Volatility and evaporation kinetics of atmospheric aerosols
It has been widely recognized that regional air pollution models generally under-predict formation of aerosol particles in the atmosphere by a large margin, thereby hampering efforts to evaluate policy options aimed at improving air quality. One reason for this discrepancy is thought to be the poor state of knowledge regarding semi-volatile species that are important in atmospheric particle formation, a gap stemming from challenges of accurately measuring the vapor pressure and the molecular accommodation/evaporation coefficient of complex, time-varying, low vapor pressure materials.
In partnership with the Khlystov Aerosol Research Lab at Duke University we have developed from first principles thermodenuder-based methods (IV-TDMA and Equilibration Profile) to measure these properties in the lab. Our key methodological contribution to this field has been to experimentally decouple overlapping thermodynamic and kinetic constraints on particle evaporation. By doing so, we have been able to report first determinations of evaporation coefficient for a number of organic compounds and mixtures relevant to atmospheric air pollution, and have revised previously reported values of their vapor pressures. We have also reported first determinations of the effective evaporation coefficient of concentrated atmospheric particles sampled in Beirut in the summer of 2010. The data showed that these are of the order 10-1, indicating that ambient aerosol may be significantly more volatile than previously estimated from field measurements made under the assumption that the coefficient is unity. This difference would, at least in part, account for “missing aerosol” in ambient pollution models.
Using computational and experimental means we have also studied the problem of growth and evaporation of hygroscopic aerosol particles in bounded flows with wall heat and mass transfer, a problem fundamental to predicting the fraction and location inhaled particles deposit in the human respiratory system. This is relevant to understanding health effects of inhaled pollutants and to designing respiratory drug delivery devices.
