Fine-scale simulation of ammonium and nitrate over the south coast air basin and san joaquin valley of California during CalNex-2010

National ambient air quality standards (NAAQS) have been set for PM_2.5 due to its association with adverse health effects. PM_2.5 design values in the South Coast Air Basin (SoCAB) and San Joaquin Valley of California exceed NAAQS levels, and NH₄⁺ and NO₃¯ make up the largest fraction of total PM_2.5 mass on polluted days. Here we evaluate fine-scale simulations of PM_2.5 NH₄⁺ and NO₃¯ with the Community Multiscale Air Quality model using measurements from routine networks and the California Research at the Nexus of Air Quality and Climate Change 2010 campaign. The model correctly simulates broad spatial patterns of NH₄⁺ and NO₃¯ including the elevated concentrations in eastern SoCAB. However, areas for model improvement have been identified. NH₃ emissions from livestock and dairy facilities appear to be too low, while those related to waste disposal in western SoCAB may be too high. Analyses using measurements from flights over SoCAB suggest that problems with NH₃ predictions can influence NO₃¯ predictions there. Offline ISORROPIA II calculations suggest that overpredictions of NH_x in Pasadena cause excessive partitioning of total nitrate to the particle phase overnight, while underpredictions of Na⁺ cause too much partitioning to the gas phase during the day. Also, the model seems to underestimate mixing during the evening boundary layer transition leading to excessive nitrate formation on some nights. Overall, the analyses demonstrate fine-scale variations in model performance within and across the air basins. Improvements in inventories and spatial allocations of NH₃ emissions and in parameterizations of sea spray emissions, evening mixing processes, and heterogeneous ClNO₂ chemistry could improve model performance.