetaldehyde, propionaldehyde and i-butyraldehyde by a mechanism involving a primary dissociation step which leads to the formation of free radicals. The product analysis for photolysis experiments of butyraldehyde, pentanal and longer chain aldehydes indicates that for these molecules two primary photodissociation steps occur which gives either stable molecules or free radicals. Integrated quantum yields can be calculated from the ratio of the theoretical photolysis frequency using the measured radiation data and known absorption cross-sections by assuming a quantum yield of unity and the measured photolytic decay rate. The results obtained can be employed in numerical models which describe the tropospheric degradation of these compounds in order to assess the importance of the additional radical production on the atmospheric oxidation capacity and ozone formation potential of the precursors VOCs.

The investigation of the influence of fuel formulation on atmospheric processes, in particular, formation of photooxidants in the low troposphere should provide kinetic and mechanistic data needed for the development of abatement strategies based on the use of various fuel compositions. One of the objectives of the work was the investigation of the influence of fuel formulation on the exhaust gas reactivity, which was assessed by smog chamber experiments.

A motor test bed, which consists of a FORD diesel engine and an air-cooled eddy current brake has been successfully installed at the EUPHORE smog chamber. A matrix for diesel fuel and gasoline has been developed.

From the photooxidation experiments using carbon monoxide in clean air with different NO_x concentrations, the first order rate constant describing the OH formation on the chamber walls due to the heterogeneous reaction of NO₂ and water was evaluated. In addition, the smog chamber characteristics with respect to heterogeneous HONO formation was investigated using a wet effluent diffusion denuder. Experiments performed with gasoline clearly showed that the fuel composition has a strong impact on ozone formation. In a recent measurement campaign the impact of the exhaust of the different diesel fuels on the NO_y chemistry, in particular, HONO formation was investigated. The results from these campaigns will be presented and discussed.

Towards the end of the 1940s the air’s contamination began in Mexico. It just happened when Mexico starts its industrial growing. During these years the population increased notably. Industrial growing in the 1950s was carried through an urbanization process (Restrepo, 1992). Mexico City was the first because it had industries, services, government offices, culture attractions and higher education. It was followed by suburbs of the State of Mexico, which are close to the Mexico City. Nowadays it is called Mexico City’s metropolitan area (MCMA) and it lies in a high valley surrounded by mountains located 2240 meters above the sea level capable of holding in pollutants released by its more than 20 million inhabitants. This altitude causes a fuel’s incomplete combustion because of low oxygen concentration in the air. In addition to average low velocities (< 1.5 m/s) almost during 7 months in a year (Martínez, 1996). Due to that, MCMA has a behavior close to a smog chamber and has suffered during the past two decades from increasingly severe smog events with very high ozone (O₃) levels. On the other hand, the major non-methane hydrocarbons (NMHC) components and nitrogen oxides (NOx) of the contaminated atmospheric mixture proceed from the approximately 3 million vehicles in the MCMA and of industrial operation (inventario de emisiones, 1996).

Taking in account this problem was necessary to do research look into the ambient air the conditions under it was happening. Some papers were consulted in order to begin an investigation. The ozone formation depends not only on the HC and its atmospherics reactions, but also on the conditions of the system where the HC are reacting (Carter, 1994). So with a budge cut out, it was decided to looking for ozone control strategies under real conditions. Several experiments were conducted to simulate potential O₃ control strategies involving the effect of adding NMHC or NOx and reductions of them in outdoor smog chambers. It was made a similar experiment as Nelson A. Kelly carried out in Los Angeles (Kelly and Gunst, 1990).

Captive-air under natural irradiation (CAI) was used to evaluate the effects of HC and NOx changes on peak springtime O₃ levels. The experiments were performed at the Mexican Petroleum Institute (IMP). It is located in northwest of Mexico City near large sources of HC and NOx emissions. At this site. The CAI experiments were run on 17 days from 3 to 22 of April in 1995.

