Determining the Controls on Buffering Capacity and pH Evolution of Inorganic Aqueous Droplets Using Aerosol Optical Tweezers

GRAHAM THORNHILL, Hallie Boyer Chelmo, Luke Monroe, Yucheng Zhang, Ryan Sullivan, Carnegie Mellon University

     Abstract Number: 574
     Working Group: Aerosol Chemistry

Abstract
Aerosol physicochemical properties such as composition and morphology along with the multiphase chemical reactions that aerosols undergo are all directly affected by the acidity of each particle. The direct determination of aerosol pH, however, is challenging due to wide aerosol size distributions, tiny sample volumes, varying complex compositions, and the need to account for non-ideal thermodynamics in high solute concentration droplets. Given these challenges, the driving factors and sensitivities of changes in atmospheric aerosol pH and buffering capacity are not well understood. Recently our lab developed a method for directly determining aerosol pH via the cavity-enhanced Raman spectrum obtained continuously from single levitated droplets using aerosol optical tweezers. This method allows for continuous pH determination in inorganic sodium bisulfate droplets over a range of -0.36 to 0.76, with an accuracy of ±0.03-0.06 pH units. Here we present an expansion of this pH determination method to more complex inorganic droplets containing sulfates, nitrate, and ammonium in equilibrium with ammonia vapor. The buffering capacity and pH sensitivity to initial aerosol composition and the partial pressures of water vapor and ammonia were determined by varying the gas-phase concentrations in equilibrium with the tweezed droplet. The temperature of the tweezed droplet was also varied to determine its effect on the buffering capacity and pH. These experiments provide new insights into the controlling parameters of the evolution of the pH of inorganic aerosol droplets and its relationship to the surrounding gas-phase composition.