Fine-scale simulation of ammonium and nitrate over the South Coast Air Basin and San Joaquin Valley of California during CalNex-2010

摘要

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 NH_4~+ and NO_3~- make up the largest fraction of total PM_(2.5) mass on polluted days. Here we evaluate fine-scale simulations of PM_(2.5) NH_4~+ and NO_3~- 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 NH_4~+ and NO_3~- including the elevated concentrations in eastern SoCAB. However, areas for model improvement have been identified. NH_3 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 NH_3 predictions can influence NO_3~- 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 NH_3 emissions and in parameterizations of sea spray emissions, evening mixing processes, and heterogeneous ClNO_2 chemistry could improve model performance.