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 (RO 2 ) 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 RO 2 autoxidation becomes increasingly important, potentially increasing PM 2.5 , while oxidant availability driving RO 2 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-RO 2 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.