Effect of Aerosol Nitrate Photolysis on Wintertime Air Quality

Recent field and laboratory studies suggest that aerosol nitrate can undergo photolysis over the marine environment to generate gaseous nitrous acid and nitrogen dioxide. In this study, we examine the effect of aerosol nitrate photolysis on air quality using the hemispheric Community Multiscale Air Quality (CMAQ) model. Consistent with other air quality models, CMAQ currently does not contain the photolysis of aerosol nitrate. We add the photolysis of aerosol nitrate to the CMAQ model using recently published rate expression developed based on laboratory experiments and apply the photolysis of aerosol nitrate only over marine environments. Simulations are performed without and with the photolysis of aerosol nitrate over the Northern Hemisphere. Model results suggest that the photolysis of aerosol nitrate decreases annual mean surface aerosol nitrate over the ocean by ~60%, enhances annual mean surface nitrous acid by ~60%, and enhances annual mean surface ozone by ~17%. Enhanced ozone over marine environment is transported to land areas which increases annual mean surface ozone by 1–7 ppbv over large land-based areas. Simulated nitrous acid with the photolysis of aerosol nitrate agree better with observed data. Simulated ozone with the photolysis of aerosol nitrate reduces mean bias in cooler months but increases the bias in warmer months compared to observations in the United States and Japan.