M. Simmel

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 (σ) 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 σ depression during cloud activation may affect droplet number ( Nd) as much as a 2-fold increase in particle number, whereas by considering also the size distribution of particulate SAS, Nd 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 PM10 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 Nd.

The Role of Aerosol Characteristics in the Evolution of Clouds and Precipitation

A spectral microphysics scheme is implemented into the three- dimensional Lokalmodell of the German Weather Service in order to investigate the role of aero- sol characteristics and cloud droplet nucleation for the evolution of clouds and the formation of precipitation. The scheme includes a combined spectral description of wetted aerosols including cloud condensation nuclei as well as cloud droplets and rain drops. An initial spectrum is prescribed which can freely evolve afterwards. Sensitivity studies on an artificial test case are undertaken showing the impact of various aerosol characteristics, such as particle number distribution, on the cloud properties. The studies show, e.g., that an increased initial particle number with suf- ficiently large particles leads to diminished cloud droplet sizes and a relative humid- ity near 100%, while fewer particles lead to supersaturations up to 15% and larger droplets. This results in a larger amount of precipitation for fewer but not too small particles.