Natural Chemicals Transform Man-Made Particulates

Competition between two natural chemicals to coat and change atmospheric particles from fossil fuel combustion could improve accuracy of climate and air quality simulations.

Natural Chemicals
Oak and other trees and shrubs emit the gas isoprene into the atmosphere. In the presence of human-made sulfate particles, isoprene transforms into an aerosol particle, a type of air pollutant that is implicated in serious health problems, including lung and heart disease. Image courtesy of Pacific Northwest National Laboratory

The Science

Tiny particles in the air play an important role in our climate, air quality, and health. A recent study sheds light on a complex competition between plant-derived gases. These gases emitted by trees and other plants compete to coat and change particles from fossil fuel combustion. The scientists found that other plant-derived chemicals significantly impede the transformation of the plant-derived gas isoprene into a coating for sulfate particles.

The Impact

Scientists could use the information from this collaborative study to improve the accuracy of models used to predict the effect of atmospheric particles on climate and air quality.

Summary

Secondary organic aerosols (SOA) are air pollutants 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 organic chemicals. 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, before 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 organic chemicals. 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. The team showed, 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 the formation of IEPOX-derived SOA is affected when sulfate particles are coated with atmospherically relevant organic chemicals such as 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. 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. Scientists could incorporate these findings 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.