Impacts of summertime photochemical aging on the physicochemical properties of aerosols in a Paris suburban forest region

Abstract

Organic aerosols (OA) play a significant role in influencing both climate and human health. However, in source–receptor modelling, a large fraction of OA is typically attributed to highly aged, atmospherically processed species collectively referred to as oxygenated organic aerosol (OOA). Nevertheless, the formation pathways and evolution of OOA as well as their impacts on aerosol optical properties, remain poorly understood. To address this knowledge gap, an experiment was conducted in a suburban site in the Paris region to study the evolution of OOA and their optical properties. Our results show that in regionally transported air masses with mixed biogenic and anthropogenic emissions, the formation of OOA through photochemical processes explains most of the increase in submicron particle mass. Meteorological conditions played a critical role: under dry and strong solar radiation conditions, enhanced formation of more-oxidized OOA (MO-OOA) was observed. BrC absorption increased concurrently, with short-wavelength absorption rising by ∼ 35 % over relatively ∼ 24 h of photochemical aging. Conversely, under humid, low-radiation conditions, the OA composition shifted toward less-oxidized OOA (LO-OOA). Suppressed photochemistry limited MO-OOA production, resulting in a lower overall OA oxidation state. These findings highlight the role of photochemistry in shaping both the chemical evolution and resultant optical properties of OA, underscoring the need to consider meteorological dynamics when evaluating aerosol–climate interactions in suburban forest environments.