The experimental program was designed with the primary objective of finding out the response of maximum ozone levels, O₃ (max), to changes in the initial concentrations HC and NOx. The experimental design consisted of four levels each for HC and NOx; two levels of reduction below the ambient level (-25% and -50%). The ambient level, and one level of increase above the ambient level (+25%). So that, 16 HC-NOx experiments were possible each day, however only eight chambers could be operated each day. The eight HC-NOx combination to be tested on a given day were selected to meet two objective more: to provide replicates, with identical prepared mixtures and to minimize the confounding effects of day to day variations. In this scheme, at least one unperturbed chamber was included each day. The selection of the chamber to be used for each experiment in an eight-chamber block of experiments was made randomly during the study. A total number of 136 experiments were performed on 20 days. Control chamber experiments were run every day. Those experiments comprised the largest fraction of the total.

Gas-phase smog chamber experiments are being performed at EPA in order to develop databases that can be used to evaluate a number of current chemical mechanisms for inclusion in EPA’s regulatory and research models. This work differs from that being performed in other institutions because the emphasis at EPA is on examining a variety of different mechanisms, rather than developing a single mechanism, and evaluating the appropriateness of each one for the various types of applications of air quality models at EPA. The mechanisms that we are currently studying include the Carbon Bond IV 99 (CB4-99) (Adelman, 1999), the SAPRC99 mechanism (Carter, W.P.L., 1999, private communication), and an updated version of the RACM mechanism (Stockwell, et. al., 1997). In addition, the database will provide information for studying a variety of mechanisms formulated using the newly-developed Morphecules method (Jeffries et al., 1999). In keeping with this objective, the experimental conditions in the chambers are chosen to stress the mechanisms and enhance the differences among mechanisms. By providing slightly different chamber conditions and experimental conditions than are found in other existing smog chambers, the EPA chamber data expands the database of experiments that are available for robust mechanism evaluation.

To better understand fates of aromatics hydrocarbon species in the atmosphere, we have investigated the transformation chemistry of butenedial (CHOCH=CHCHO), 4-oxo-2-pentenal (CH3COCH=CHCHO), and 3-hexene-2, 5-dione (CH3COCH=CHCOCH3).  These 1,4-unsaturated dicarbonyls are known to be products of aromatic photochemical oxidation.  Both hydroxyl radical (OH) and ozone (O3) initiated smog chamber experiments under atmospheric conditions were conducted in the University of North Carolina 300,000-liter dual  outdoor smog chamber.   In daytime outdoor experiments, we began with oxides of nitrogen and butenedial, or 4-oxo-2-pentenal, or 3-hexene-2,5-dione in the chamber.  In nighttime experiments, ozone was injected at a constant rate into each side of the chamber for the duration of the experiment.  One side was filled with an initial amount of the compound to be studied and about 100-150 ppmV cyclohexane as the OH radical scavenger.  The other side served as a reference for O3 concentration.  The results show that these compounds are highly reactive.

Carbonyl intermediates and products were measured using the O- (2,3,4,5,6-pentafluorobenzyl)-hydroxylamine derivatization method followed by gas chromatography /ion trap mass spectrometry analysis.  By comparison with their corresponding standards, four categories of products are found in the OH and O3 initiated reactions, namely, simple aldehydes, dicarbonyls, unsaturated carbonyls, and hydroxy carbonyls.  Many of the products are on the EPA’s HAPs list.  Carbonyl products detected and identified by comparison with standards in the OH-initiated photooxidation of butenedial include formaldehyde, acrolein, glycolaldehyde, glyoxal, and malonaldehyde (CHOCH2CHO).  For 4-oxo-2-pentenal, the carbonyl products were formaldehyde, methyl vinyl ketone, glycolaldehyde, hydroxyacetone, glyoxal, methylglyoxal, and malonaldehyde.  For 3-hexene-2, 5-dione the products were formaldehyde, acetaldehyde, acetone, hydroxyacetone, and methylglyoxal. Carbonyl products detected in the O3-initiated experiments with cyclohexane as the OH scavenger were formaldehyde and glyoxal in butenedial; formaldehyde, glyoxal, methylglyoxal, and malonaldehyde in 4-oxo-2-pentenal; and formaldehyde and methylglyoxal in 3-hexene-2, 5-dione.  The new carbonyls we detected belong to categories of unsaturated carbonyls (acrolein, methyl vinyl ketone), hydroxyl carbonyls (glycolaldehyde and hydroxyacetone), and dicarbonyls (malonaldehyde).
We also observed a few compounds whose molecular weights were obtained by mass spectra, but whose identities could not unambiguously be determined because of lack of standards. Their possible structures are discussed in the text. Possible identifies of unknowns are listed as well as compounds known not to be unknowns.

