Abstract:

Multiphase chemistry of isoprene photooxidation products has been shown to be one of the major sources of secondary organic aerosol (SOA) in the atmosphere. A number of recent studies, including those conducted in our lab, indicate that aqueous aerosol phase provides a medium for reactive uptake of isoprene photooxidation products, and in particular, isomeric isoprene epoxydiols (IEPOX), with reaction rates, yields, and particle properties being dependent on aerosol acidity, water content, sulfate concentration, and organic coatings. However, very few studies focused on chemistry occurring within actual cloud droplets. I will present the aircraft-based single-particle measurements of the size and mixing state of individual below-cloud particles, interstitial aerosol particles, and cloud droplet residuals characterized during two contrasting seasons. Our measurements reveal enhanced contribution from larger and sulfate-rich particles in cloud droplet residuals and provide direct evidence for sulfate and IEPOX-derived SOA formation in cloud droplets. We observe a strong dependence of the size and mixing state of below-cloud aerosol on their cloud droplet activation fraction especially during the spring campaign, when the observed dynamic range in aerosol properties was large. Ultimately, the combined cloud, aerosol, and gas-phase measurements are used to develop and evaluate model treatments of aqueous-phase isoprene SOA formation and aerosol-cloud-climate interactions.