Nitrate radicals suppress biogenic new particle formation from monoterpene oxidation

Abstract Highly oxygenated organic molecules (HOMs) are a major source of new particles affecting Earth’s climate 1,2 . HOM production from the oxidation of volatile organic compounds (VOCs) occurs during both day and night, and can lead to new particle formation (NPF) 3,4 . However, NPF involving organic vapors has been reported much more often during daytime 3-6 than during nighttime 7,8 . Here, we show that the nitrate radicals (NO 3 ) - which arise predominantly at night – inhibit NPF during the oxidation of monoterpenes based on three lines of observational evidence: NPF experiments in the CLOUD chamber at CERN; radical chemistry experiments using an oxidation flow reactor; and field observations in a wetland that occasionally exhibits nocturnal NPF. Nitrooxy-peroxy radicals formed from NO 3 chemistry suppress the production of ultra-low volatility organic compounds (ULVOCs) responsible for biogenic NPF, which are covalently bound RO 2 dimer association products. The ULVOC yield of α-pinene in the presence of NO3 is one-fifth of that resulting from ozone chemistry alone. Even trace amounts of NO 3 radicals, at sub parts per trillion level, suppress the NPF rate by a factor of 4. Ambient observations further confirm that when NO 3 chemistry is involved, monoterpene NPF is completely turned off. Our results explain the frequent absence of nocturnal biogenic NPF in monoterpene-rich environments.