Rate constants of reactions of O3 with these unsaturated dicarbonyls were computed from O3 loss in these systems with OH radicals suppression. The measurement technique is described.  The rate constant computed for O3 with butenedial is (1.6 ? 0.1) ? 10-18 cm3molecule-1s-1 at 294-298 K, with 4-oxo-2-pentenal is (4.8 ? 0.8) ? 10-18 cm3molecule-1s-1 at 293-297 K and with 3-hexene-2,5-dione is  (3.6 ? 0.3) ? 10-18 cm3molecule-1s-1 at  295-297 K.  Also reported are time series of some carbonyl products.  Calibration techniques including preparation of gas-phase standards are described.  For high boiling point polar compounds, e.g., hydroxyacetone, glyoxal, methylglyoxal, and glycolaldehyde, direct injection into the chamber is difficult. To solve this problem, we used a Collison nebulizer for injection.  We assumed that after injected in the chamber the relative ratio of each compound in the solution remains the same as in the nebulizer solution.  By using a nebulizer injection internal standard, the concentration of other compounds could be calculated according to the carbonyl’s ratio and the known internal standard concentration. C5-hydroxy carbonyls such as 4-OH-3-methyl-2-butanone and 3-OH-3-methyl-2-butanone were found to be appropriate standards.  They are polar enough so that they will not be off-gassed when injected by nebulizer and yet, they have sufficiently low boiling points that they can be directly injected into the chamber.

UV absorption spectra were measured.   Reaction mechanisms are proposed and discussed in the last part of the paper.  The evidence for unknown temperature sensitive nitrates is presented and discussed.

Gas-phase chemical mechanisms for use in regional or global tropospheric atmospheric chemistry applications require validation against measurement data in order to assure the modeller and policy-makers that the mechanism adequately represents the chemical processes of the real atmosphere.  One means of mechanism validation is through the use of smog-chamber data, in which atmospheric chemistry experiments are performed under controlled conditions, and eliminate the potential confounding effects of non-chemical processes in the ambient atmosphere.  The absence of the ambient atmosphere’s confounding factors (in which some of the chemical changes may be due to advection, diffusion, emissions, deposition, convection, rainout and washout) allow a worthwhile comparison of chemical effects alone.

The need for a means of mechanism verification was identified as one of the components of the Atmospheric Environment Service Unified Regional Air-Quality Modelling System (AURAMS).  This is a regional gas and particulate matter modelling system which is the subject of current research at the AES Laboratory in Downsview, Ontario.   One of the key components of this modelling system is the gas-phase chemistry module, which includes an operational simulation system used within the regional model itself, and a mechanism validation module to be used in determining the validity and expected performance of a mechanism when used within the regional model.   A description of the validation module its first applications are the subject of the current work.

Air quality models have traditionally been validated by comparing ambient data with model simulated concentrations for ozone (O3) and a few other trace gas  species. There is, however, an increasing awareness that these approaches are insufficient and that models must also be evaluated in terms of their ability to simulate the fundamental chemical processes that control O3 formation and the sensitivity of O3 to emissions reductions. Important new approaches for validating models include a process analysis in which ambient data are used to validate model representation of the budgets of radicals, odd nitrogen, and odd oxygen; and photochemical indicators which are used to validate the accuracy of model simulated sensitivity of O3 to changes in emissions of precursor species. There are considerable uncertainties, however, in the usefulness of these methods due to uncertainties on each of the many  sink and source terms that can contribute to production or loss of trace species.  Before these approaches can be used with confidence in ambient air, they should be characterized in the relatively well controlled conditions of an environmental chamber. This paper summarizes each of these approaches and suggests chamber studies that should be performed to empirically test their usefulness.

