Effects of NOx on the Volatility of Secondary Organic Aerosols Generated by Isoprene Photooxidation
Isoprene, an organic compound that is produced and emitted into the atmosphere by many species of trees, plays an important role in tropospheric ozone chemistry and in the formation of secondary organic aerosol (SOA) particles. SOAs can affect Earth’s radiation balance directly through scattering or absorption of sunlight and indirectly through forming cloud condensation nuclei. Accurately simulating the impacts of SOAs on climate requires understanding the chemical processes that lead to SOA formation in the atmosphere under a wide variety of environmental conditions. Previous laboratory studies have demonstrated different SOA oxidation yields and properties at “low” (near-zero) and “high” levels of nitrogen oxides (NOx). Current models use a linear combination of these extreme conditions to predict altered SOA formation in the presence of anthropogenic emissions. Unfortunately, data from recent laboratory and field studies have not been consistent with this simple model. A team of U.S. Department of Energy researchers conducted laboratory experiments in an environmental chamber to investigate the effects of NOx on the volatility and chemical composition of SOAs generated by isoprene photooxidation. Volatility is a key property of organic aerosols because it determines the partitioning between the gas and particle phases, and thus SOA particle formation. The team found that the volatility and oxidation state of isoprene SOAs are sensitive to, and exhibit a nonlinear dependence on, NOx levels. The dependence of SOA yield, volatility, and oxidation state on the NOx level likely arises from gas-phase chemistry of organic peroxy radicals (RO2) and succeeding particle-phase reactions. This observation helps reconcile the seemingly contradictory observations of the NOx effect on isoprene SOA volatility reported in previous literature studies. These results indicate that the nonlinear effects of NOx on SOA formation need to be included in the next generation of models to accurately predict the dynamics of SOA formation and composition in ambient environments where RO2 fate varies considerably.
Xu, L., M. S. Kollman, C. Song, J. E. Shilling, and N. L. Ng. 2014. “Effects of NOx on the Volatility of Secondary Organic Aerosol from Isoprene Photooxidation,” Environmental Science and Technology 48(4), 2253-62. DOI:10.1021/es404842g.