PNNL

Abstract

Aerosol particles strongly influence global climate by modifying the properties of clouds. An accurate assess-ment of the aerosol impact on climate requires knowledge of the concentration of cloud condensation nuclei (CCN), a subset of aerosol particles that can activate and form cloud droplets in the atmosphere. Atmospheric particles typically consist of a myriad of organic species, which frequently dominate the particle composition. As a result, CCN concen-tration is often a strong function of the hygroscopicity of or-ganics in the particles. Earlier studies showed organic hygro-scopicity increases nearly linearly with oxidation level. Such an increase in hygroscopicity is conventionally attributed to higher water solubility for more oxidized organics. By sys-tematically varying the water content of activating droplets, we show that for the majority of secondary organic aerosols (SOAs), essentially all organics are dissolved at the point of droplet activation. Therefore, for droplet activation, the or-ganic hygroscopicity is not limited by solubility but is dic-tated mainly by the molecular weight of organic species. In-stead of increased water solubility as previously thought, the increase in the organic hygroscopicity with oxidation level is largely because (1) SOAs formed from smaller precursor molecules tend to be more oxidized and have lower aver-age molecular weight and (2) during oxidation, fragmenta-tion reactions reduce average organic molecule weight, lead-ing to increased hygroscopicity. A simple model of organic hygroscopicity based on molecular weight, oxidation level, and volatility is developed, and it successfully reproduces the variation in SOA hygroscopicity with oxidation level ob-served in the laboratory and field studies.