Optimized Simulation of HONO Sources and Its Impact on Nitrate Formation in Guangzhou
Nitrous acid (HONO) is a critical precursor of hydroxyl radicals (·OH) in the atmosphere, influencing oxidative capacity and secondary pollutant formation. However, model simulations often underestimate HONO concentrations, and its role in nitrate formation remains unclear. This study investigates a typical winter haze episode in Guangzhou (January 2021, peak PM2.5: 243.0 μg·m−3) using observational data and a box model to quantify HONO sources and assess their impact on ·OH and particulate nitrate. HONO concentrations increased from (1.0±1.0) μg·m−3 during clean periods to (9.2±3.8) μg·m−3 during polluted periods, while nitrate rose from (6.4±3.4) to (43.3±20.0) μg·m−3 (6.8-fold). Incorporating seven additional HONO sources improved simulated daytime HONO from (0.3±0.1) to (6.5±2.3) μg·m−3, matching observations. Source apportionment showed direct vehicle emissions dominated (49.7%), followed by heterogeneous photosensitized reaction of NO2 on aerosol surfaces (23.0%), ground surface reaction (10.7%), and nitrate photolysis (8.7%). With optimized HONO, simulated daytime ·OH increased from (0.6±0.3)×10^6 to (1.5±0.8)×10^6 molec·cm−3 (1.2-fold), and nitrate production via ·OH+NO2 increased from (3.4±1.2) to (15.3±8.5) μg·m−3·h−1 (3.5-fold). The simulated-to-observed nitrate ratio improved from 21% to 81%. Sensitivity tests indicated that setting nitrate photolysis enhancement to 100 times gaseous nitric acid yielded better HONO and nitrate simulations. This study underscores the importance of refining HONO sources for accurate simulation of atmospheric oxidation and nitrate formation, aiding pollution control strategies.