Determination of Evaporation Coefficients of Ambient and Laboratory-Generated Semivolatile Organic Aerosols from Phase Equilibration Kinetics in a Thermodenuder

Accurately predicting formation and partitioning of ambient organic aerosols remains a challenge despite decades of sustained effort in this domain. A major source of uncertainty is the poorly characterized volatility of these aerosols. This uncertainty stems in large part from difficulty separating the overlapping effects of aerosol thermodynamic properties and evaporation coefficients in thermodenuder volatility studies. For lack of other information, it is commonly assumed that the evaporation coefficient is unity when interpreting thermodenuder data, leading to potentially large biases in inferred volatility of the sampled aerosol. In this paper, we present a novel thermodenuder-based approach for determining evaporation coefficients of pure compound and complex aerosols without knowledge of their thermodynamic properties. The method involves tracing the normalized dynamic response of an aerosol system to a step change in temperature as it flows through a heated tube. The approach is validated using pure compounds and a mixture of laboratory-generated dicarboxylic acids, and is applied to concentrated ambient aerosols sampled in Beirut, Lebanon. Three valid data sets were obtained frommore than 200 h of ambient air sampling during the month of August 2010, yielding values of 0.34, 0.46, and 0.28 for an assumed binary gas diffusion coefficient of 7.8 x 10(-6) m(2)/s at 60 degrees C.