Aerosol Acidity Size-Dependence in a Remote Marine Environment

KAYLEIGH REILLY, Tiantian Zhu, Cara Waters, Clare Murphy, Andreas Schmitt, Ruhi Humphries, Christina S. McCluskey, Kerri Pratt, Andrew P. Ault, University of Michigan

     Abstract Number: 364
     Working Group: Remote and Regional Atmospheric Aerosol

Abstract
Aerosol acidity plays a crucial role in the formation of secondary aerosols and their physicochemical properties as a result of pH-dependent processes. However, aerosol pH has proven difficult to accurately measure or predict in the ambient atmosphere due to their complex composition, partitioning of semi-volatile gases, and the non-conservative nature of the H+ ion. Thermodynamic models and gas-aerosol partitioning studies have provided important acidity insights, but do not account well for potential pH size-dependence due to assumptions about mixing state and thermodynamic equilibrium. Thus, field-deployable acidity measurement techniques are critically important to understand aerosol acidity in the ambient atmosphere. Herein, we collected direct size- and time-resolved colorimetric pH spot measurements to explore the size-dependence of aerosol acidity under different atmospheric conditions. With this method, aerosols are directly impacted onto pH paper and an imaging processing script then calculates the pH by relating the average blue-green pixel intensity of the spot to a calibration curve, using a previously published method. Aerosol pH data was collected over the course of two months at kennaook Cape Grim (KCG) Research Station during the CAPE-k campaign and COAST-k research cruise. KCG is located on the coast of western Tasmania and experiences a unique marine environment that frequently experiences pristine air masses from the Southern Ocean. Over the course of this measuring period, acidic aerosols were measured for submicron particles during all air mass histories, while more neutral pH values were observed in the coarse mode. The transition size to more acidic aerosols varied, shifting to larger sizes when the aerosol composition was dominated by sea spray. This detailed and unique data set of direct atmospheric environmental pH measurements enables a greater understanding of the factors that determine aerosol pH, which will improve our ability to predict secondary aerosol formation and its impacts on climate.