The climate-cooling haze that covers the southeastern United States in summer comes largely from an underappreciated source, new research shows. Large numbers of aerosol particles are forming, say scientists, when organic gases released mainly by trees react with pollution released by human activity.
Allen Goldstein and colleagues at the University of California, Berkeley used satellite and ground-based measurements to examine concentrations of aerosols in the atmosphere over the entire United States. They found that in the southeastern US the observed patterns could not be explained solely by manmade aerosols — previously thought to be the main source. Concentrations in the region were considerably higher in summer than in winter, especially over forests that release more organic gases as temperatures climb. Warmer individual days also increased both the forest emissions and the overall aerosol levels, suggesting that most of the summertime haze is created when natural and anthropogenic emissions react, forming a secondary aerosol layer.
Further warming could result in an even denser cooling haze over the region, putting a brake on increasing summertime temperatures — unless reactive pollutants emitted from tailpipes and smokestacks are reduced.
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Barnett, A. Secondary sources. Nature Clim Change 1, 67 (2009). https://doi.org/10.1038/climate.2009.50
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DOI: https://doi.org/10.1038/climate.2009.50
