Modeling retained water content in measured aerosol mass

The contribution of liquid water to measured aerosol mass was estimated using a numerical approach applied to European measurement data. Aerosol mass is determined after conditioning of the filter under dry conditions for a predefined period according to a standard procedure. After this procedure, water in aerosols is assumed to be at very low concentrations. However, we find indications that the equilibrium of aerosol is not fully established and water is not completely evaporated after the drying operation. To estimate the aerosol humidity, a mass transfer coefficient was derived for evaporation from the filters. Data from a few available measurements of aerosol humidity were applied with a numerical kinetic approach, using thermodynamic equilibrium as boundary conditions. The calculated coefficient ranged from 10−5 to 10−4 (s−1), with an average value of 3.2×10−5 s−1. Using this average value the expected water content of aerosol was assessed for sites in Austria. Most of the samples are expected to still contain 1–5% water. In winter, the air is humid and the concentration of inorganic hygroscopic compounds is also high, resulting in high uptake of water from the air. Under such conditions, approximately 70% of unidentified non-carbonaceous components, or about 10% of PM2.5 mass, were considered to be water.