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

Abstract

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.

Keywords: HONO; NO(2); O(3); Particulate nitrate; Photolysis.