Insights into factors affecting nitrate in PM2.5 in a polluted high NOx environment through hourly observations and size distribution measurements

Nitrate, a major PM2.5 component in polluted environments, could be greatly elevated during pollution episodes. In this study, nitrate and other inorganic ions on PM2.5 were measured half hourly at a residential location in Hong Kong in December 2009. Hourly nitrate concentrations in PM2.5 varied from 0.8 to 40.5 µg m−3. In an episode during which hourly visibility was down to 3.7 ± 1.0 km and NO2 was 80.7 ± 14.4 ppb, PM2.5 NO3− reached 27.8 ± 8.0 µg m−3, ~6 times the level during the normal hours. Nitrate was fully balanced by NH4+, indicating abundant presence of NH3. Size‐segregated measurements showed 84% of nitrate was in the fine mode during the episode and also suggested that less acidic fine particles and less abundant sea‐salt particles were the contributing factors to the dominant presence in the fine mode. An observation‐based model for secondary inorganic aerosols was applied to investigate the relative importance of homogeneous and heterogeneous reactions to production of NO3− potential (sum of HNO3 (g) and aerosol nitrate). The modeling analysis shows that both formation pathways were significantly more active during the episode. Gas phase production of HNO3 through reaction of NO2 + OH dominated during the initial rapid buildup of nitrate around noon time, but the heterogeneous N2O5 hydrolysis pathway made a sizable contribution in the subsequent few hours due to sustained high‐NO2 concentrations combined with reduced photolysis loss of N2O5. This case study illustrates the important role of NH3 and NO2 in elevating PM2.5 in a high‐NOx environment through the formation of nitrate.