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Photochemical formation of C1-C5 alkyl nitrates in suburban Hong Kong and over South China Sea Lewei Zeng, Xiaopu Lyu, Hai Guo, Shichun Zou, and Zhenhao Ling Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b00256 • Publication Date (Web): 17 Apr 2018 Downloaded from http://pubs.acs.org on April 17, 2018
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Environmental Science & Technology
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Photochemical formation of C1-C5 alkyl nitrates in
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suburban Hong Kong and over South China Sea
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Lewei Zeng, † Xiaopu Lyu, † Hai Guo,*, † Shichun Zou,**, ‡ Zhenhao Ling§
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†
Air Quality Studies, Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hong Kong, China
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‡
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§
School of Marine Sciences, Sun Yat-sen University, Guangzhou 510275, China
School of Atmospheric Sciences, Sun Yat-sen University, Guangzhou 510275, China
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*Corresponding author.
[email protected] 10
**Corresponding author.
[email protected] 11
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ABSTRACT: Alkyl nitrates (RONO2) are important reservoirs of atmospheric nitrogen,
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regulating nitrogen cycling and ozone (O3) formation. In this study, we found that propane and
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n-butane were significantly lower at the offshore site (WSI) in Hong Kong (p0.05). Stronger oxidative capacity at
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WSI led to more efficient RONO2 formation. Relative incremental reactivity (RIR) was for the
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first time used to evaluate RONO2-precursors relationships. In contrast to consistently volatile
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organic compounds (VOCs)-limited regime at TC, RONO2 formation at WSI switched from
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VOCs-limited regime during O3 episodes to VOCs and nitrogen oxides (NOx) co-limited regime
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during non-episodes. Furthermore, unlike the predominant contributions of parent hydrocarbons
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to C4-C5 RONO2, the productions of C1-C3 RONO2 were more sensitive to other VOCs like
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aromatics and carbonyls, which accounted for ~40-90% of the productions of C1-C3 alkylperoxy
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(RO2) and alkoxy radicals (RO) at both sites. This resulted from the decomposition of larger
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RO2/RO and the change of OH abundance under the photochemistry of other VOCs. This study
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advanced our understandings on the photochemical formation of RONO2, particularly the
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relationships between RONO2 and their precursors which were not confined to the parent
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hydrocarbons.
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Environmental Science & Technology
1 Introduction
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Alkyl nitrates (RONO2) comprise a critical fraction of reactive nitrogen (NOy) in the
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atmosphere.1,2 Most RONO2 are formed as byproducts in the process of O3 formation, derived
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from reactions between volatile organic compounds (VOCs) and nitrogen oxides (NOx = NO +
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NO2) in the presence of sunlight, thus influencing the tropospheric O3 formation. They also act
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as nitrogen reservoir due to their relatively long lifetimes,3,4 hence affecting air pollutants profile
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in remote regions. Therefore, they have significant impact on local, regional and global
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atmospheric chemistry.
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C1-C5 RONO2 generally dwell in gas phase and involve in homogeneous reactions in the
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atmosphere. Previous studies on RONO2 spanned from the oceans to the continents and from
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rural areas to urban regions. The mixing ratios of individual RONO2 vary from several to
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hundreds pptv.5,6,7,8,9 In continental air plumes, such as in rural Ontario,10 in rural and urban
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Karachi 5 and in urban China, 6 C3-C4 RONO2 were generally the most abundant RONO2 (15-80
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pptv), due to the combined effect of the abundance of parent hydrocarbons, and the branching
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ratios and the lifetimes of RONO2.11,12 The cruise studies over the equatorial Pacific Ocean
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revealed higher concentrations of C1-C3 RONO2.13 For example, C1 RONO2 observed over the
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Pacific Ocean could reach 50 pptv.14 Under the influence of oceanic emissions, C1 RONO2 also
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ranked the first (5-20 pptv) among all the RONO2 in coastal England.8 In Hong Kong, C3-C4
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RONO2 generally dominated, but high mixing ratio of C1 RONO2 was also reported (~20 pptv at
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a mountainous site and ~15 pptv at a coastal site), likely indicating the characteristics of both
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continental and marine origins.15,16
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Similar to O3, Observed RONO2 usually showed a peak at midday, reflecting the dominance of
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photochemical reactions.4,17 Besides, studies have confirmed that RONO2 are also directly
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emitted from the oceans, particularly for light (
