On the Mechanisms Linking Nitrogen Oxides to Trends in Ammonium Nitrate Aerosol over the Last Decade in the San Joaquin Valley

Abstract:

Nitrogen oxide (NO_x) abundances across the U.S. have fallen steadily over the last fifteen years. Patterns in anthropogenic sources result in 2-fold lower NO_x on weekends than weekdays largely without co-occurring changes in other emissions. These trends taken together provide a near perfect NO_x constraint on the nonlinear chemistry of ozone, on the key oxidants nitrate radical (NO₃) and hydroxyl radical (OH), and on secondary aerosol formation. We use this NO_x 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 12-year routine monitoring record and the air- and ground-based DISCOVER-AQ-2013 datasets to quantify the impact of NO_x emission controls on the frequency of wintertime exceedances of the national PM2.5 standard. 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 NO_x reductions have worked to simultaneously increase daytime and decrease nighttime NH₄NO₃ production over the last 15 years. The net effect has been a substantial decrease in NH₄NO₃ via decreased NO₃-radical initiated production in the nocturnal residual layer, a layer largely separated from nighttime emissions at the surface. Whereas NO₃^– production in the nocturnal residual layer drove NH₄NO₃ chemistry over the last decade, OH-initiated chemistry at the surface is poised to be the most important source of NH₄NO₃ in the next decade.