Articles | Volume 16, issue 10
https://doi.org/10.5194/acp-16-6355-2016
https://doi.org/10.5194/acp-16-6355-2016
Research article
 | 
24 May 2016
Research article |  | 24 May 2016

Formation of reactive nitrogen oxides from urban grime photochemistry

Alyson M. Baergen and D. James Donaldson

Abstract. Impervious surfaces are ubiquitous in urban environments and constitute a substrate onto which atmospheric constituents can deposit and undergo photochemical and oxidative processing, giving rise to “urban grime” films. HNO3 and N2O5 are important sinks for NOx in the lower atmosphere and may be deposited onto these films, forming nitrate through surface hydrolysis. Although such deposition has been considered as a net loss of NOx from the atmosphere, there is increasing evidence that surface-associated nitrate undergoes further reaction. Here, we examine the gas phase products of the photochemistry of real, field-collected urban grime using incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS). Gas phase nitrogen oxides are emitted upon illumination of grime samples and their production increases with ambient relative humidity (RH) up to 35 % after which the production becomes independent of RH. These results are discussed in the context of water uptake onto and evaporation from grime films.

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Short summary
"Urban grime" films form from the deposition of atmospheric constituents on urban surfaces exposed to the atmosphere. An important component is nitrate, which is thought to represent a final loss channel of nitrogen oxides from the atmosphere. In this study we show that nitrate photolysis on grime is a significant loss process and gives rise to gas phase nitrogen oxides, effectively recycling them to the atmosphere and thus impacting local ozone levels.
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