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

Nitrate, a major PM_2.5 component in polluted environments, could be greatly elevated during pollution episodes. In this study, nitrate and other inorganic ions on PM_2.5 were measured half hourly at a residential location in Hong Kong in December 2009. Hourly nitrate concentrations in PM_2.5 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, PM_2.5 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 PM_2.5 in a high-NO_x environment through the formation of nitrate.