Mass Accommodation Coefficients of Water on Organics from Complementary Photoacoustic and Light Scattering Measurements on Laser-Trapped Droplets

Abstract

The mass accommodation coefficient, $\alpha_M$, describes evaporation and condensation kinetics at the liquid-vapor interface. In spite of numerous experimental efforts, reliable values of $\alpha_M$ are still not available for many substances. Here, we present a novel experimental technique, photothermal single-particle spectroscopy (PSPS), that allows for a robust retrieval of mass accommodation coefficients from three simultaneous independent measurements. PSPS combines resonant photoacoustic absorption spectroscopy with modulated Mie scattering measurements on single particles. We study the mass transport of water on organic aerosol droplets that are optically trapped using counter-propagating tweezers. We find the mass accommodation coefficient of water on a pure model organic that is fully miscible with water to be 0.021 at 296K, and to decrease by more than an order of magnitude when the temperature increases to 309K. The experimentally observed temperature dependence of $\alpha_M$ shows an Arrhenius behavior. Furthermore, the water content of the droplets is found to have a profound effect on $\alpha_M$. No concentration dependence of $\alpha_M$ is observed at low water concentrations, while at elevated water concentrations we observe a fivefold increase in $\alpha_M$. The technique presented in this work has the potential to become a reliable method for the retrieval of $\alpha_M$ values at liquid-vapor interfaces, which are essential for accurate global climate and pharmaceutical aerosol inhalation modeling, to mention but a few.