Chemical Evolution of Monoterpene-Derived Secondary Organic Aerosols during Dark Aging

Secondary organic aerosols (SOA), a major component of atmospheric particles, significantly impact both climate and human health. Monoterpenes (C10H16), a prevalent class of volatile organic compounds, are key SOA precursors. However, the aging processes that alter SOA’s physicochemical properties─particularly under high relative humidity (RH)─remain poorly understood. In this study, we used advanced mass spectrometry techniques to investigate the gas- and particle-phase composition of SOA formed via O3/OH-initiated oxidation of β-pinene, limonene, and α-pinene. Fresh SOA showed different levels of dimeric species, with β-pinene SOA containing the most (37%). The dimer fractions in SOA increased during dark aging from 12 to 37% to 33–51%, driven by particle-phase accretion rather than gas-particle partitioning. Simultaneously, parametrizations used to estimate the evolution of SOA physical properties showed that aging led to a decrease in SOA volatility (from 1.4 to 2.4 to 0.5–1.1 μg m–3) while the viscosity increased (11.2–14.9 to 17.9–19.0 Pa s) over time. The rise in unsaturated dimers and decline in carboxylic acid monomers suggest esterification as a potentially important mechanism. These findings highlight dark aging as a crucial factor in SOA evolution.