In this communication, we present experimental results from our environmental chamber studies to suggest that anthropogenic sources of molecular chlorine (Cl₂), a photolytic source of Cl·, may contribute significantly to ozone formation in some urban environments. Many studies have focused on the sources and chemistry of halogen atoms in pristine marine environments, often attributing a net consumption of ground-level ozone to reactions with halogen atoms, primarily bromine and to a lesser extent, chlorine. However, little attention has been directed at determining the effect of Cl· on ozone formation in urban environments, where oxides of nitrogen (NO_x) and volatile organic compounds (VOCs) are ubiquitous and there exist many anthropogenic sources of Cl₂. Large anthropogenic sources of Cl₂ emissions include chemical production facilities, water treatment plants, and paper production operations. Chlorine atoms, formed by the photolysis of Cl₂, react with alkanes at up to two orders of magnitude faster than do hydroxyl radicals and can promote the formation of O₃ in the presence of VOCs and NO_x.

An advanced third-generation air quality modeling system has been developed by the Atmospheric Modeling Division of the U.S. EPA. The air quality simulation model at the heart of the system is known as the Community Multiscale Air Quality (CMAQ) Model. It is comprehensive in scope and allows for the simulation of ozone and photochemical oxidants, acid deposition, and fine and coarse particles at spatial scales ranging from urban to regional. The model is contained within a computational framework, Models-3 (for 3^rd generation), that enables users to interact with the modeling system through a high-level graphical user interface that also facilitates data transmission among the components of the system and provides for analysis, graphics, and visualization capabilities for model simulation results. The modeling system is available from the U.S. EPA (see web site: www.epa.gov/asmdnerl/models3/), and is currently being evaluated for photochemical oxidants and fine particles using field study databases from the eastern United States from 1990 and 1995. The CMAQ is also being extended to include the modeling of selected air toxics, including atmospheric mercury and atrazine (a pesticide).

Incremental reactivity estimates for many high molecular weight organic compounds used in consumer products are viewed as uncertain because of limited data on their reaction mechanisms and products.  This study performs a systematic uncertainty analysis of the calculated incremental reactivities of two such compounds: n-butyl acetate and 2-butoxy ethanol. 2-butoxy ethanol is a relatively well-studied compound for which incremental reactivity experiments have been performed and product data are available for most reaction pathways.  In contrast, there are incremental reactivity experiments but essentially no product data for n-butyl acetate.  The uncertainty analysis accounts for uncertainties in the environmental chamber experiments used to estimate key parameters of the 2-butoxy ethanol and n-butyl acetate mechanisms, as well as in the parameters of the base SAPRC-97 chemical mechanism used to calculate their incremental reactivities.  Uncertainties in the 2-butoxy ethanol reactivity estimates are lower than those estimated previously for many other volatile organic compounds (VOCs).  In contrast, the uncertainties in the n-butyl acetate reactivity estimates are at the upper end of the range of uncertainties estimated for other VOCs.  The chamber-derived parameters of the n-butyl acetate and 2-butoxy ethanol mechanisms contribute at most about 7% of the uncertainty in their incremental reactivity estimates.

Summary

Modeling of ozone formation requires accurate emissions inventories and chemical mechanisms. Consideration of equilibrium vapor pressures and partition between gas phase and aerosols suggested that many low-vapor-pressure (LVP) VOC in consumer, commercial, and agricultural products would be present predominately in the gas phase. However, this conclusion may not be correct in view of studies indicating alternate fates for those VOCs. The tropospheric concentration of a VOC is affected by both the rate and extent of release from an emission source and by the rate of removal through a variety of competing processes (e.g. photooxidation, deposition, horizontal and vertical transport, aerosol formation). There are many ways that compounds of low volatility, especially those which are hydrophilic, may be prevented from entering the atmosphere or removed once they enter, but quantitative assessments are rare.

The kinetic, mechanistic and smog chamber studies upon which calculations of MIRs are based do not include transport to water or soil, which are present in the natural environment. Not only the compound to be evaluated, but also its oxidation products, may not participate in ozone formation to the extent predicted if they have other environmental fates besides oxidation in air and advection in air.

