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Enhanced wintertime oxidation of VOCs via sustained radical sources in the urban atmosphere

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posted on 2021-02-25, 17:06 authored by Roberto Sommariva, Leigh R Crilley, Stephen M Ball, Rebecca L Cordell, Lloyd DJ Hollis, William J Bloss, Paul S Monks
Daytime atmospheric oxidation chemistry is conventionally considered to be driven primarily by the OH radical, formed via photolytic sources. In this paper we examine how, during winter when photolytic processes are slow, chlorine chemistry can have a significant impact on oxidative processes in the urban boundary layer. Photolysis of nitryl chloride (ClNO2) provides a significant source of chlorine atoms, which enhances the oxidation of volatile organic compounds (VOCs) and the production of atmospheric pollutants.

We present a set of observations of ClNO2 and HONO made at urban locations in central England in December 2014 and February 2016. While direct emissions and in-situ chemical formation of HONO continue throughout the day, ClNO2 is only formed at night and is usually completely photolyzed by midday. Our data show that, during winter, ClNO2 often persists through the daylight hours at mixing ratios above 10–20 ppt (on average). In addition, relatively high mixing ratios of daytime HONO (65 ppt) provide a strong source of OH radicals throughout the day.

The combined effects of ClNO2 and HONO result in sustained sources of Cl and OH radicals from sunrise to sunset, which form additional ozone, PAN, oxygenated VOCs, and secondary organic aerosol. We show that radical sources such as ClNO2 and HONO can lead to a surprisingly photoactive urban atmosphere during winter and should therefore be included in atmospheric chemical models.

History

Citation

Environmental Pollution Volume 274, 1 April 2021, 116563

Author affiliation

School of Geography, Geology and the Environment

Version

  • AM (Accepted Manuscript)

Published in

Environmental Pollution

Volume

274

Pagination

116563

Publisher

Elsevier BV

issn

0269-7491

Acceptance date

2021-01-19

Copyright date

2021

Available date

2022-01-25

Language

en

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