Direct Determination of Aerosol Acidity in Model Continental Aerosol Particles
KOMAL JASWAL, Victoria Adekunle, Miriam Freedman, The Pennsylvania State University
Abstract Number: 278
Working Group: Aerosol Chemistry
Abstract
The acidity of aerosol particles can have a significant impact on atmospheric visibility, climate, heterogeneous chemistry, and human health. Continental aerosol particles have been predicted by thermodynamic models to be highly acidic based on their composition. The small size of aerosol particles makes pH measurement challenging. Here, we present a novel approach using carbon quantum dots (CQDs) as in situ fluorophores to directly measure the acidity of individual droplets and droplet phases in the range of pH 1.5 – pH 3.5. The method used pH-sensitive CQDs and pH-insensitive CQDs (internal standard) which were incorporated into models for continental aerosol particles. Model continental aerosol particles were synthesized in terms of two distinct systems: one consisting solely of organic compounds with varying pKa values, and another comprising mixtures of these organic compounds with different inorganic compounds. Fluorescence microscopy was used to construct a calibration curve using the ratioed fluorescence intensities of droplets of standard solutions to develop an assay of fluorescence emission intensities as a function of pH. The calibration curve was used to determine the pH of model continental aerosol particles in the highly acidic range and compared with bulk pH measurements. The droplet pH values of the aerosol particles showed a good agreement with the bulk pH values, demonstrating the efficacy of this direct approach for evaluating the acidity of aerosol particles. Moreover, the effect of the pKa of the organic compounds on the droplet pH was investigated, and it was observed that with an increase in the pKa, there is a smaller increase in the pH of aerosol particles. Our method demonstrates the capability of CQDs for real-time and direct measurement of the acidity of aerosol particles, which holds a significant potential to parametrize acidity in models and refine predictions of aerosol properties such as heterogeneous chemistry.