Experimental Study of the Formation of Organosulfates from α-Pinene Oxidation. Part I: Product Identification, Formation Mechanisms and Effect of Relative Humidity

In the present study, quasi-static reactor and atmospheric simulation chamber experiments were performed to investigate the formation of α-pinene-derived organosulfates. Organosulfates (R-OSO₃H) were examined for the reactions between acidified ammonium sulfate particles exposed to an individual gaseous volatile organic compound, such as α-pinene and oxidized products (α-pinene oxide, isopinocampheol, pinanediol and myrtenal). Molecular structures were elucidated by liquid chromatography interfaced to high-resolution quadrupole time-of-flight mass spectrometry equipped with electrospray ionization (LC/ESI-HR-QTOFMS). New organosulfate products were detected and identified for the first time in the present study. Reaction with α-pinene oxide was found to be a favored pathway for organosulfate formation (C₁₀H₁₈O₅S) and to yield organosulfate dimers (C₂₀H₃₄O₆S and C₂₀H₃₄O₉S₂) and trimers (C₃₀H₅₀O₁₀S₂) under dry conditions (RH < 1%) and high particle acidity and precursor concentrations (1 ppm). The role of relative humidity on organosulfate formation yields and product distribution was specifically examined. Organosulfate concentrations were found to decrease with increasing relative humidity. Mechanistic pathways for organosulfate formation from the reactions between α-pinene, α-pinene oxide, isopinocampheol, or pinanediol with acidified ammonium sulfate particles are proposed.