Despite improvements in urban air quality in recent years, unacceptable levels of ground-level ozone and atmospheric particulate matter continue to be a persistent problem in both urban and rural areas of California. To meet National Ambient Air Quality Standards, state and local agencies must develop State Implementation Plans and adopt additional regulations to control emissions of these pollutants and their precursors. It is therefore necessary to understand the physical and chemical processes that form these pollutants under various atmospheric conditions. Photochemical air quality models are the primary tool for determining the limiting precursors for various secondary pollutants in California airsheds. Chemical mechanisms are an integral part of these photochemical air quality models and must represent the state-of-the-science understanding of how ozone and other secondary pollutants are formed. This research project extended the existing Statewide Air Pollution Research Center (SAPRC) chemical mechanism for ozone prediction to allow for prediction of secondary organic aerosol (SOA) through the incorporation of a Statistical Oxidation Model (SOM) framework into the reaction scheme. Results show that SOA concentrations predicted by the UCD-SOM model are very similar to those predicted by the standard two-product model and that the chamber data used to parameterize the models captures the majority of the SOA mass formation from multi-generational oxidation under the conditions tested. Additionally, air quality meteorological scenarios were updated and combined with recent volatile organic compound (VOC) surrogate updates to provide a framework for reactivity modeling assessments. The Maximum Incremental Reactivity (MIR) ozone formation potential under high NOx conditions decreased by approximately 40% across 39 cities in the U.S. when conditions were updated from 1988 to 2010. Changes to the meteorology, emissions, initial conditions, background concentrations and composition profiles all contributed to the decrease in MIR. Finally, the SAPRC chemical mechanism was updated to represent explicit reactions between biogenic VOCs and nitrogen oxides (NOX) that may influence predicted concentrations of SOA and nitrate. The results indicate these reactions have little effect in winter, but yield modest increases of ~1 μg/m3 in SOA concentrations during summer conditions. Overall, the results of this research project provide ARB with improved and more up-to-date mechanisms for gas-phase and SOA prediction that underlie ARB’s air quality planning efforts.
