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

Abstract

The mass accommodation coefficient, α M, describes evaporation and condensation kinetics at the liquid–vapor interface. In spite of numerous experimental efforts, reliable values of α 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 296 K and to decrease by more than an order of magnitude when the temperature increases to 309 K. The experimentally observed temperature dependence of α M shows an Arrhenius behavior. Furthermore, the water content of the droplets is found to have a profound effect on α M. No concentration dependence of α M is observed at low water concentrations, while at elevated water concentrations, we observe a 5-fold increase in α M. The technique presented in this work has the potential to become a reliable method for the retrieval of α M values at liquid–vapor interfaces, which are essential for accurate global climate and pharmaceutical aerosol inhalation modeling, to mention but a few.

Photoacoustics of single aerosol droplets immobilised by counter-propagating optical tweezers

“…where ρ a is the particle density. From our previous work 33, 34,69 we know that T increases significantly with particle radius. The equations for HF and MF in the traditional model 73,74,77 start to fail when used for elevated particle temperatures and need to be adapted to compensate for temperature-dependent variables.…”

“…Here, only a brief description of the set-up is provided, as the details can be found elsewhere. 33,34,69 We use a Proc. of SPIE Vol.…”

“…Our model is based on the theory proposed by Kulmala et al, 25,[73][74][75][76] which was also adopted by Murphy 77 and successfully used in our former work. 33,34,69 It comprises a locally fixed, spherical particle composed of aqueous TEG with its composition varying with relative humidity, and an IR laser tuned to be absorbed efficiently by TEG, trapped inside an acoustic resonator with constant ambient temperature T ∞ , relative humidity ϕ and pressure p. The IR laser is intensity-modulated in a sinusoidal fashion, thus making the heating of the particle periodic as well. The equation for the energy conservation can be expressed as:…”

“…27,29,35,68 Using the relative humidity dependence of the PA signal, we were also able to retrieve the mass accommodation coefficients of water on organics. 69,70 In this work we further investigate the influence of relative humidity on the acoustic properties of the PA cells and we present a new theoretical model, which allows for simulations of particles with elevated temperatures at different levels of relative humidity. We use this newly revised model to retrieve the mass accommodation coefficients from our previously collected experimental data.…”

Response to reviewer comments

“…The composition of the droplet surface can influence the mass transport and chemical reactions that occur at the surface (e.g. Cosman et al, 2008;Park et al, 2009;Roy et al, 2020). The acidity of organic acids on water surfaces has been measured to be much lower than predicted for the bulk aqueous phase (Enami et al, 2010;Werner et al, 2018).…”