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Understanding the role of nitrogen oxides in the nocturnal atmosphere

Posted on 29 June 2017

Researchers in the Wolfson Atmospheric Chemistry Laboratory (WACL) and several US institutions have carried out work to understand the importance of nighttime chemistry in controlling air pollutants.

Photo credit: Shona Smith

They discovered that the interaction between emissions from human activity and organic compounds emitted from vegetation in the nighttime atmosphere could have large impacts on both air pollution and climate.

Emissions of nitrogen oxides (NOx) from combustion processes, such as power generation have decreased significantly in the Southeast U.S. over recent years. The reduction in air pollutants such as aerosols, however, has not followed the same trend. One possible contributor to this is the highly understudied chemistry that occurs in the night-time atmosphere.

The importance of daytime chemical cycles involving NOx in controlling air pollutants such as ozone and secondary organic aerosol is well known. The night-time chemistry of the atmosphere, however, is much less well studied. This work shows that nocturnal chemistry has its own distinct chemical cycles, and that these could explain recent trends in daytime air pollutants in the Southeast US.

This paper combines aircraft measurements and models to elucidate important chemical processes occurring in the nocturnal atmosphere, and finds the Southeast U.S. to be in a transition between a highly NOx controlled recent past and a future more similar to the pre-industrial atmosphere.

Dr Pete Edwards, lead author, said: “The difficulty in accessing the nocturnal atmosphere means its chemistry has seen very little attention compared to its daytime counterpart.

Without sunlight to drive the chemistry a different set of processes take over at night, transforming chemicals emitted during the day.

In this paper we show that understanding this chemistry is essential if we are to predict the response of air pollutants to changes in emissions”.

This paper provides a blueprint for understanding the influence of emissions trends on this atmospheric chemistry in other regions with different BVOC emission strengths and NOx emission histories, such as Asia.

Notes to editors:

  • Transition from high- to low-NOx control of night-time oxidation in the southeastern US is published in Nature Geoscience. To read, visit
  • Dr Pete Edwards’ affiliations for this work are: Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado, USA; Chemical Sciences Division, NOAA Earth System Research Laboratory, Boulder, Colorado, USA; and Wolfson Atmospheric Chemistry Laboratories, University of York, York, UK