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Scientists peer into the dark for insights on daytime air pollution
Theo Stein

Scientists peer into the dark for insights on daytime air pollution

Scientists have long known that when common pollutants from traffic exhaust get cooked by the sun, they get transformed into ozone and particulates, two “secondary” pollutants that are harmful to human health. 

But what happens when the sun goes down? As it turns out, the chemistry of air pollution at night is much less well understood, even though it primes the atmosphere for what happens the next day.  

To date, only a limited number of modeling studies have assessed how the production of ozone and particulate matter is influenced by nighttime atmospheric chemistry, said Steven Brown, a researcher with NOAA’s Chemical Sciences Laboratory. Together with researchers in China, Brown helped develop a new analysis method scientists can use to assess the influence of nighttime atmospheric chemistry on air quality around the world.

“We hope this work will lead to further research that explores how air quality and climate are affected by what happens when the sun goes down,” said Brown.

Their work was published in the journal Nature Geoscience.

The new analysis focuses on production of the nitrate radical, or  NO3, a potent oxidant that originates with the emissions of nitrogen oxides from sources such as tailpipes and smokestacks. Oxidation is a type of reaction that is responsible for the production of some secondary air pollutants created by reactions in the atmosphere, as well as the ultimate removal of many other types of air pollutants. NO3 is present mainly at night because it is rapidly destroyed by sunlight. 

The research found that while the rate of nighttime oxidant production was very rapid in polluted regions of the U.S. and Europe in the past, it has moderated due to the implementation of air quality regulations. Data shows a slightly decreasing trend in the past decade, but it's not declining as rapidly as other air pollutants, such as nitrogen oxides  

Nighttime oxidation rates in China, the monitoring data suggest, are still rapidly increasing, despite recent controls on nitrogen oxide pollutants there. The analysis also demonstrated for the first time that nightime chemistry involving nitrogen oxides, or NOx, is not linear, just like daytime chemistry. Other factors control oxidation rates, which can either increase or decrease with decreasing NOx. 

Based on the 40-years of data from Los Angeles since 1980, and air quality measurements capturing transportation sector pollution declines in response to COVID-19, the researchers suggest reducing emissions of volatile organic compounds would simultaneously reduce daytime ozone production and nighttime oxidation. 

As nations around the globe strive to improve air quality and reduce global warming, Brown said, it will be important to have a clearer understanding of how different pollution control strategies influence nighttime atmospheric chemistry.

The research team included scientists from Peking University and Sun Yat-sen University.

For more information, contact Theo Stein, NOAA Communications:

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