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Volume 16, issue 10
Atmos. Chem. Phys., 16, 6355–6363, 2016
https://doi.org/10.5194/acp-16-6355-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.
Atmos. Chem. Phys., 16, 6355–6363, 2016
https://doi.org/10.5194/acp-16-6355-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 24 May 2016

Research article | 24 May 2016

Formation of reactive nitrogen oxides from urban grime photochemistry

Alyson M. Baergen1 and D. James Donaldson1,2 Alyson M. Baergen and D. James Donaldson
  • 1Department of Chemistry, University of Toronto, Toronto, M56 3H6, Canada
  • 2Department of Physical and Environmental Science, University of Toronto at Scarborough, Toronto, M1C 1A4, Canada

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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"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.
"Urban grime" films form from the deposition of atmospheric constituents on urban surfaces...
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