Photochemical formation of C1-C5 alkyl nitrates in suburban Hong

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Characterization of Natural and Affected Environments

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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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]

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**Corresponding author. [email protected]

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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 (