利用真空FTIR比较研究油酸薄膜和油酸包覆粉煤灰分别与臭氧的非均相化学反应动力学

摘要

气溶胶老化是目前大气颗粒物物理化学领域的研究热点之一。通过真空型傅里叶红外光谱仪原位观测并对比了油酸(OA)薄膜和油酸包覆粉煤灰两种样品分别与臭氧发生的化学反应动力学，并评估了其中粉煤灰在非均相化学反应中所起到的作用。除了指纹区特征峰的不同，在反应前纯油酸薄膜和纯油酸包覆粉煤灰样品的主要红外特征吸收峰是相似的。当两种样品分别处于20 ppm 臭氧浓度、室温和干燥的反应环境中，—CH 的特征吸收峰(3050 cm-1)强度降低而—OH 伸缩振动特征峰(3430 cm-1)强度增加。此外，随着臭氧氧化反应的进行，处于1710cm-1的羧酸 C—O 伸缩振动特征峰强度下降，反之，产物酯类物质的伸缩振动特征峰在1740 cm-1出现并增强。通过红外光谱的变化趋势能够得到两组样品的 OA 成分在反应中逐步消耗并转化为含有羟基和酯类官能团的产物。通过位于1740 cm-1处的 C—O 特征峰的变化能够得到准一级反应速率常数Kapp和摄取系数γ。粉煤灰样品的准一级反应速率常数大体是油酸薄膜反应体系的两倍。对于比表面积，粉煤灰样品要大于油酸薄膜，而两种反应体系的臭氧浓度一致，油酸薄膜和油酸包覆粉煤灰的γ值分别为(2.70±0.11)×10-4和(3.70±0.13)×10-4。较大的催化表面积和较快的催化效率是导致粉煤灰样品较油酸薄膜反应快的原因。由此证明了当粉煤灰与不饱和有机酸结合并处在臭氧氧化环境中时会常易促进二次有机气溶胶的形成。%Aerosol aging is the one of the research hotspots in the field of physical chemistry of atmospheric particulates at present.This study evaluates the impact of flyash in heterogenous reaction through the compari-sion of ozone oxidation reaction kinetics between the oleic acid (OA)thin film and oleic acid coated flyash which are observed by in situ vacuum FTIR for the first time.Except for the fingerprint region,the main bands of infrared spectrum of the fresh OA thin film and the fresh OA coated flyash are similar.When the two samples are exposed in the 20 ppm ozone concentration,room temperature and dry (RH~0%)conditions re-spectively,the —CH absorption peak (3 050 cm-1 )decreases while The —OH stretch band (3 430 cm-1 ) increases.Moreover,The carboxylic C—O stretch band at 1 710 cm-1 decreases whereas a new ester group stretch band at 1 740 cm-1 appears and increases with increasing ozone exposure time.The OA component of the two samples can be gradually consumed and converted to some products containing hydroxyl and ester groups during the reactions through the changing trends of the infrared spectrum.The pseudo-first-order rate constant Kapp and the overall uptake coefficientγ,are obtained through changes in the absorbance of C—O stretching bands at 1 740 cm-1 .The ozonolysis reaction rate constant of OA coated flyash is nearly double to the OA thin film.Since the surface area-to-volume ratio of OA coated flyash is larger than the OA thin film and the ozone concentrations are the same for the both reactions,theγvalues for ozone uptake on OA thin film and OA coated flyash are (2.70±0.11)×10-4 and (3.70±0.13)×10-4.Rapid reaction rate of the flyash sample is due to the larger catalytic surface area and more valid catalytic effect compared to the OA thin film. This demonstrates that the flyash often easily leads to the secondary organic aerosols (SOAs)when the flyash unites with unsaturated organic acids and exposes to the ozone oxidation environment.