Kinetic study of esterification of sulfuric acid with alcohols in aerosol bulk phase

Abstract. In this study, we hypothesize that the formation of organosulfates through the reactions between sulfuric acid and alcohols in the aerosol bulk phase is more efficient than that in solution chemistry. To prove this hypothesis, the kinetics of the organosulfate formation was investigated for both aliphatic alcohol with single OH group (e.g., 1-heptanol) and the multialcohols ranging from semivolatiles (e.g., hydrated-glyoxal and glycerol) to nonvolatiles (e.g., sucrose) using analytical techniques directly monitoring aerosol bulk phase. Both the forward (k₁) and the backward (k₋₁) reaction rate constants of organosulfate formation via the particle phase esterification of 1-heptanol with sulfuric acid were estimated using a Fourier Transform Infrared (FTIR) spectrometer equipped with a flow chamber under varying humidities. Both k₁ and k₋₁ are in orders of 10⁻³ L mol⁻¹ min⁻¹, which are three orders of magnitude higher than the reported values obtained in solution chemistry. The formation of organosulfate in the H₂SO₄ aerosol internally mixed with multialcohols was studied by measuring the proton concentration of the aerosol collected on the filter using a newly developed Colorimetry integrated with a Reflectance UV-Visible spectrometer (C-RUV). The formation of organosulfate significantly decreases aerosol acidity due to the transformation of H₂SO₄ into dialkylsulfates. The forward reaction rate constants for the dialkylsulfate formation in the multialcohol-H₂SO₄ aerosols were also three orders of magnitude greater than the reported values in solution chemistry. The water content (M_H2O) in the multialcohol-H₂SO₄ particle was monitored using the FTIR spectrometer. A large reduction of M_H2O accords with the high yield of organosulfate in aerosol. Based on this study, we conclude that organosulfate formation in atmospheric aerosol, where both alcohols and sulfuric acid are found together, is significant.