New research is needed to develop models that can incorporate all that is known about a chemical's tropospheric fate and removal. Environmental fate modeling, used extensively to track persistent organic pollutants, may be applicable in assessing tropospheric ozone-forming potential of VOCs as well.

This presentation will touch on four aspects of California’s reactivity program.  First, it will cover ARB’s existing Low Emission Vehicle/Clean Fuels regulation and a consumer products regulation which is under development.  Second, it will provide a very brief summary of some of the research the California Air Resources Board (ARB) is currently funding in the field of reactivity.  Lastly, it will cover the internal reactivity team and external advisory groups.

The mission of the RRWG is to provide an improved scientific basis for reactivity-related regulatory policies. That is being accomplished by bringing together all parties actively interested in sponsoring, planning, performing or assessing policy-relevant scientific research on the reactivities of organic compounds emitted to ambient air, as related to the formation of ozone, PM2.5, and regional haze. This is for the purposes of coordinating such research and defining potential applications, while continuously involving key policymakers.

The German Programme of control concepts and measures for ozone is a result of a research project funded by the German Federal Environmental Agency. The central issue was the investigation of the effects of several measures to reduce ozone precursor substances on emission levels and ozone concentrations. The whole title of the project, elaborated by scientists of the Prognos enterprise in Basel as well as of the Meteorological Institutes of the Free University Berlin and the University Colon, was: „Determination and evaluation of the effects of regional-scale and of local emission control measures on elevated ground-level ozone concentrations in mid-summer episodes“.

The project was divided into two different parts. The first part included the analysis of the efficiency of measures at a regional, nation-wide level. This analysis was performed by using the photochemical model systems EURAD and REM3 with  meso-scale grids. The corresponding modelling domain covered most parts of Europe with horizontal grid distances of 30 km.

The subject of the second part was the investigation of the efficiency of measures at a local level. This analysis was performed for three selected regions in Germany, Berlin, Dresden and Rhein/Main, using the photochemical model systems EURAD and CALGRID/REM3 with  a  higher spatial  resolution.  Horizontal grid distances of 2 km were applied to the model simulations.

A broad variety of scenarios was investigated in this study. The scenarios can be divided into long-term and temporary scenarios. Moreover scenarios for Europe, Germany and the three regions in Germany were considered.

The scenario Trend 2005 includes emission reductions of NOx and VOC, fully achieved in 2005 by control measures which are already implemented due to existing European and national Directives.  This scenario was applied to the entire, large modelling domain (most parts of Europe).The Reduction 2005 scenario is basically the same like Trend 2005, but for Germany further reduction potentials concerning traffic, industry and solvent emissions are added. Both scenarios are long-term scenarios.

Other control scenarios are temporary scenarios applied nation-wide and local to the three selected regions. These scenarios include various restrictions like speed limits, driving bans for cars without a regulated 3-way catalytic converter and for non-low emission diesel cars.

Another type of scenarios includes a combination of several measures. It is connected with speed limits, other driving restrictions, temporary closure of large emission sources as well as public appeals to business and households.  These measures are assumed to be applied  in the three local regions and adjacent areas, that means in adjacent federal states.

Altogether 8 scenarios in the regional scale and nation-wide and 10 scenarios in the three areas in Germany were considered.

The most effective scenarios in emission reductions of NOx and VOC are Reduction 2005 and the combination scenario (with a variety of measures in local regions and adjacent areas). These two scenarios result in a reduction of NOx emissions  of nearly 65 % and in a reduction of VOC emissions between 60 to 70 % (these numbers are related to Germany for Reduction 2005 and to the corresponding region for the combination scenario).

The reduction rates concerning Trend 2005 for Germany and for the small, local areas are similar. NOx reductions between 20 and 30%  and VOC reductions around 45 % can be achieved. Driving bans result in similar emission reductions like Trend 2005.

A temporary measure which is less effective are speed limits. NOx reductions between 10 and 15 % can be found. There is no significant effect on VOC emissions.

