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Impact of aerosol nitrate photolysis on air quality over the Northern Hemisphere
Citation:
Sarwar, G., C. Hogrefe, B. Henderson, AND R. Mathur. Impact of aerosol nitrate photolysis on air quality over the Northern Hemisphere. Annual CMAS Conference, 2023, Chapel Hill, NC, October 16 - 18, 2023.
Impact/Purpose:
Recent field and laboratory studies suggest that aerosol nitrate can undergo photolysis to generate gaseous nitrous acid and nitrogen dioxide. We use the Community Multiscale Air Quality (CMAQv5.4) model to examine the potential impact of aerosol nitrate photolysis on air quality over the Northern Hemisphere. In this study, we perform simulations without and with the photolysis of aerosol nitrate and examine its impact on air quality over the Northern Hemisphere.
Description:
Recent field and laboratory studies suggest that aerosol nitrate can undergo photolysis to generate gaseous nitrous acid (HONO) and nitrogen dioxide (NO2). We use the Community Multiscale Air Quality (CMAQv5.4) model to examine the potential impact of aerosol nitrate photolysis on air quality over the Northern Hemisphere. Similar to other air quality models, CMAQv5.4 does not consider aerosol nitrate photolysis; however, it does consider the photolysis of nitric acid (HNO3). We calculate the photolysis frequency of aerosol nitrate by multiplying the photolysis frequency of HNO3 with an enhancement factor that varies between 10 and 100 and depends on aerosol nitrate and sea-salt aerosols concentrations. We perform simulations without and with aerosol nitrate photolysis for 2018. Preliminary results suggest that aerosol nitrate photolysis decreases aerosol nitrate concentrations and increases HONO, NO2, and subsequently ozone (O3) mixing ratios. Mean surface O3 mixing ratios averaged over the entire Northern Hemisphere increase in each month and range between 2.9-6.2 ppbv with the minimum enhancement occurring in July and the maximum in April. However, larger enhancements occur over some areas. For example, annual mean O3 mixing ratios increase by 8-10 ppbv over a large portion of the western U.S. We compare model NO2 and O3 data with the Ozone Monitoring Instrument (OMI) retrievals and find that the seasonal mean model NO2 and O3 column data without aerosol nitrate photolysis are lower than the OMI retrievals while the data with the aerosol nitrate photolysis agree better with the OMI retrievals. We compare model O3 with available surface observed data from the U.S., Japan, the Tropospheric Ozone Assessment Report – Phase II, and OpenAQ. The model without aerosol nitrate photolysis underestimates the observed data in winter and spring seasons and the model with aerosol nitrate photolysis improves the comparison in both seasons largely removing the pronounced underestimation in spring. Compared to measurements from the western U.S., model O3 mixing ratios with aerosol nitrate photolysis agree better with observed data in all months due to the persistent underestimation of O3 without aerosol nitrate photolysis. Model comparisons with observed data in other seasons provide mixed results deteriorating the comparison in some areas while improving the comparison in other areas. We also compare model predictions with ozonesonde measurements and find that model O3 mixing ratios with aerosol nitrate photolysis agree better with observed data than the model results without aerosol nitrate photolysis. The presentation will provide a detailed comparison of model predictions with observed aerosol nitrate, HONO, NO2, and O3 data from available surface, satellite retrievals, and ozonesonde measurements.