Size-resolved surface active substances of atmospheric aerosol: reconsideration of the impact on cloud droplet formation.

Our current understanding of the importance of surface-active substances (SAS) on atmospheric aerosol cloud-forming efficiency is limited, as explicit data on the content of size-resolved ambient aerosol SAS, which are responsible for lowering the surface tension (sigma) of activating droplets, are not available. We report on the first data comprising seasonal variability of size-segregated SAS concentrations in ambient aerosol particulate matter (PM). To assess the impact of SAS distribution within PM on cloud droplet activation and growth, a concept of surfactant activity was adopted and a parametrization developed; i.e., surfactant activity factor (SAF) was defined, which allowed translation of experimental data for use in cloud parcel modeling. The results show that SAS-induced sigma depression during cloud activation may affect droplet number (N-d) as much as a 2-fold increase in particle number, whereas by considering also the size distribution of particulate SAS, N-d may increase for another 10%. This study underscores the importance of size-resolved SAS perspective on cloud activation, as data typically obtained from aqueous extracts of PM2.5 and PK10 may result in misleading conclusions about droplet growth due to large mass fractions of supermicron particles with SAS deficit and little or no influence on CCN and N-d.