The Global Importance of Gas-phase Peroxy Radical Accretion Reactions

Abstract. Secondary organic aerosol (SOA) is an important class of atmospheric species with influences on air quality and climate. One understudied SOA formation pathway is gas-phase peroxy radical (RO₂) accretion reactions, where two peroxy radicals combine to form a dimer species. This work makes use of recent advances in the theoretical understanding of RO₂ accretion reactions to assess their contribution to SOA. After evaluation in a chemical box model, a reduced representation of RO₂ accretion reactions was added to a global chemical transport model (GEOS-Chem) to assess the contribution to global SOA and the associated radiative impact. The results of this work suggest that RO₂ accretion products comprise 30–50 % of particulate matter (PM_2.5) in tropical forested environments, and a smaller proportion in more temperate regions like the south-eastern USA (≈5 %). This work confirms that biogenic volatile organic compounds (BVOCs) are the main precursors to accretion products globally, but suggests that a notable fraction of aerosol-phase accretion products come from aromatic-derived RO₂ and small acyl-peroxy radicals. Contrary to previous assumptions that accretion products are organic peroxides, the box modelling investigations suggest that non-peroxide accretion products (ethers and esters) could comprise the majority of accretion products in both the gas and aerosol phase. This work provides justification for more extensive measurements of RO₂ accretion reactions in laboratory experiments and RO₂ accretion products in the ambient atmosphere in order to better constrain the representation of this chemistry in atmospheric models, including a greater level of mechanistic chemical representation of SOA formation processes.