Abstract
The droplet surface plays important roles in the interaction between organic aerosols with clouds and climate. Surface active organic compounds can partition to the droplet surface, depleting the solute from the droplet bulk or depressing the droplet surface tension. This may in turn affect the shape of the droplet growth curve, threshold of aerosol activation into cloud droplets, activated droplet size distributions, and cloud radiative effects. In this work, a new monolayer model along with a traditional Gibbs adsorption isotherm model was used in conjunction with equilibrium Köhler theory to predict cloud condensation nuclei (CCN) activation of both simple and complex surface active model aerosol systems. For the surface active aerosol considered, the monolayer droplet model produces similar results to the Gibbs model as well as comparable results to CCN measurements from the literature, even for systems where specific molecular identities and aqueous properties are unknown. The monolayer model is self-contained and fully prognostic, and provides a versatile, conceptually simple, yet physically based model for understanding the role of organic surfactants in cloud droplet formation.
Highlights
We focus on pollenkitt extracted from the pollen of black poplar (Populus nigra) and common ragweed (Ambrosia artemisiifolia), which were found to be the most and least cloud condensation nuclei (CCN) and surface active, respectively, among six pollenkitts studied by Prisle et al.[12]
Combined with equilibrium Kohler theory, the CCN activity of these surface active model aerosol systems was calculated for a range of dry particle sizes and surface active organic mass fractions, using both the monolayer model and a traditional Gibbs surface framework
Underpinning the CCN activity of surface active aerosols in the two models is an interplay of several mechanisms, including species and droplet mixing state-dependent surface partitioning, surface tension reduction, dilution, and changing surface/ bulk volume ratios of aqueous droplets as they grow and are activated
Summary
The role of surfactants in cloud droplet activation has been a recurring theme for modelling of organic aerosol cloud climate interactions.[1,2,3,4,5,6,7,8,9,10,11,12] While it has been established that surface activity can impact single droplet activation through both lowering surface tension and from diminishing the solute effect by surface partitioning,[3,4,5,6,12,13] the relative impact of these effects and their potential synergies under different conditions are currently not well constrained.[9,10,14,15] This balance has been shown to signi cantly impact the predictions of global cloud droplet numbers and radiative forcing[16] and is important to understand in a fully prescriptive way.
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