American Association for Aerosol Research - Abstract Submission

AAAR 38th Annual Conference
October 5 - October 9, 2020

Virtual Conference

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Tracking pH Changes in Real-time in Single Levitated Organic or Inorganic Acidic Droplets Undergoing Ammonia Uptake

HALLIE BOYER CHELMO, Walker Cage, Ryan Sullivan, University of North Dakota

     Abstract Number: 59
     Working Group: Aerosol Chemistry

Abstract
Aerosol pH is one of the most consequential properties dictating the chemical processing atmospheric particles undergo, yet direct measurements of in situ aerosol droplet pH have remained an important challenge. pH can change considerably during atmospheric aging, requiring a real-time pH micro-probe to directly study this chemistry. We recently developed a novel method of pH measurement of levitated microdroplets using a custom-built aerosol optical tweezers (AOT) for a test system of partially dissociating bisulfate, achieving an accuracy of ±0.03 and a pH range of -0.36 to 0.76. Here, we apply the method to a new probe molecule, 3-methylglutaric acid, and monitor droplet growth and pH change by ammonia and water co-condensation. We demonstrate the new capacity to track droplet pH changes during exposure to ammonia by trapped organic and inorganic acidic droplets. Direct measurements of the rate (kinetics) and extent (thermodynamics) of ammonia uptake will be presented. The pH measurement method uses two independently measured quantities available in the AOT: spontaneous Raman spectra of the acid and base modes and the stimulated Whispering Gallery Modes. Together, these quantities enable measurements of the acid/base concentration ratio and the refractive index, where refractive index is used to determine total anion concentration. The ammonia vapor induces a change of up to 1 measurable pH unit in a bisulfate droplet, a large increase relative to the ~0.1 pH unit change due to changes in relative humidity alone. This work advances our ability to assess aerosol buffering capacity by closely tracking pH in aerosol droplets, such as during important atmospherically relevant processes of gas-particle partitioning, liquid–liquid phase separation, and multiphase chemical reactions.