Ranney April P, Ziemann Paul J
Air Pollution Research Center and Interdepartmental Graduate Program in Environmental Toxicology, University of California , Riverside, California 92521, United States.
Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado , Boulder, Colorado 80309, United States.
J Phys Chem A. 2016 Apr 28;120(16):2561-8. doi: 10.1021/acs.jpca.6b01402. Epub 2016 Apr 14.
Previous studies have shown that 1,4-hydroxycarbonyls, which are often major products of the atmospheric oxidation of hydrocarbons, can undergo acid-catalyzed cyclization and dehydration in aerosol particles to form highly reactive unsaturated dihydrofurans. In this study the kinetics of dehydration of cyclic hemiacetals, the rate-limiting step in this process, was investigated in a series of environmental chamber experiments in which secondary organic aerosol (SOA) containing cyclic hemiacetals was formed from the reaction of n-pentadecane with OH radicals in dry air in the presence of HNO3. A particle beam mass spectrometer was used to monitor the formation and dehydration of cyclic hemiacetals in real time, and SOA and HNO3 were quantified in filter samples by gravimetric analysis and ion chromatography. Measured dehydration rate constants increased linearly with increasing concentration of HNO3 in the gas phase and in SOA, corresponding to catalytic rate constants of 0.27 h(-1) ppmv(-1) and 7.0 h(-1) M(-1), respectively. The measured Henry's law constant for partitioning of HNO3 into SOA was 3.7 × 10(4) M atm(-1), ∼25% of the value for dissolution into water, and the acid dissociation constant was estimated to be <8 × 10(-4), at least a factor of 10(4) less than that for HNO3 in water. The results indicate that HNO3 was only weakly dissociated in the SOA and that dehydration of cyclic hemiacetals was catalyzed by molecular HNO3 rather than by H(+). The Henry's law constant and kinetics relationships measured here can be used to improve mechanisms and models of SOA formation from the oxidation of hydrocarbons in dry air in the presence of NOx, which are conditions commonly used in laboratory studies. The fate of cyclic hemiacetals in the atmosphere, where the effects of higher relative humidity, organic/aqueous phase separation, and acid catalysis by molecular H2SO4 and/or H(+) are likely to be important, is discussed.
先前的研究表明，1,4 - 羟基羰基化合物通常是碳氢化合物大气氧化的主要产物，它们可在气溶胶颗粒中发生酸催化的环化和脱水反应，形成高活性的不饱和二氢呋喃。在本研究中，通过一系列环境舱实验研究了环半缩醛脱水反应的动力学，该反应是此过程中的限速步骤。在这些实验中，在干燥空气中，正十五烷与OH自由基在HNO₃存在下反应生成含环半缩醛的二次有机气溶胶（SOA）。使用粒子束质谱仪实时监测环半缩醛的形成和脱水情况，并通过重量分析和离子色谱法对过滤器样品中的SOA和HNO₃进行定量分析。测得的脱水速率常数随气相和SOA中HNO₃浓度的增加呈线性增加，相应的催化速率常数分别为0.27 h⁻¹ ppmv⁻¹和7.0 h⁻¹ M⁻¹。测得的HNO₃分配到SOA中的亨利定律常数为3.7×10⁴ M atm⁻¹，约为溶解于水中值的25%，酸解离常数估计小于8×10⁻⁴，至少比水中HNO₃的酸解离常数小10⁴倍。结果表明，HNO₃在SOA中仅微弱解离，环半缩醛的脱水反应是由分子HNO₃而非H⁺催化的。此处测得的亨利定律常数和动力学关系可用于改进在NOx存在下干燥空气中碳氢化合物氧化形成SOA的机制和模型，这些条件常用于实验室研究。文中还讨论了环半缩醛在大气中的归宿，在大气中较高的相对湿度、有机/水相分离以及分子H₂SO₄和/或H⁺的酸催化作用可能很重要。
