Anthropogenic enhancements to production of highly oxygenated molecules from autoxidation.

Abstract

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 (PM_2.5). 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 NO_x emissions decrease, the chemical regime of the atmosphere changes to one in which RO₂ autoxidation becomes increasingly important, potentially increasing PM_2.5, while oxidant availability driving RO₂ formation rates simultaneously declines, possibly slowing regional PM_2.5 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 NO_x decreases, absolute HOM production rates decrease due to oxidant reductions, leading to an overall positive coupling between anthropogenic NO_x 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 PM_2.5 abatement strategies come with NO_x emission reductions and have implications for aerosol-climate interactions due to changes in global SOA resulting from NO_x interactions since the preindustrial era.