Nitrogen oxide (NOx) abundances across the U.S. have fallen steadily over the last fifteen years. Patterns in anthropogenic sources result in 2-fold lower NOx on weekends than weekdays largely without co-occurring changes in other emissions. These trends taken together provide a near perfect NOx constraint on the nonlinear chemistry of ozone, on key oxidants hydroxyl (OH) and nitrate radical (NO3), and on secondary aerosol formation. We use this NOx constraint to interpret trends in wintertime PM2.5 over the last decade in San Joaquin Valley, California, a location with severe aerosol pollution and where a large portion of the total aerosol mass is ammonium nitrate (NH4NO3). We combine the 15-year routine monitoring record and the air- and ground-based DISCOVER-AQ-2013 dataset to quantify the impact of NOx emission controls on wintertime PM2.5 concentrations. Nitrate ion (NO3�) is the oxidation product of NO2 and is formed by distinct daytime and nighttime pathways, both of which are nonlinear functions of the NO2 abundance. We present observationally derived decadal trends in both pathways and show that NOx reductions have dually worked to increase and decrease NH4NO3 production over the last 15 years. The net effect is a substantial decrease in NO3� via decreased production in the nighttime residual layer. We quantify the impact of future NOx controls on PM2.5 exceedances.

Linking NO<sub>x</sub> chemistry and aerosol over the last decade in San Joaquin Valley, California

248th National Meeting (2014)

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Linking NOx chemistry and aerosol over the last decade in San Joaquin Valley, California

Linking NO_x chemistry and aerosol over the last decade in San Joaquin Valley, California