Nocturnal Atmospheric Synergistic Oxidation Reduces the Formation of Low-volatility Organic Compounds from Biogenic Emissions

Abstract. Volatile organic compounds (VOCs) are often subject to synergistic oxidation by different oxidants in the atmosphere. However, the exact synergistic oxidation mechanism of atmospheric VOCs and its role in particle formation remain poorly understood. In particular, the reaction kinetics of the key reactive intermediates, organic peroxy radicals (RO₂), during synergistic oxidation is rarely studied. Here, we conducted a combined experimental and kinetic modelling study of the nocturnal synergistic oxidation of α-pinene (the most abundant monoterpene) by O₃ and NO₃ radicals as well as its influences on the formation of highly oxygenated organic molecules (HOMs) and particles. We find that in the synergistic O₃ + NO₃ regime, where OH radicals are abundantly formed via decomposition of ozonolysis-derived Criegee intermediates, the production of C_xH_yO_z-HOMs is substantially suppressed compared to that in the O₃-only regime, mainly because of the termination of α-pinene RO₂ derived from ozonolysis and OH oxidation by those arising from NO₃ oxidation. Measurement-model comparisons further reveal that the termination reactions between ozonolysis- and NO₃-derived RO₂ are on average 10 – 100 times more efficient than those of OH- and NO₃-derived RO₂. Despite a strong production of organic nitrates in the synergistic oxidation regime, the substantial decrease of C_xH_yO_z-HOM formation leads to a significant reduction in ultralow- and extremely low-volatility organic compounds, which significantly inhibits the formation of new particles. This work provides valuable mechanistic and quantitative insights into the nocturnal synergistic oxidation chemistry of biogenic emissions and will help to better understand the formation of low-volatility organic compounds and particles in the atmosphere.