Direct Measurement of the Acidity of Model Marine Aerosol Particles Using Carbon Quantum Dots
VICTORIA ADEKUNLE, Komal Jaswal, Miriam Freedman, The Pennsylvania State University
Abstract Number: 220
Working Group: Aerosol Chemistry
Abstract
pH is a central component of aqueous and multiphase aerosol chemistry in the atmosphere and has significant implications for heterogeneous chemistry, climate, and human health. Atmospheric aerosol particles are known to be acidic, and the composition of aerosol particles contributes to their pH. Sea spray aerosol particles (SSAs) have the most significant atmospheric emission by mass, and nascent sea spray aerosol particles have been shown to be acidic. Therefore, understanding the influence of the composition of SSA and relating it to aerosol acidity is essential, mainly due to the impact of aerosol acidity on heterogeneous chemistry. In this study, we develop an in situ fluorescence-based technique for directly measuring aerosol pH based on carbon quantum dots (CQDs) by incorporating a pH-sensitive CQD probe and a pH-insensitive CQD internal standard into laboratory-generated aerosol particles and carrying out ratiometric fluorescence microscopy. We measure the pH of aerosol particles containing sodium chloride, medium-chain saturated fatty acids, and various marine-relevant organic compounds with different pKa values as models for SSAs to assess the factors influencing their acidity. Our droplet pH results range from approximately 2.0 to 3.7, showing a general decrease in droplet pH with increasing concentrations and decreasing pKa values of organic acids present in our model SSAs, as confirmed by pH paper measurements. Notably, although the model SSAs primarily consist of saturated fatty acids, we observe a pH decrease of up to 1 unit with just a 6% increase in dicarboxylic acid concentrations. Our findings demonstrate the potential of using CQDs for in situ measurements of particle physicochemical properties and suggest that the pKa and composition of organic acids may significantly influence the acidity of SSAs. Understanding the origins of the acidity of sea spray aerosol particles will aid in developing parameterizations for acidity to aid in modeling their heterogeneous atmospheric chemistry.