Particle-Phase Peroxides Form Carboxylic Acids in Pinene Secondary Organic Aerosol

CHRISTOPHER KENSETH, Jing Chen, Olivia Hakan, Donterrio Moore, Matthieu Riva, Michael Kamrath, Yuzhi Chen, John Shilling, Henrik Kjaergaard, Joel A. Thornton, University of Washington

     Abstract Number: 587
     Working Group: Aerosol Chemistry

Abstract
Carboxylic acids are major components of atmospheric secondary organic aerosol (SOA), which plays a pivotal role in climate, air quality, and health. Their production is currently understood to occur predominantly through gas-phase chemistry. However, despite extensive study, the identities and thus the formation mechanisms of carboxylic acids in SOA remain largely unvalidated. In this work, we identify the most abundant carboxylic acids in SOA from ozonolysis of α-pinene and β-pinene—substantial global SOA sources—through independent synthesis of authentic standards. Based on targeted laboratory experiments featuring isotopically labeled compounds, tailored precursors, and isomer-resolved mass spectrometry, together with quantum chemical calculations of multigenerational peroxy and alkoxy radical chemistry, we show that these acids are formed in the particle phase through the decomposition of acylperoxyhemiacetals. These findings, together with past work on the formation of dimer esters in SOA, demonstrate that the particle-phase production of acylperoxyhemiacetals, and their subsequent conversion to carboxylic acids and dimer esters via Baeyer-Villiger decomposition and nucleophilic addition of alcohols, represents a key process in SOA formation.