Influence of organic compounds on the cloud droplet activation: A model investigation considering the volatility, water solubility, and surface activity of organic matter

[1] The influence of organic compounds on the activation of cloud droplets was investigated using a numerical model. The droplet activation process was simulated with an adiabatic air parcel model, coupled with a detailed treatment of aerosol dynamics. To cover the whole range of organic matter present in atmospheric aerosols, four different groups of organic compounds were considered: water-soluble and water-insoluble organics that are nonvolatile and slightly soluble and water-soluble organics that are semivolatile and thereby able to move reversibly between the gas and the particle phase. The modeling results suggest that water-soluble organics are able to influence the droplet concentrations by increasing the solute mass of the droplets and also by reducing their surface tension. Compared to these substances, slightly soluble compounds clearly have a minor role in the cloud droplet activation. In fact, according to the conducted simulations the clear majority of semivolatile, slightly soluble compounds have practically no influence on the activation process. Concerning nonvolatile organics, it was found that the cloud droplet concentrations are most sensitive to these species when they constitute a dominant fraction of the Aitken mode. Since atmospheric organic compounds are not well characterized at present, the model contained only a relatively simplified treatment of these species.