Abstract. The present paper reviews existing knowledge with regard to Organic Aerosol (OA) of importance for global climate modelling and defines critical gaps needed to reduce the involved uncertainties. All pieces required for the representation of OA in a global climate model are sketched out with special attention to Secondary Organic Aerosol (SOA): The emission estimates of primary carbonaceous particles and SOA precursor gases are summarized. The up-to-date understanding of the chemical formation and transformation of condensable organic material is outlined. Knowledge on the hygroscopicity of OA and measurements of optical properties of the organic aerosol constituents are summarized. The mechanisms of interactions of OA with clouds and dry and wet removal processes parameterisations in global models are outlined. This information is synthesized to provide a continuous analysis of the flow from the emitted material to the atmosphere up to the point of the climate impact of the produced organic aerosol. The sources of uncertainties at each step of this process are highlighted as areas that require further studies.

Abstract. The present paper reviews existing knowledge with regard to Organic Aerosol (OA) of importance for global climate modelling and defines critical gaps needed to reduce the involved uncertainties. All pieces required for the representation of OA in a global climate model are sketched out with special attention to Secondary Organic Aerosol (SOA): The emission estimates of primary carbonaceous particles and SOA precursor gases are summarized. The up-to-date understanding of the chemical formation and transformation of condensable organic material is outlined. Knowledge on the hygroscopicity of OA and measurements of optical properties of the organic aerosol constituents are summarized. The mechanisms of interactions of OA with clouds and dry and wet removal processes parameterisations in global models are outlined. This information is synthesized to provide a continuous analysis of the flow from the emitted material to the atmosphere up to the point of the climate impact of the produced organic aerosol. The sources of uncertainties at each step of this process are highlighted as areas that require further studies.

The density and surface tension of 0.12 to 0.75 acid mass fraction in aqueous sulfuric acid solutions have been measured in the temperature region from (220 to 300) K. The density has been expressed as a polynomial of the acid mass fraction and temperature, fitting the new measurements together with the tabulated values from International Critical Tables (1928). Older literature data for the surface tension of aqueous sulfuric acid has been corrected for systematic errors and used with the new data to derive a parametrization of the surface tension as a function of acid mass fraction and temperature.

Abstract. Refractive and absorption indices in the UV and visible region of selected aqueous organic acids relevant to tropospheric aerosols are reported. The acids investigated are the aliphatic dicarboxylic acids oxalic, malonic, tartronic, succinic and glutaric acid. In addition we report data for pyruvic, pinonic, benzoic and phthalic acid. To cover a wide range of conditions we have investigated the aqueous organic acids at different concentrations spanning from highly diluted samples to concentrations close to saturation. The density of the investigated samples is reported and a parameterisation of the absorption and refractive index that allows the calculation of the optical constants of mixed aqueous organic acids at different concentrations is presented. The single scattering albedo is calculated for two size distributions using measured and a synthetic set of optical constants. The results show that tropospheric aerosols consisting of only these organic acids and water have a pure scattering effect.

Stratospheric sulfate aerosols have a cooling effect on the Earth's surface. Sulfur aerosols from large volcanic eruptions are often the dominant source, while non‐volcanic background stratospheric sulfate aerosols are supposed to mainly originate from carbonyl sulfide (OCS). Several recent studies indicate, however, that this latter source is too small to account for the observed background stratospheric aerosol concentration. Based on model calculations we suggest that most of the lower stratospheric sulfate aerosol concentration is of anthropogenic origin. We estimate a global mean radiative forcing due to the anthropogenic influence on the stratospheric aerosol layer of −0.05 W m−2. This represents a new climate forcing mechanism and emphasizes anthropogenic sulfur emission as an important cooling mechanism.