While OH radicals and O3 play a key role in the oxidation of monoterpenes and ultimately new particle formation (NPF), NO3 radicals can be an important or even dominant oxidant in the atmosphere. NO3 initiated oxidation of biogenic volatile organic compounds (VOCs) can produce a wide variety of multifunctional nitrate-containing compounds such as organonitrates (ONs). Results from field measurements indicate that 70% of the ONs observed in the boreal forest are formed from the NO3-initiated oxidation of biogenic VOCs, with NO3 chemistry being important even during daytime. Due to their semi-volatile/low-volatility nature, ONs can partition into the particle phase either by condensation onto pre-existing aerosol particles or by forming new particles. However, field and laboratory studies have suggested that formation might ONs reduce and/or hinder NPF.<br/><br/>To constrain, the fate and distribution of compounds produced from the NO3-initiated oxidation of monoterpenes, laboratory experiments were performed using a flow reactor at IRCELYON (CNRS, France) and using The Cosmics Leaving Outdoor Droplets (CLOUD) chamber located at CERN. Due to its importance in the atmosphere, α-pinene was selected as a model compound. A Q-Exactive Orbitrap mass spectrometer (Thermo Scientific, US) coupled to an atmospheric pressure chemical ionization (CI) inlet was used for online analysis of the oxygenated VOCs generated from the oxidation of α-pinene by NO3 radicals using two different chemical ionization methods, i.e., nitrate-ion-based (NO3–) and ammonium-ion-based (NH4+) schemes. As O3 and NO3 chemistry coexist in the atmosphere, the experiments were performed under atmospheric relevant conditions with the injection of NO2 to mimic different O3/NO3 conditions (i.e., from O3 dominating to NO3 dominating chemistry). Overall, our results show that the presence of NO3 radical greatly impacts the distribution of α-pinene derived ozonolysis products, including the formation of nitroxydimers, dinitroxydimers, and ONs while suppressing the formation of the conventional dimeric products and highly oxygenated organic compounds (i.e., HOMs). The presence of NO3 radicals yields a substantial drop in the formation rate of new particles. Ambient observations further support laboratory findings and reveal the pivotal role of the oxidation capacity of NO3 radicals in the formation of new particles.

Nitrate radical chemistry hinders new particle formation from monoterpene oxidation

ACS Spring 2022

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