Mechanisms for Air Quality Modeling: Development and Applications of the Regional Atmospheric Chemistry Mechanism

The Regional Atmospheric Chemistry Mechanism (RACM) is a highly revised version of the RADM2 mechanism (RACM; Stockwell et al., 1997). 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, peroxy radical reactions, aromatic chemistry and better treatment of the chemistry of isoprene and terpenes. The RACM mechanism has been applied to evaluate incremental reactivities for the production of ozone from volatile organic compounds and to assess the aerosol particle formation reactivity of nitrogen oxide (NOx) emissions (Stockwell et al., 1999a;b). The concept of incremental reactivity was extended to multi-day scenarios. Ethane and acetone are regarded as unreactive compounds but for a five day scenario their reactivity is appreciable. The maximum ozone incremental reactivities (MOIR) of ethane, acetone and dimethyoxymethane (a proposed low reactivity replacement solvent) have incremental reactivities that are about equal for a scenario with a duration of six days. A similar approach was developed to assess the aerosol particle formation reactivity of nitrogen oxide (NOx) emissions for wintertime conditions in central California (Stockwell et al., 1999b). Our calculations found that about 33% of emitted NOx were converted to particulate nitrate on a molar basis and about 0.6 g of ammonium nitrate is produced for each gram of NOx emitted (the mass of NOx calculated as NO2). This estimate is in reasonable agreement with field measurements. INTRODUCTION The gas-phase chemical mechanism is one of the most important components of an atmospheric chemistry model. It is difficult to include all significant chemical reactions in an air quality model because the organic chemistry of the polluted atmosphere is very complicated. There are large numbers of emitted volatile organic compounds (VOC) and each compound’s degradation mechanism may include many chemical intermediates and reactions. The Regional Atmospheric Chemistry Mechanism (RACM; Stockwell et al., 1997) is a compromise between chemical detail and accurate chemical predictions. The RACM is a new version of the Regional Acid Deposition Model mechanism (RADM2; Stockwell et al., 1990). There were relatively minor changes made to the inorganic chemistry but the mechanisms for many organic compounds were highly revised. The alkane chemistry was revised to decrease the ratio of the yields of aldehydes to ketones. The yield of HO from the reaction of alkenes with ozone was increased. The branching ratios for the reaction of acetyl peroxy radicals with NO and NO2 were revised and the reactions of organic peroxy radical + NO3 reactions were added and these changes cause predicted concentrations of peroxyacetyl nitrate (PAN) from RACM to be lower than RADM2. The reactions of unbranched alkenes with NO3 produce relatively large amounts of nitrates during the nighttime and this is now included. The RACM mechanism has a new condensed mechanism for aromatics, isoprene and terpenes but further improvement will be necessary in view of recent laboratory measurements. The RACM is becoming widely used in Europe, Asia and the United States. It has been accepted as the baseline mechanism for the German Tropospheric Research Program. The RADM2 and the new RACM mechanisms were tested against the same set of representative environmental chamber experiments used previously to evaluate the RADM2 (Stockwell et al., 1990). An agreement of ±30% in peak ozone concentrations is within the limits imposed by uncertainties in the chamber experimental characteristics including wall loss, wall radical sources, actinic flux and initial conditions. The peak ozone concentrations predicted by the RACM mechanism had a mean normalized deviation of 19% from the chamber experiments while the mean normalized deviation was 13% for the RADM2 mechanism (Stockwell et al., 1997). The RACM and the RADM2 mechanisms predict peak NO2 concentrations and the timing that the peak occurs very well. The mean normalized deviations of NO2 peak concentrations from the chamber experiments are both near 10%. The mean normalized deviations of timing of the NO2 peak concentrations for both mechanisms are near 3%. The concentrations of O3, H2O2, H2SO4, HNO3 and PAN predicted by the RACM and the RADM2 mechanisms were compared for a set of urban and rural conditions (Stockwell et al., 1997). Although the differences between the two mechanisms for O3, H2O2, H2SO4 and HNO3 are small the differences in predicted PAN concentrations are very significant. The average predicted noontime PAN concentration ratios of RACM to RADM2 were 0.58. MECHANISM APPLICATIONS AND METHODS The RACM mechanism has been applied to the calculation of incremental reactivity and the equivalence between NOx emissions and ammonium nitrate particle formation. Incremental reactivity, IR, is the change in the peak ozone concentration, ∆[O3], divided by an incremental change in the VOC present in the atmosphere, ∆[VOC], (Carter, 1994).