Chemical and physical modification of aerosol particles by cloud processing

While it is well established that sulphate is formed in clouds, only recently it has been suggested that also secondary organic mass can be formed in clouds from a variety of anthropogenic and biogenic organic precursors. A multiphase mechanism will be presented that includes detailed gas and aqueous phase reactions and predicts the formation of various organic compounds that are exclusively formed in clouds (e.g. macromolecular compounds, oxalate). The amount of organic mass formed in clouds depends on a variety of parameters such as initial NOx/VOC ratios, and cloud properties (liquid water content). The importance of these parameters for organic aerosol mass yields will be discussed. Even though the resulting composition of the organic fraction can be very complex, it will be shown that, in general, the hygroscopic properties of the water-soluble organic fraction can be greatly simplified in models and robust predictions of cloud properties (droplet number concentrations) in subsequent cloud cycles are possible.