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

Wednesday, 17 December 2014
Sally E Pusede1,2, Qi Zhang3, Caroline Parworth3, Hwajin Kim3, Alexis Shusterman1, Amina Saleh1, Kaitlin Duffey1, Paul J Wooldridge1, Lukas C Valin4, Alan Fried5, John B Nowak6, James H Crawford2 and Ronald C Cohen1, (1)UC Berkeley, Berkeley, CA, United States, (2)NASA Langley Research Center, Hampton, VA, United States, (3)University of California Davis, Davis, CA, United States, (4)Lamont Doherty Earth Observatory, Palisades, NY, United States, (5)University of Colorado at Boulder, Boulder, CO, United States, (6)Aerodyne Research Inc., Billerica, MA, United States
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 the key oxidants nitrate radical (NO3) and hydroxyl radical (OH), 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 12-year routine monitoring record and the air- and ground-based DISCOVER-AQ-2013 datasets to quantify the impact of NOx 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 NOx 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.