The complex effects of reduced mobile source emissions on submicron particulate matter concentrations in Los Angeles

Despite considerable reductions in mobile source emissions, annual average aerosol concentrations measured in Los Angeles using Federal Reference Methods (FRM) have not appreciably declined over the last decade. Here, we use submicron aerosol measurements and zero-dimensional modeling to quantify the impacts of these emissions reductions on aerosol formation in Pasadena, CA during the late spring and summer of 2022. Reductions in secondary organic aerosol (SOA) concentrations expected from reduced mobile source emissions appear to have been largely offset by increases in hydroxyl radical concentrations, an indirect effect of reduced nitrogen oxide (NO_x) emissions. As a result, while the predicted contribution of mobile sources to the SOA burden has declined from ~50% in 2010 to only ~25% in 2022, concentrations of locally-formed SOA have remained relatively constant. In contrast, reductions in mobile source NO_x emissions have likely reduced overnight production of nitric acid and ammonium nitrate (AN) aerosol. We provide indirect evidence that FRM measurements may have failed to capture the reduction in AN since 2010 due to evaporation of semi-volatile species from FRM filter samples. Our results suggest that given the effectiveness of historical regulatory efforts aimed at mobile sources, and on-road sources in particular, additional reductions in submicron aerosol concentrations in Los Angeles will likely require increased focus on abating emissions from non-road and area sources.