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Insights into factors affecting nitrate in PM 2.5 in a polluted high NO x environment through hourly observations and size distribution measurements
Author(s) -
Xue Jian,
Yuan Zibing,
Lau Alexis K. H.,
Yu Jian Zhen
Publication year - 2014
Publication title -
journal of geophysical research: atmospheres
Language(s) - English
Resource type - Journals
eISSN - 2169-8996
pISSN - 2169-897X
DOI - 10.1002/2013jd021108
Subject(s) - nitrate , aerosol , environmental chemistry , homogeneous , inorganic ions , sea salt , chemistry , pollution , environmental science , ion , mineralogy , ecology , physics , organic chemistry , biology , thermodynamics
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 NO 2 was 80.7 ± 14.4 ppb, PM 2.5 NO 3 − reached 27.8 ± 8.0 µg m −3 , ~6 times the level during the normal hours. Nitrate was fully balanced by NH 4 + , indicating abundant presence of NH 3 . 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 NO 3 − potential (sum of HNO 3 (g) and aerosol nitrate). The modeling analysis shows that both formation pathways were significantly more active during the episode. Gas phase production of HNO 3 through reaction of NO 2 + OH dominated during the initial rapid buildup of nitrate around noon time, but the heterogeneous N 2 O 5 hydrolysis pathway made a sizable contribution in the subsequent few hours due to sustained high‐NO 2 concentrations combined with reduced photolysis loss of N 2 O 5 . This case study illustrates the important role of NH 3 and NO 2 in elevating PM 2.5 in a high‐NO x environment through the formation of nitrate.