A comparison of CMAQ HONO predictions with observations from the Northeast Oxidant and Particle Study

Predictions of nitrous acid from the Community Multiscale Air Quality modeling system are compared with the measurements from the 2001 Northeast Oxidant and Particle Study. Four different sources of nitrous acid were considered in the study: gas-phase reactions, direct emissions, a heterogeneous reaction, and a surface photolysis reaction. When only gas-phase reactions were considered in the model, the diurnally averaged mean bias, the normalized mean bias, the root mean square error, and the normalized mean error of the model were −1.01 ppbv, −98%, 1.05 ppbv, and 98%, respectively. However, the diurnally averaged mean bias, normalized mean bias, the root mean square error, and the normalized mean error of the model improved to −0.42 ppbv, −41%, 0.45 ppbv, and 41%, respectively, when all sources were considered. Model results suggest that the heterogeneous reaction and the surface photolysis reaction are the most important sources of nitrous acid in the atmosphere, accounting for about 86% of the predicted nitrous acid. Emissions and the gas-phase reactions were relatively minor sources and accounted for only 14% of the predicted nitrous acid. Model predictions suggest that the heterogeneous reaction is the most significant source of nitrous acid at night, while the surface photolysis reaction is the most significant source during the day. The addition of these sources increased the diurnally averaged hydroxyl radicals and ozone by 10% and 1.4 ppbv, respectively.