Studying the growth and cloud activation of sodium decanoate and sodium chloride particles as surrogates of organic marine aerosols

Abstract

Aerosol particles transform into cloud condensation nuclei (CCN) above a critical value of the ambient water vapor supersaturation. Global climate models (GCMs) use parameterizations to estimate number concentrations of these CCN in clouds. Surfactants are organic compounds that can affect the droplet growth by e.g. decreasing surface tension of the droplet. Marine aerosols could contain an unaccounted amount of these kind of surfactants. Surfactant induced effects should be considered into calculation modules of droplet activation, droplet growth and droplet light extinction. In this thesis the cloud droplet activation of surfactant enriched aerosols is evaluated with the “simple” Köhler model (Köhler, 1936) and Köhler model combined with the monolayer model (Malila & Prisle, 2018) for surface thermodynamics. The main goal was the assessment of a modelling framework that accounts for complex surfactant effects on the droplet activation of sodium decanoate/sodium chloride particles as surrogates of marine aerosols.

It is questioned that the estimations of the cloud forming potential of secondary marine aerosols with the conventional setting of the Köhler model could be misled by using the corrected water mole fraction as a measure of the water activity.

The new composition-dependent parametrizations of water activity and density including micelles we used show a different perspective of the activation efficiency of surfactant-enriched aerosols. Even when these are surrogates of primary marine aerosols (enriched in NaCl not in SO4 as the secondary marine aerosols), the interaction organic/inorganic controlling the surfactant behavior are very similar.

The study confirmed that the critical radius of particles values varies between 20 nm–150 nm in normal conditions but activated droplets can be as small as 10 nm–15 nm in radius. Our calculations agree with the results from previous studies as both models predict the minimum critical particle radius to be around 15 nm–70 nm.

The more particles there are taking up water in the cloud the less likely it gets for an individual droplet to become a CCN. This would result in a decrease in precipitation. Our study gives insight to what is to be expected when using the conventional Köhler models for predicting aerosol-cloud inter-actions in marine environments. More precise and computationally efficient ways of property calculation are needed to understand the role of biogenic aerosols in the radiative forcing of marine clouds.