Key drivers of cloud response to surface-active organics

This is the final version. Available on open access from Nature Research via the DOI in this recordData availability: The data used to produce Figs. 1–5 are available from the Bolin database (https://bolin.su.se/data/lowe-2019) and/or upon request from the authors. The observational data in Fig. 4 has been acquired from the EBAS database (www.ebas.nilu.no).Code availability: Plotting, data analysis and simulation setup scripts are available at https://github.com/SamJLowe/NatComms_OrgSurfaceCloudsAerosol-cloud interactions constitute the largest source of uncertainty in global radiative forcing estimates, hampering our understanding of climate evolution. Recent empirical evidence suggests surface tension depression by organic aerosol to significantly influence the formation of cloud droplets, and hence cloud optical properties. In climate models, however, surface tension of water is generally assumed when predicting cloud droplet concentrations. Here we show that the sensitivity of cloud microphysics, optical properties and shortwave radiative effects to the surface phase are dictated by an interplay between the aerosol particle size distribution, composition, water availability and atmospheric dynamics. We demonstrate that accounting for the surface phase becomes essential in clean environments in which ultrafine particle sources are present. Through detailed sensitivity analysis, quantitative constraints on the key drivers – aerosol particle number concentrations, organic fraction and fixed updraft velocity – are derived for instances of significant cloud microphysical susceptibilities to the surface phase.Knut and Alice Wallenberg foundationChemical Sciences Geosciences and Biosciences Division, Office of Basic Energy Sciences, U.S. Department of Energ