Epoxide-driven secondary organic aerosol formation is modulated by aerosol-cloud cycling

Isoprene-derived epoxydiols secondary organic aerosol (IEPOX SOA) and sulfate particles are major contributors to atmospheric particulate matter and play central roles in transforming gaseous emissions into secondary organic and inorganic aerosol. Although both species form through aqueous-phase reactions, the timing and lifecycle controls governing aqueous secondary organic aerosol (SOA) formation remain poorly constrained, in contrast to the relatively well-understood in-cloud formation of sulfate. Here, we combine single-particle measurements and a particle-resolved model of aerosol-cloud cycling to quantify processes governing aqueous sulfate and IEPOX SOA chemistry in aerosol particles and cloud droplets observed during the HI-SCALE field campaign, representing the first comparison of single-particle observations with particle-resolved simulations of cloud-processed aerosol. . We identify fundamental differences in the processes controlling their formation: sulfate production is governed by aqueous oxidation kinetics within cloud droplets, whereas IEPOX SOA forms rapidly during droplet evaporation and is strongly controlled by cloud-water acidity and the transient concentration of nucleophiles. These results demonstrate that sulfate and IEPOX SOA form at distinct stages of the aerosol–cloud life cycle and suggest that models neglecting droplet-scale concentration effects may substantially underestimate IEPOX SOA formation. More broadly, our findings indicate that evaporation-driven concentration dynamics during cloud cycling likely regulate other aqueous SOA pathways that depend on nucleophile availability.