Department of Atmospheric Sciences, Texas A&M University, College Station, Texas 77843, United States.
J Phys Chem A. 2012 Jun 21;116(24):6078-90. doi: 10.1021/jp2112704. Epub 2012 Mar 2.
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. To evaluate the role of epoxides in the formation of secondary organic aerosols (SOA), the kinetics and mechanism of heterogeneous reactions of two model epoxides, isoprene oxide and α-pinene oxide, with sulfuric acid, ammonium bisulfate, and ammonium sulfate have been investigated using complementary experimental techniques. Kinetic experiments using a fast flow reactor coupled to an ion drift-chemical ionization mass spectrometer (ID-CIMS) show a fast irreversible loss of the epoxides with the uptake coefficients (γ) of (1.7 ± 0.1) × 10(-2) and (4.6 ± 0.3) × 10(-2) for isoprene oxide and α-pinene oxide, respectively, for 90 wt % H(2)SO(4) and at room temperature. Experiments using attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR) reveal that diols are the major products in ammonium bisulfate and dilute H(2)SO(4) (<25 wt %) solutions for both epoxides. In concentrated H(2)SO(4) (>65 wt %), acetals are formed from isoprene oxide, whereas organosulfates are produced from α-pinene oxide. The reaction of the epoxides with ammonium sulfate is slow and no products are observed. The epoxide reactions using bulk samples and Nuclear Magnetic Resonance (NMR) spectroscopy reveal the presence of diols as the major products for isoprene oxide, accompanied by aldehyde formation. For α-pinene oxide, organosulfate formation is observed with a yield increasing with the acidity. Large yields of organosulfates in all NMR experiments with α-pinene oxide are attributed to the kinetic isotope effect (KIE) from the use of deuterated sulfuric acid and water. Our results suggest that acid-catalyzed hydrolysis of epoxides results in the formation of a wide range of products, and some of the products have low volatility and contribute to SOA growth under ambient conditions prevailing in the urban atmosphere.
环氧化物最近被确定为碳氢化合物气相氧化过程中的重要中间体，并且它们的水解产物已在环境气溶胶中被观察到。为了评估环氧化物在二次有机气溶胶（SOA）形成中的作用，使用互补的实验技术研究了两种模型环氧化物（异戊二烯氧化物和α-蒎烯氧化物）与硫酸、硫酸氢铵和硫酸铵的非均相反应动力学和机理。使用快速流动反应器与离子漂移-化学电离质谱仪（ID-CIMS）耦合的动力学实验表明，这两种环氧化物以（1.7±0.1）×10（-2）和（4.6±0.3）×10（-2）的吸收系数（γ）快速不可逆地损失，分别对应于 90wt%硫酸和室温下的异戊二烯氧化物和α-蒎烯氧化物。使用衰减全反射-傅里叶变换红外光谱（ATR-FTIR）的实验表明，二醇是这两种环氧化物在硫酸氢铵和稀硫酸（<25wt%）溶液中的主要产物。在浓硫酸（>65wt%）中，缩醛由异戊二烯氧化物形成，而有机硫酸盐则由α-蒎烯氧化物形成。环氧化物与硫酸铵的反应缓慢，没有观察到产物。使用块状样品和核磁共振（NMR）光谱的环氧化物反应表明，二醇是异戊二烯氧化物的主要产物，同时伴随着醛的形成。对于α-蒎烯氧化物，观察到有机硫酸盐的形成，并且随着酸度的增加，其产率增加。所有使用氘代硫酸和水的α-蒎烯氧化物 NMR 实验中，有机硫酸盐的大产率归因于动力学同位素效应（KIE）。我们的结果表明，酸催化的环氧化物水解导致形成广泛的产物，其中一些产物的挥发性低，在城市大气中普遍存在的环境条件下有助于 SOA 的增长。
