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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., Competing Interests: The authors declare no conflict of interest.

Organic peroxy radicals (RO 2 ) are key intermediates in the atmospheric degradation of organic matter and fuel combustion, but to date, few direct studies of specific RO 2 in complex reaction systems exist, leading to large gaps in our understanding of their fate. We show, using direct, speciated measurements of a suite of RO 2 and gas-phase dimers from O 3 -initiated oxidation of α-pinene, that ∼150 gaseous dimers (C 16-20 H 24-34 O 4-13 ) are primarily formed through RO 2 cross-reactions, with a typical rate constant of 0.75-2 × 10 -12 cm 3 molecule -1 s -1 and a lower-limit dimer formation branching ratio of 4%. These findings imply a gaseous dimer yield that varies strongly with nitric oxide (NO) concentrations, of at least 0.2-2.5% by mole (0.5-6.6% by mass) for conditions typical of forested regions with low to moderate anthropogenic influence (i.e., ≤50-parts per trillion NO). Given their very low volatility, the gaseous C 16-20 dimers provide a potentially important organic medium for initial particle formation, and alone can explain 5-60% of α-pinene secondary organic aerosol mass yields measured at atmospherically relevant particle mass loadings. The responses of RO 2 , dimers, and highly oxygenated multifunctional compounds (HOM) to reacted α-pinene concentration and NO imply that an average ∼20% of primary α-pinene RO 2 from OH reaction and 10% from ozonolysis autoxidize at 3-10 s -1 and ≥1 s -1 , respectively, confirming both oxidation pathways produce HOM efficiently, even at higher NO concentrations typical of urban areas. Thus, gas-phase dimer formation and RO 2 autoxidation are ubiquitous sources of low-volatility organic compounds capable of driving atmospheric particle formation and growth., Competing Interests: The authors declare no conflict of interest.

The chemical complexity of atmospheric organic aerosol (OA) has caused substantial uncertainties in understanding its origins and environmental impacts. Here, we provide constraints on OA origins through compositional characterization with molecular-level details. Our results suggest that secondary OA (SOA) from monoterpene oxidation accounts for approximately half of summertime fine OA in Centreville, AL, a forested area in the southeastern United States influenced by anthropogenic pollution. We find that different chemical processes involving nitrogen oxides, during days and nights, play a central role in determining the mass of monoterpene SOA produced. These findings elucidate the strong anthropogenic-biogenic interaction affecting ambient aerosol in the southeastern United States and point out the importance of reducing anthropogenic emissions, especially under a changing climate, where biogenic emissions will likely keep increasing., Competing Interests: The authors declare no conflict of interest.