Effect of NOx on secondary organic aerosol concentrations

The secondary organic aerosol (SOA) module in PMCAMx, a three-dimensional chemical transport model, has been updated to incorporate NOx-dependent SOA yields. Under low-NOx conditions, the RO2 radicals react with other peroxy radicals to form a distribution of products with lower volatilities, resulting in higher SOA yields. At high-NOx conditions, the SOA yields are lower because aldehydes, ketones, and nitrates dominate the product distribution. Based on recent laboratory smog chamber experiments, high-NOx SOA parametrizations were created using the volatility basis-set approach. The organic aerosol (OA) concentrations in the Eastern US are simulated for a summer episode, and are compared to the available ambient measurements. Changes in NOx levels result in changes of both the oxidants (ozone, OH radical, etc.) and the SOA yields during the oxidation of the corresponding organic vapors. The NOx dependent SOA parametrization predicts a maximum average SOA concentration of 5.2 mu g m(-3) and a domain average concentration of 0.6 mu g m(-3). As the NOx emissions are reduced by 25%, the domain average SOA concentration does not significantly change, but the response is quite variable spatially. However, the predicted average SOA concentrations increase in northern US cities by around 3% but decrease in the rural southeast US by approximately 5%. A decrease of the average biogenic SOA by roughly 0.5 mu g m(-3) is predicted for the southeast US for a 50% reduction in NOx emissions.