Expanding Aerosol Acidity Measurements to the Water-Limited Regime and Complex Chemical Systems: A Coupled Condensational Growth and Colorimetric Approach
CARA WATERS, Mia Kauffman, Evan Dalton, Zhenfa Zhang, Avram Gold, Jason Surratt, Andrew P. Ault, University of Michigan
Abstract Number: 337
Working Group: Aerosol Chemistry
Abstract
Aerosol acidity is a critical variable that influences the formation of particulate matter in the atmosphere. Particle pH affects the rate of heterogeneous reactive uptake of carbonyl- and epoxide-containing species, aqueous-phase accretion reaction rates, dissolution of transition metal ions, and sulfate oxidation pathways. However, directly measuring aerosol pH is analytically challenging due to the low liquid water content and high ionic strength of aerosol particles. The colorimetric method for bulk aerosol pH determination is low-cost and easily field-deployable, but pH measurements cannot be obtained for highly acidic particles (pH < 0) or particles with low liquid water content that do not saturate the pH indicator paper. We have developed a new approach that extends aerosol pH measurements into the water-limited regime, using a condensational growth tube to induce water uptake onto submicron aerosol prior to impaction onto pH paper. Aerosol pH prior to condensational growth is calculated based on the volume of water accumulated over the growth tube and the shift in relevant acid-base equilibria. Herein, we use this water uptake colorimetric method to explore a range of different conjugate acid-base pairs and measure aerosol pH for particles of increasing chemical complexity, including chamber-generated SOA. Aerosols are generated from bulk solutions at a moderate relative humidity (60%), simulating the conditions of “wet”-seeded chamber experiments and a common atmospheric RH value. Aerosol pH can be more acidic than the original solution by up to 3 pH units, with this difference being highly dependent on the initial pH and conjugate acid-base pair of the bulk solution. These results highlight the importance of accurate aerosol acidity measurements to understand the relationship between aerosol pH and secondary aerosol mass formation, both in complex laboratory studies and ambient locations where aerosol pH remains highly uncertain.