PNNL

Abstract

Aerosol particles strongly influence global climate by modifying the albedo and lifetime of clouds. An accurate assessment 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 concentration is often a strong function of the hygroscopicity of organics in the particles. Laboratory and field studies show organic hygroscopicity often increases nearly linearly with oxidation level (i.e., atomic O:C ratio). Such increase in hygroscopicity has long been thought a result of higher polarity and therefore increased water solubility of more oxygenated organics. In this study, we systematically characterized organic hygroscopicity of secondary organic aerosol (SOA) formed from representative precursors. We show that for the majority of SOA, organic hygroscopicity is not limited by water solubility, but is controlled mainly by the molecular weight of organic species. Instead of increased water solubility as previously thought, the increase of the organic hygroscopicity with O:C is largely because (1) SOA formed from smaller precursor molecules tend to have lower average molecular weight and higher O:C and (2) during oxidation, fragmentation reactions reduce average organic molecule weight, leading to increased hygroscopicity. A simple model of organic hygroscopicity based on molecular weight, O:C, and volatility is developed, and it successfully reproduces the variation of SOA hygroscopicity with O:C observed in the laboratory and field studies.