Abstract
Abstract. Organic nitrates are important atmospheric species that significantly affect the cycling of NOx and ozone production. However, characterization of particulate organic nitrates and their sources in polluted atmosphere is a big challenge and has not been comprehensively studied in Asia. In this study, an aerodyne high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) was deployed at an urban site in China from 2015 to 2016 to characterize particulate organic nitrates in total nitrates with a high time resolution. Based on the cross-validation of two different data processing methods, organic nitrates were effectively quantified to contribute a notable fraction of organic aerosol (OA), namely 9 %–21 % in spring, 11 %–25 % in summer, and 9 %–20 % in autumn, while contributing a very small fraction in winter. The good correlation between organic nitrates and fresh secondary organic aerosol (SOA) at night, as well as the diurnal trend of size distribution of organic nitrates, indicated a key role of nighttime local secondary formation of organic nitrates. Furthermore, theoretical calculations of nighttime SOA production of NO3 reactions with volatile organic compounds (VOCs) measured during the spring campaign were performed, resulting in three biogenic VOCs (α-pinene, limonene, and camphene) and one anthropogenic VOC (styrene) identified as the possible key VOC precursors to particulate organic nitrates. The comparison with similar studies in the literature implied that nighttime particulate organic nitrate formation is highly relevant to NOx levels. This study proposes that unlike the documented cases in the United States and Europe, modeling nighttime particulate organic nitrate formation in China should incorporate not only biogenic VOCs but also anthropogenic VOCs for urban air pollution, which needs the support of relevant smog chamber studies in the future.
Highlights
Organic nitrates (ONs) in aerosols have an important impact on the fate of NOx and ozone production (Lelieveld et al, 2016), which can be formed in a minor channel of the reaction between peroxy radicals (RO2) and NO (Reactions R1 and R2; usually, an increased fraction of this reaction leads to the formation of alkoxy radicals (RO) and NO2 – Reaction R3) or via the NO3-induced oxidation of unsaturated hydrocarbons (R=R ) (Reaction R4)
The analytical outcomes by the NO+/NO+2 ratio method and by the positive matrix factorization (PMF) method consistently suggest that organic nitrates had the highest ambient concentration (0.34–0.53 μg m−3) and proportion in total nitrates (41 %–64 %) in summer compared to the different seasons
Based on the production potential evaluation above, we further explore secondary organic aerosol (SOA) yield of NO3 plus the six notable volatile organic compounds (VOCs) precursors according to the analysis method of particulate organic nitrate formation in Xu et al (2015a)
Summary
Organic nitrates (ONs) in aerosols have an important impact on the fate of NOx and ozone production (Lelieveld et al, 2016), which can be formed in a minor channel of the reaction between peroxy radicals (RO2) and NO (Reactions R1 and R2; usually, an increased fraction of this reaction leads to the formation of alkoxy radicals (RO) and NO2 – Reaction R3) or via the NO3-induced oxidation of unsaturated hydrocarbons (R=R ) (Reaction R4). Field and laboratory studies around the world indicated that particulate organic nitrates could contribute a large portion of secondary organic aerosol (SOA; Rollins et al, 2012; Xu et al, 2015a; Fry et al, 2013; Ayres et al, 2015; Boyd et al, 2015; Lee et al, 2016). Relevant Chinese datasets are still scarce and not included in this review This was because (1) the contributions of organic nitrates in SOA and total nitrates in the Chinese atmosphere remain poorly understood and (2) the anthropogenic and biogenic precursor emissions in China are significantly different from those in the United States and Europe and cannot be estimated. Few studies have investigated the concentrations and formation pathways of particulate organic nitrates in China. Xu et al (2017) estimated the mass concentration of organic nitrogen in Beijing using AMS, but in this study, they ignored the contribution of NO+x family, which are the major fragments of organic nitrates
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