Modeling NH4NO3 Over the San Joaquin Valley During the 2013 DISCOVER‐AQ Campaign

Abstract

The San Joaquin Valley (SJV) of California experiences high concentrations of particulate matterNH4NO3during episodes of meteorological stagnation in winter. A rich data set of observations related toNH4NO3formation was acquired during multiple periods of elevated NH4NO3during the DerivingInformation on Surface Conditions from Column and Vertically Resolved Observations Relevant to Air Quality(DISCOVER-AQ)field campaign in SJV in January and February 2013. Here NH4NO3is simulated during the SJVDISCOVER-AQ study period with the Community Multiscale Air Quality (CMAQ) model, diagnostic modelevaluation is performed using the DISCOVER-AQ data set, and integrated reaction rate analysis is used toquantify HNO3production rates. Simulated NO3 generally agrees well with routine monitoring of 24-hraverage NO3 , but comparisons with hourly average NO3 measurements in Fresno revealed differences athigher time resolution. Predictions of gas-particle partitioning of total nitrate (HNO3+NO3 ) and NHx(NH3+NH4+) generally agree well with measurements in Fresno, although partitioning of total nitrate toHNO3is sometimes overestimated at low relative humidity in afternoon. Gas-particle partitioning resultsindicate that NH4NO3formation is limited by HNO3availability in both the model and ambient. NH3mixingratios are underestimated, particularly in areas with large agricultural activity, and additional work on thespatial allocation of NH3emissions is warranted. During a period of elevated NH4NO3, the model predictedthat the OH + NO2pathway contributed 46% to total HNO3production in SJV and the N2O5heterogeneoushydrolysis pathway contributed 54%. The relative importance of the OH + NO2pathway for HNO3productionis predicted to increase as NOx emissions decrease.