Dual Fluorophore System for Direct Measurement of Acidity of Aerosol Particles

Atmospheric aerosol particles exhibit diverse physical and chemical properties, depending on their composition and source, which are challenging to measure directly due to their chemical complexity. Aerosol acidity is a key chemical property that has significant implications for climate, heterogeneous chemistry, and human health. Here, we present a novel approach to measure aerosol acidity using carbon dots (CDs) as in situ fluorophores. The method uses a dual fluorophore system consisting of pH-sensitive CDs and pH-insensitive CDs incorporated into model aerosol particles, with the pH-insensitive CDs serving as an internal standard. Fluorescence microscopy was used to construct a calibration curve by plotting the ratio of fluorescence intensities of droplets of fluorophore-containing standard solutions against pH. This calibration curve was used to determine the pH of model aerosol particles in the range of pH 1.5 to pH 3.5, demonstrating the efficacy of this approach for direct measurement of acidity. This approach holds significant potential to parameterize acidity in models, refine our understanding and predictions of aerosol properties such as heterogeneous chemistry, and aid in assessing the impact of aerosol particles on climate and health.