Heterogeneous Reactions of Epoxides in Acidic Media

Epoxides have recently been identified as important intermediates in the gas-phase oxidation of hydrocarbons, and their hydrolysis products have been observed in ambient aerosols. We investigate the kinetics and mechanism of heterogeneous reactions for isoprene oxide and α-pinene oxide with sulfuric acid, ammonium bisulfate, and ammonium sulfate, using two complementary experimental techniques. Kinetic experiments in a fast flow reactor coupled to an ion drift-chemical ionization mass spectrometer (ID-CIMS) reveal a fast irreversible uptake of both epoxides on sulfuric acid, with a temperature-dependent reaction mechanism. For 90 wt % H2SO4, the uptake coefficients (γ) of isoprene oxide and α-pinene oxide at room temperature are (1.7 ± 0.1) × 10-2 and (4.6 ± 0.3) × 10-2, respectively. Measurements with attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR) show that acetals are formed from isoprene oxide, whereas organosulfates are generated from α-pinene oxide when reactions occurred in concentrated H2SO4(>65 wt %). For ammonium bisulfate and dilute H2SO4 (<25 wt %) solutions, diols are mainly formed from both epoxides. The reaction of epoxides with ammonium sulfate is slow and no products are observed. This study suggests that acid-catalyzed hydrolysis of epoxides results in the formation of a wide range of products, some of which have low volatility and can contribute to SOA growth under ambient conditions prevailing in the urban atmosphere.