Competition not concentration matters when forming cloud-influencing aerosols

Results of this study indicate that nitrogen oxides cannot be assumed to have a linear effect on SOA formation. The complexities of the relationship need to be included in the next generation of models to accurately predict the dynamics of SOA formation and composition in different environments.

Isoprene, an organic compound produced and emitted into the atmosphere by many tree species, plays an important role in tropospheric ozone chemistry and in formation of SOA particles. The particles 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 SOA 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. To help address this challenge, 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 SOA generated by isoprene photooxidation. Volatility is a key property of organic aerosol because it determines the partitioning between the gas and particle phases, and thus the formation of SOA particles. Researchers found that the volatility and oxidation state of isoprene SOA are sensitive to, and exhibit a nonlinear dependence on, NOx levels. The dependence likely arises from gas-phase chemistry of organic peroxy radicals, which compete with NOx in reactions, 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.