The theoretical modelling of the mean activity coefficient of atmospheric liquid aerosols has been afforded in various ways, but so far only inorganic+water systems have been extensively investigated. Indeed, in most treatments, organic compounds are neglected since their fraction is assumed to be sufficiently small. However, realistic situations can involve large fractions of organic compounds, up to about 65% within particulate matter. To deal with water + organic+ inorganic mixtures of interest in environmental chemistry, some models have been proposed in recent years, which are mainly based on UNIFAC. However, many adjustable parameters are required by such models, these parameters being not directly measurable from experiments but only predictable by means of best-fitting numerical techniques from experimental data. In this context, the pseudo-lattice approach [E. Moggia, B. Bianco, J. Phys. Chem. B, 2007, vol. 111] provides a one-parameter equation available for wide ranges of concentrations and temperatures. Moreover, the unique required parameter is a suitable value of solute concentration, which is experimentally known at least for many simple electrolytes. To use the pseudo-lattice approach for atmospheric aerosols, one starts from considering the organic + water component as a two-component mixture solvent, whose dielectric constant can be either measured or calculated starting from the known permittivities of the components. The application of the model to H2SO4 + water + organic and to HNO3+water+organic solutions will be presented and discussed.