Looking at the effect of regional and local measures on simulated hourly ozone concentrations it can be found that the effect of speed limits is the smallest . Driving bans are a little bit more effective than speed limits.

The highest ozone reductions result for Reduction 2005 which is a long-term scenario applied to a larger region.

Obviously two basic results of the research project become evident: First, all regional measures are more effective than their corresponding measures at the local level. Second, long-term measures seem to be more effective than temporary measures.

To come to the conclusions the research project has shown that the long-term and European-wide applied scenarios Trend and Reduction 2005 are most effective. Provided that these scenarios are fully implemented a reduction of peak ozone concentrations by 40 % can be achieved. Temporary and local measures are less effective in decreasing high ozone levels. For each single measure, the decrease of peak ozone concentrations would be at most 5 %.  Only the combined application of all possible local measures results in a reduction of NOx and VOC emissions of approximately 50 %. Then, the corresponding decrease of peak ozone concentrations would reach at most 20 %.

Long-term control measures implemented on a large region (national, international) are even with lower overall percentage emissions reduction more effective in reducing high ozone concentrations than temporary and locally restrictive measures.

Further information about the results of this research project can be found in the internet. The address is: www.umweltbundesamt.de/ozon-e.

A large daylight atmosphere simulation chamber for the investigation of tropospheric photochemistry under natural conditions (SAPHIR) is currently being build at the Forschungszentrum Juelich. The main scientific issues planned to be investigated in this chamber are:

Mexico City’s air pollution is a persistent and pervasive environmental problem that has imposed health and economic costs on society. With Mexico’s expenditures on controls calculated in several millions of pesos per year and slow progress toward compliance with air quality standards, a critical need exits for better and more effective abatement strategies. With the current Federal Environmental Policy on fossil fuel burning for power utilities and industrial processes shifting from heavy fuel oils to low sulfur natural gas, it becomes important to characterize industrial emissions as well as residential emissions that use gaseous fuels for establishing their impact on the Mexican airshed. Concurrently, it is recognized that an efficient strategy to abate air pollution in Mexico City consists of improving the properties of Mexican liquid fuels for transportation. Although liquid hydrocarbons have been substantially improved to comply with environmental regulations not much has been done to evaluate exhaust gas emissions and their role in ozone and aerosol formation.

The need to address the above-interrelated topics demands great efforts in reducing air pollution. Common obstacles in this pursuit include insufficient understanding of the relationship among the underlying scientific, economic, and social issues, and also in addressing the problem with limited availability of resources, and infrastructure. Although addressing all these issues collectively is out of the scope of the present investigation, much can be accomplished by devising more prominent scientific methodologies and experimental techniques that can shed new light in mitigating and controlling air pollution in Mexico’s populated cities. An integrated assessment will provide the opportunity to understand the air pollution problem and its coupling to regional and global impacts. Mexican researchers at the IMP are initiating multidisciplinary work in which the city’s environmental problems are being addressed. The major objective of this investigation is aim at acquiring scientific knowledge to address the still prevalent high concentrations of fine particles and ozone formation in the Mexico City Metropolitan Area (MCMA).

A two-step approach is devised to carry out the objectives of this investigation. The first task relies on experimentation to gather enough information that can be used later on for tuning parameters and refining chemical reaction mechanisms in ozone formation modules embedded in advanced urban and regional models for regulatory purposes. Modeling of important episodes and historic events will be a key element in evaluating the impact of emission sources into the air. During the experimentation phase smog chambers will be used for determining gas precursor contribution from both gaseous fuels and tail pipe emissions from combusted liquid hydrocarbon fuels on ozone formation. This phase will characterize Volatile Organic Compound (VOC’s) reactivates including exhaust gas and evaporative liquid fuel as well. The adopted methodology will be based on reactivity indexes to estimate VOC’s reactivity in controlled atmospheres inside radiated environmental chambers. The incremental reactivities of a series of representative VOC’s are expected to differ from those measured by Carter in previous studies since the VOC to NOx ratio, VOC’s speciation and radiation fluxes typical of the Mexican airshed vary considerably from that of northern latitudes. In this line of work computer modeling of environmental chamber measurements of incremental reactivity of VOC will be pursued in order to evaluate detailed atmospheric photochemical mechanisms for developing ozone reactivity scales for VOC’s.

