In situ aerosol acidity measurements using a UV–Visible micro-spectrometer and its application to the ambient air

Abstract

An in situ analytical method was demonstrated to measure the proton concentration (H⁺]_C-RUV) of an aerosol particle by using colorimetry integrated with a Reflectance UV-Visible spectrometer (C-RUV). Acidic particles comprising ammonium, sulfate, and water were generated in a flow tube under varying humidity and employed to calibrate the method using the inorganic thermodynamic models (i.e., E-AIM and ISORROPIA). The predictive H⁺]_C-RUV equation derived using strongly acidic compositions was then extended to ammonia-rich aerosols, which were lacking in the database of the thermodynamic models. The predictive H⁺]_C-RUV equation was also expanded to aerosols composed of sodium, ammonium, and sulfate. H⁺]_C-RUV generally agrees with both E-AIM predicted H⁺] and ISORROPIA predicted H⁺] for highly acidic aerosols, or aerosols at high humidity. For ammonia-rich aerosols under low humidity, H⁺]_C-RUV disagrees with that predicted from inorganic thermodynamic models. C-RUV was feasible for ambient aerosols because colorimetry is specific to aerosol acidity. Most aerosols collected at the University of Florida between 2018 and 2019 were acidic. Sodium ions appeared during the spring and summer, as coastal sea breezes traveled inland. The concentrations of ammonium and nitrate were high in the winter due to the phase partitioning of nitric acid and ammonia gases. The fraction of non-electrolytic dialkyl-organosulfate (diOS) to total sulfate is estimated by comparing the actual particle H⁺] measured by C-RUV to the H⁺] predicted using the inorganic composition and the inorganic thermodynamic models. The diOS fraction varied from 0% to 60% and was higher in the summer months when H⁺] is high.

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