Impact of particulate nitrate photolysis on air quality over the Northern Hemisphere

We use the Community Multiscale Air Quality (CMAQv5.4) model to examine the potential impact of particulate nitrate (pNO3−) photolysis on air quality over the Northern Hemisphere. We estimate the photolysis frequency of pNO3− by scaling the photolysis frequency of nitric acid (HNO3) with an enhancement factor that varies between 10 and 100 depending on pNO3− and sea-salt aerosol concentrations and then perform CMAQ simulations without and with pNO3−photolysis to quantify the range of impacts on tropospheric composition. The photolysis of pNO3− produces gaseous nitrous acid (HONO) and nitrogen dioxide (NO2) over seawater thereby increasing atmospheric HONO and NO2 mixing ratios. HONO subsequently undergoes photolysis, producing hydroxyl radicals (OH). The increase in NO2 and OH alters atmospheric chemistry and enhances the atmospheric ozone (O3) mixing ratio over seawater, which is subsequently transported to downwind continental regions. Seasonal mean model O3 vertical column densities without pNO3− photolysis are lower than the Ozone Monitoring Instrument (OMI) retrievals, while the column densities with the pNO3− photolysis agree better with the OMI retrievals of tropospheric O3 burden. We compare model O3 mixing ratios with available surface observed data from the U.S., Japan, the Tropospheric Ozone Assessment Report – Phase II, and OpenAQ; and find that the model without pNO3− photolysis underestimates the observed data in winter and spring seasons and the model with pNO3− photolysis improves the comparison in both seasons, largely rectifying the pronounced underestimation in spring. Compared to measurements from the western U.S., model O3 mixing ratios with pNO3−photolysis agree better with observed data in all months due to the persistent underestimation of O3 without pNO3−photolysis. Compared to the ozonesonde measurements, model O3 mixing ratios with pNO3−photolysis also agree better with observed data than the model O3 without pNO3−photolysis.