The present experiments will complement studies under high NOx maximum reactivity conditions and enable our research group to gain sufficient knowledge in this area of expertise. The scope of the present work is not only centered on the design and characterization of smog chambers but also to adopt existing technologies to carry out the activities herewith described and to study heterogeneous processes related to chamber wall effects.

Another branch of research that the project contemplates is aerosols. Fine particle formation within the MCMA when strong photochemical activity takes place severely affects not only the urban airshed but also air quality of downwind rural areas. In the Mexico City basin airborne particles contribute significantly to light scattering and absorption and hence, visibility reduction, which is related to both light-particle interaction and gas-to-particle processes, is drastically hinder by the intense human activity of the region. The complex heterogeneous chemical reactions between gaseous pollutants and suspended particles that occur during aerosol formation are not well understood. Further information is necessary to explain aerosol formation as favored through photochemical reaction of gases and vapors in semiarid atmospheres. Although the mechanisms responsible for inorganic aerosol formation seem to be well characterized, very little is known on the chemistry of organic aerosols. Mexican authorities have recognized the urgency to address issues relevant to visibility reduction and aerosol formation in order to improve Mexico’s air quality. To resolve such problems a better understanding of the complex heterogeneous atmospheric chemistry is required. It is therefore of paramount interest for researchers at IMP to initiate chamber studies of aerosol formation and to actively participate with other research groups in related projects. This collaborative effort will fortify common areas of research and will enable us to provide the answers that are needed to mitigate pollution in megacities.

A key element distinguishing this project from other air pollution research is its multidisciplinary nature and the myriad areas of research that it encompasses. Air quality modeling is the focus of the second phase of this investigation. Advanced air quality models will be used to simulate photochemisty along with transport and formation of aerosols. Models will be developed to study the formation of inorganic and organic aerosols. Chemical mechanism development and evaluation for low vapor organic compounds that are formed in photochemical atmospheres are considered as a fundamental part of the investigation. Gas-particle equilibrium models for VOC’s are fundamental to investigate the gas to particle pathways and particle growth that characterize aerosol formation in the basin’s atmosphere. There is also a great need to study VOC accumulation on soot particles, as the number of sources that emit soot is considerably large in the region. To determine the origin, relative contribution and distribution of particles and volatile organic compounds receptor models will be used. Receptor models will aid in characterizing the VOC sources while providing a tool to evaluate the effectiveness of pollution control strategies.

The impact of aerosols on the radiation budget will be evaluated as a first estimate on regional climate change. Linking the local and regional air quality issues that are most important to local decision-makers to factors affecting global change, the proposed assessment offers a new opportunity to advance effective polices on both fronts.

With the scientific platform on which this project rests air quality models will be successfully used to estimate future scenarios that may arise as a result of a number of modifications or implementations on a regional scale. For instance a shift form residual fuel oils to natural gas applied on the industrial sector. Improved liquid fuels introduced into the market for transportation. Addition or removal of various types of emission sources in the area as well as the effect of adding air pollution control systems for power generation units, or other commercial combustion applications. The project contemplates the simulation of different scenarios to evaluate the ambient impact of aerosol when using reformulated fuels according to a refinery system upgrade. The complementary tasks to support the project’s activities in the second phase, such as reactivity indexes and the development of explicit reaction mechanisms will derive from a new-planed experimental facility designed for high-volume environmental chambers.

Because of the complexity of the chemical processes involved, data from environmental chambers are essential to assuring that these models give correct predictions. However, current environmental chamber technology is not adequate for testing models under conditions representative of rural atmospheres or the cleaner urban atmospheres we expect to attain as we approach attainment of air quality standards. This involves design, construction and operation of a next-generation environmental chamber facility capable of obtaining the data needed for evaluating models under conditions relevant to today’s control strategy problems. The funding available is approximately $3 million, with approximately half that being for design, construction and characterization of the facility, and the remainder to support its operations for four years.