Naturally derived organic coatings on sulfate particles from fossil fuel combustion affect the formation of secondary organic aerosols.
Atmospheric aerosol particles play an important role in climate, air quality, and health. A recent study sheds light on how complex interactions between manmade and natural emissions affect the formation of a major type of air pollutant.
The study’s findings show that transformation of plant-derived isoprene into atmospheric organic aerosols by sulfate particles emitted from human activities is significantly impeded by the presence of other plant-derived organic coatings on sulfate particles. This information could be used to improve the accuracy of models simulating the effect of organic aerosols on climate and air quality.
Secondary organic aerosols (SOA) are air pollutants that have been implicated in serious health problems such as lung and heart disease. They are produced through a complex interaction among sunlight; volatile organic compounds from trees, plants, cars, or industrial emissions; and other atmospheric organics. Aside from methane, the most abundant hydrocarbon released into the atmosphere is isoprene—a volatile organic compound emitted by oak, poplar, eucalyptus, and other trees. In many regions of the United States, a major contributor to SOA formation is a complex reaction between isoprene byproducts called isoprene epoxydiols (IEPOX) and acidic sulfate aerosols generated by the combustion of fossil fuels. However, prior to this study, researchers did not know whether the reaction occurs on the particles’ surfaces or inside the particles. Moreover, past studies investigated this reaction using pure sulfate particles rather than realistic atmospheric sulfate particles, which are usually coated with other organics.
To investigate these complex processes, a team of researchers from the University of North Carolina at Chapel Hill; Pacific Northwest National Laboratory (PNNL); Aarhus University; University of California, Berkeley; and Imre Consulting used a unique single particle mass spectrometer, known as SPLAT II, at the Department of Energy’s (DOE) Environmental Molecular Sciences Laboratory. They started by studying the IEPOX uptake by pure sulfate particles, showing, for the first time, that the IEPOX reaction with uncoated sulfate particles is volume controlled, leading to a situation in which all particles have the same amount of IEPOX-derived products. In another set of experiments, the team examined how formation of IEPOX-derived SOA is affected when sulfate particles are coated with atmospherically relevant organics like α-pinene SOA—mainly produced from pine tree emissions. These studies show reactions between IEPOX and sulfate particles strongly depend on how much coating material is present. The rate of IEPOX uptake by coated sulfate particles compared with pure sulfate particles is significantly reduced even at very low coating concentrations, and higher concentrations completely stopped the reaction, eliminating SOA formation. Notably, unlike for the pure sulfate case, the coatings yield small particles with less IEPOX-derived SOA than larger ones. These findings could be incorporated into models to enable more accurate representations of the most abundant particles in the atmosphere and to simulate their effect on climate and air quality.
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This work was supported by DOE’s Office of Science (Offices of Biological and Environmental Research and Basic Energy Sciences, including support of EMSL, a DOE user facility). This work was also funded in part by the National Science Foundation, Carlsberg Foundation, Centre of Excellence Cryosphere-Atmosphere Interactions in a Changing Arctic Climate funded by NordForsk, and the Camille and Henry Dreyfus Postdoctoral Fellowship Program in Environmental Chemistry.
Riva, M., D. M. Bell, A.-M. K. Hansen, G. T. Drozd, Z. Zhang, A. Gold, D. Imre, J. D. Surratt, M. Glasius, and A. Zelenyuk. 2016. “Effect of Organic Coatings, Humidity and Aerosol Acidity on Multiphase Chemistry of Isoprene Epoxydiols,” Environmental Science and Technology 50(11), 5580-88. DOI: 10.1021/acs.est.5b06050. (Reference link)
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