Anthropogenic enhancements to production of highly oxygenated molecules from autoxidation.
Citation:
Pye, H., E. D’Ambro, B. Lee, S. Schobesberger, M. Takeuchi, Y. Zhao, F. Lopez-Hilfiker, J. Liu, J. Shilling, J. Xing, R. Mathur, A. Middlebrook, J. Liao, A. Welti, M. Graus, C. Warneke, J. Gouw, J. Holloway, T. Ryerson, I. Pollack, AND J. Thornton. Anthropogenic enhancements to production of highly oxygenated molecules from autoxidation. PNAS (PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES). National Academy of Sciences, WASHINGTON, DC, 116(14):6641-6646, (2019). https://doi.org/10.1073/pnas.1810774116
Impact/Purpose:
As ozone and particulate matter abatement strategies reduce anthropogenic nitrogen oxide (NOx) emissions, the chemical regime of the atmosphere changes to one in which autoxidation of organic peroxy radicals (RO2) becomes increasingly important. Developed countries including the United States have experienced large declines in nitrogen oxides due to controls on emissions, and the possibility for increased aerosol exists as autoxidation results in highly oxygenated organic molecules (HOM) which efficiently form particulate matter. Using the reaction of a common biogenic volatile organic compound (VOC), α-pinene, with hydroxyl radicals (OH) as an archetype system, we find that although autoxidation becomes more competitive as NOx decreases, HOM production currently increases with increasing NOx. This effect is observed in the Atlanta, Georgia urban plume where HOM is enhanced in the presence of elevated NO, and predictions for Guangzhou, China where increasing HOM-RO2 production coincides with increases in NO from 1990 to 2010. Results shown here demonstrate that anthropogenic emissions can enhance HOM and the resulting biogenic aerosol formation through oxidant abundance for low to moderate NOx levels. HOM production is also shown to be nonlinear with the maximum production rates occurring at intermediate NOx.
Description:
Atmospheric oxidation of natural and anthropogenic volatile organic compounds (VOCs) leads to secondary organic aerosol (SOA), which constitutes a major and often dominant component of atmospheric fine particulate matter (PM2.5). Recent work demonstrates that rapid autoxidation of organic peroxy radicals (RO2) formed during VOC oxidation results in highly oxygenated organic molecules (HOM) that efficiently form SOA. As NOx emissions decrease, the chemical regime of the atmosphere changes to one in which RO2 autoxidation becomes increasingly important, potentially increasing PM2.5, while oxidant availability driving RO2 formation rates simultaneously declines, possibly slowing regional PM2.5 formation. Using a suite of in situ aircraft observations and laboratory studies of HOM, together with a detailed molecular mechanism, we show that although autoxidation in an archetypal biogenic VOC system becomes more competitive as NOx decreases, absolute HOM production rates decrease due to oxidant reductions, leading to an overall positive coupling between anthropogenic NOx and localized biogenic SOA from autoxidation. This effect is observed in the Atlanta, Georgia, urban plume where HOM is enhanced in the presence of elevated NO, and predictions for Guangzhou, China, where increasing HOM-RO2 production coincides with increases in NO from 1990 to 2010. These results suggest added benefits to PM2.5 abatement strategies come with NOx emission reductions and have implications for aerosol–climate interactions due to changes in global SOA resulting from NOx interactions since the preindustrial era.
URLs/Downloads:
DOI: Anthropogenic enhancements to production of highly oxygenated molecules from autoxidation.