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

The San Joaquin Valley (SJV) of California experiences high concentrations of particulate matter NH₄NO₃ during episodes of meteorological stagnation in winter. A rich data set of observations related to NH₄NO₃ formation was acquired during multiple periods of elevated NH₄NO₃ during the Deriving Information 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 NH₄NO₃ is simulated during the SJV DISCOVER-AQ study period with the Community Multiscale Air Quality (CMAQ) model, diagnostic model evaluation is performed using the DISCOVER-AQ data set, and integrated reaction rate analysis is used to quantify HNO₃ production rates. Simulated NO₃^- generally agrees well with routine monitoring of 24-hr average NO₃^-, but comparisons with hourly average NO₃^- measurements in Fresno revealed differences at higher 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 to HNO₃ is sometimes overestimated at low relative humidity in afternoon. Gas-particle partitioning results indicate that NH₄NO₃ formation is limited by HNO₃ availability in both the model and ambient. NH₃ mixing ratios are underestimated, particularly in areas with large agricultural activity, and additional work on the spatial allocation of NH₃ emissions is warranted. During a period of elevated NH₄NO₃, the model predicted that the OH + NO₂ pathway contributed 46% to total HNO₃production in SJV and the N₂O₅ heterogeneous hydrolysis pathway contributed 54%. The relative importance of the OH + NO₂ pathway for HNO₃ production is predicted to increase as NOx emissions decrease.