Representing particulate nitrate photolysis over seawater improves CMAQ ozone predictions over the contiguous United States.

We implement particulate nitrate (pNO) photolysis into the Community Multiscale Air Quality (CMAQv5.5) model and examine the impact of pNO photolysis on air quality over the contiguous U.S. using 12-km horizontal grids for May-September 2018. Model results show that pNO photolysis increases ozone in each month compared to simulations without the pNO photolysis and increases monthly mean of 24-h surface ozone over the modeling domain by 9.3 ppb (32 %) in May, 8.0 ppb (29 %) in June, 5.6 ppb (20 %) in July, 5.1 ppbv (17 %) in August and 3.6 ppbv (13 %) in September. These increases are larger over the western U.S. than over the eastern U.S. and improve the negative ozone bias over the western U.S. Over the eastern U.S., incorporating pNO photolysis improves the underestimation of ozone in May but slightly deteriorates the positive ozone bias in June-September. However, the deterioration of the ozone bias occurs only at the lower end of observed ozone. Incorporating the effect improves the bias at the higher end of observed ozone and improves the comparison of model diurnal ozone with observed data over the western U.S. but deteriorates it over the eastern U.S. Model sensitivity results suggest that boundary condition effect of pNO photolysis contributes 68 % and pNO photolysis within the limited area domain contributes 32 % of the total impact of pNO photolysis on ozone over the U.S. in May.