AAAR 37th Annual Conference October 14 - October 18, 2019 Oregon Convention Center Portland, Oregon, USA
Abstract View
In Situ pH Measurements of Individual Microdroplets Using Aerosol Optical Tweezers to Study the Interplay between Acidity, Phase Separation, Morphology, and Reactivity
Hallie Boyer, Kyle Gorkowski, RYAN SULLIVAN, Carnegie Mellon University
Abstract Number: 687 Working Group: Aerosol Chemistry
Abstract The pH of aerosol particles controls numerous physicochemical processes and can change considerably during atmospheric aging, requiring a real-time pH micro-probe to directly study this chemistry. Highly accurate real-time pH measurements of microdroplets are obtained using our custom aerosol optical tweezers (AOT) and analysis of the Whispering Gallery Modes (WGMs) contained in the cavity-enhanced Raman spectra. Uncertainties ranging from ±0.027 to 0.057 in pH for picoliter droplets are retrieved through averaging Raman frames acquired at 0.5 Hz over 3.3 minutes. The high accuracy in pH determination is achieved by combining two independent measurements uniquely provided by the AOT approach: the anion concentration ratio from the spontaneous Raman spectra, and the total solute concentration from the refractive index retrieved from WGM analysis of the stimulated cavity-enhanced Raman spectra. pH can be determined over a range of -0.36 to 0.76 using the aqueous sodium bisulfate system. The unique ability of this technique to directly probe pH-dependent chemical and physical changes experienced by individual microparticles such as phase separations, morphology, and heterogeneous reaction kinetics was explored using mixed organic–aqueous particles as a function of relative humidity. The interplay between heterogeneous reactions that alter pH and in turn change particle morphology that in turn feedbacks on the rate of further heterogenous reactions and changes in pH will be presented. Aerosol optical tweezers provides a unique experimental platform to comprehensively explore the physical and chemical evolution of individual particles under long atmospherically-relevant timescales of many hours or days.