Photooxidation of Methacrolein in Fe(III)-Oxalate Aqueous System and Its Atmospheric Implication

Abstract

Iron and oxalic acids are widely distributed in the atmosphere and easily form ferric oxalate complex (Fe(III)-Ox). The tropospheric aqueous-phase could provide a medium to enable the photo-Fenton reaction with Fe(III)-Ox under solar irradiation. Although the photolysis mechanisms of Fe(III)-Ox have been investigated extensively, information about the oxidation of volatile organic compounds (VOC), specifically the potential for Secondary Organic Aerosol (SOA) formation in the Fe(III)-Ox system, is lacking. In this study, a ubiquitous VOC methacrolein (MACR) is chosen as a model VOC, and the oxidation of MACR with Fe(III)-Ox is investigated under typical atmospheric water conditions. The effects of oxalate concentration, Fe(III) concentration, MACR concentration, and pH on the oxidation of MACR are studied in detail. Results show that the oxidation rate of MACR greatly accelerates in the presence of oxalate when compared with only Fe(III). The oxidation rate of MACR also accelerates with increasing concentration of oxalate. The effect of Fe(III) is found to be more complicated. The oxidation rate of MACR first increases and then decreases with increasing Fe(III) concentration. The oxidation rate of MACR increases monotonically with decreasing pH in the common atmospheric water pH range or with decreasing MACR concentration. The production of ferrous and hydrogen peroxide, pH, and aqueous absorbance are monitored throughout the reaction process. The quenching experiments verify that ^·OH and $O_2^{\bar .}$ are both responsible for the oxidation of MACR. MACR is found to rapidly oxidize into small organic acids with higher boiling points and oligomers with higher molecular weight, which contributes to the yield of SOA. These results suggest that Fe(III)-Ox plays an important role in atmospheric oxidation.

摘 要

大气中广泛存在着草酸和铁元素,两者极易络合形成草酸铁络合物Fe(III)-Ox。光照下的Fe(III)-Ox可以在大气对流层水相中发生光芬顿反应。尽管Fe(III)-Ox的光解机制已被广泛研究,但Fe(III)-Ox体系内挥发性有机物(VOC)的氧化机制及其生成二次气溶胶(SOA)的潜力尚不明确。本研究选用了普遍存在的VOC甲基丙烯醛(MACR)为模型VOC,在典型的大气水相条件下研究了MACR在光照Fe(III)-Ox体系中氧化过程,包括了草酸根浓度、Fe(III)浓度、MACR浓度和pH对MACR氧化速率的影响。结果显示在有草酸根存在时,MACR的氧化速率要远远快于只有Fe(III)的光照体系,且MACR的氧化速率随草酸根浓度的增加而增加。在本研究范围内,MACR的氧化速率随溶液pH的降低和MACR初始浓度的减少而单调增加。而Fe(III)的影响机制更为复杂,MACR的氧化速率随着Fe(III)浓度的增加先增加后降低。为了研究反应机制,还监测了Fe(II)浓度、H₂O₂浓度、溶液pH、溶液吸光度的变化和不同自由基对MACR氧化的贡献。通过对产物的质谱分析发现MACR在光照Fe(III)-Ox体系中被快速氧化为小分子有机酸和高分子量的聚合物,进而贡献了大气二次气溶胶的产量。