Long-term variations of C1-C5 alkyl nitrates and their sources in Hong Kong.
Investigating the long-term trends of alkyl nitrates (RONO2) is of great importance for evaluating the variations of photochemical pollution. Mixing ratios of C1-C5 RONO2 were measured in autumn Hong Kong from 2002 to 2016, and the average level of 2-butyl nitrate (2-BuONO2) always ranked first. The C1-C4 RONO2 all showed increasing trends (p < 0.05), and 2-BuONO2 had the largest increase rate. The enhancement in C3 RONO2 was partially related to elevated propane, and dramatic decreases (p < 0.05) in both nitrogen monoxide (NO) and nitrogen dioxide (NO2) also led to the increased RONO2 formation. In addition, an increase of hydroxyl (OH) and hydroperoxyl (HO2) radicals (p < 0.05) suggested enhanced atmospheric oxidative capacity, further resulting in the increases of RONO2. Source apportionment of C1-C4 RONO2 specified three typical sources of RONO2, including biomass burning emission, oceanic emission, and secondary formation, of which secondary formation was the largest contributor to ambient RONO2 levels. Mixing ratios of total RONO2 from each source were quantified and their temporal variations were investigated. Elevated RONO2 from secondary formation and biomass burning emission were two likely causes of increased ambient RONO2. By looking into the spatial distributions of C1-C5 RONO2, regional transport from the Pearl River Delta (PRD) was inferred to build up RONO2 levels in Hong Kong, especially in the northwestern part. In addition, more serious RONO2 pollution was found in western PRD region. This study helps build a comprehensive understanding of RONO2 pollution in Hong Kong and even the entire PRD.