Cloud droplet activation of secondary organic aerosol is mainly controlled by molecular weight, not water solubility
2018
Abstract. Aerosol particles strongly influence global climate by modifying the
properties 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. Earlier studies showed
organic hygroscopicity increases nearly linearly with oxidation level. Such
an increase in hygroscopicity is conventionally attributed to higher water
solubility for more oxidized organics. By systematically 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 organic
hygroscopicity is not limited by solubility but is dictated mainly by the
molecular weight of organic species. Instead 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 average molecular weight
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, oxidation level, and
volatility is developed, and it successfully reproduces the variation in SOA
hygroscopicity with oxidation level observed in the laboratory and field
studies.
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