A New Spin on Colorimetric Measurement of Aerosol pH Using Aerosol Water Uptake
CARA WATERS, Ali Alotbi, Katherine Kolozsvari, Andrew Ault, University of Michigan
Abstract Number: 545
Working Group: Instrumentation and Methods
Abstract
Aerosol acidity is a critical property that dictates both the amount of aerosol mass and its chemical makeup. Aerosol pH influences the rate of condensed phase reactions, the uptake of gaseous organic species such as aldehydes and epoxides, and the development of phase-separated morphology. Thus, it is crucial to understand aerosol pH in different regions around the globe to accurately model the formation and evolution of aerosol particles in the atmosphere. Unfortunately, there are very few direct measurements of ambient aerosol pH due to the analytically challenging nature of these measurements. Our group has introduced a relatively low-cost method of measuring the bulk pH of size-resolved aerosol with colorimetric pH indicators. However, this method is infeasible for particles with low liquid water content that do not wet pH paper as well as particles whose pH falls outside the range of pH paper. Herein, we introduce a new collection method for the colorimetric pH method: a Spot Sampler (Aerosol Devices Inc.) growth tube increases particulate liquid water content before impaction onto pH paper. Combining the measured pH of the supersaturated aerosol and the water uptake of these particles, we can calculate the pH of aerosol particles before growth. The water uptake of particles in the Spot Sampler was quantified through a series of experiments measuring ammonium sulfate particle size distributions before and after the growth tube. To compare with previous methodology, pH measurements for lab-generated aerosol particles of varying pH were obtained with this new Spot Sampler-based colorimetric pH method and were compared with single-particle spectroscopic pH measurements based on conjugate acid-base equilibria. This work demonstrates that our expanded colorimetric method for pH measurement has significant potential for measuring the pH of bulk aerosol at sites around the world, improving our understanding of aerosol acidity and its spatial heterogeneity.