Department of Land, Air and Water Resources, ‡Agricultural and Environmental Chemistry Graduate Group, and §Department of Environmental Toxicology, University of California-Davis , 1 Shields Avenue, Davis, California 95616, United States.
Environ Sci Technol. 2014 Jan 21;48(2):1049-57. doi: 10.1021/es4045715. Epub 2014 Jan 9.
Condensed-phase chemistry plays a significant role in the formation and evolution of atmospheric organic aerosols. Past studies of the aqueous photoformation of secondary organic aerosol (SOA) have largely focused on hydroxyl radical oxidation, but here we show that triplet excited states of organic compounds ((3)C*) can also be important aqueous oxidants. We studied the aqueous photoreactions of three phenols (phenol, guaiacol, and syringol) with the aromatic carbonyl 3,4-dimethoxybenzaldehyde (DMB); all of these species are emitted by biomass burning. Under simulated sunlight, DMB forms a triplet excited state that rapidly oxidizes phenols to form low-volatility SOA. Rate constants for these reactions are fast and increase with decreasing pH and increasing methoxy substitution of the phenols. Mass yields of aqueous SOA are near 100% for all three phenols. For typical ambient conditions in areas with biomass combustion, the aqueous oxidation of phenols by (3)C* is faster than by hydroxyl radical, although rates depend strongly on pH, oxidant concentrations, and the identity of the phenol. Our results suggest that (3)C* can be the dominant aqueous oxidant of phenols in areas impacted by biomass combustion and that this is a significant pathway for forming SOA.
凝聚相化学在大气有机气溶胶的形成和演化中起着重要作用。过去对二次有机气溶胶（SOA）的水相光形成的研究主要集中在羟基自由基氧化上，但在这里我们表明，有机化合物的三重态激发态（(3)C*）也可以是重要的水相氧化剂。我们研究了三种苯酚（苯酚、愈创木酚和丁香酚）与芳香族羰基 3,4-二甲氧基苯甲醛（DMB）在水中的光反应；所有这些物质都是由生物质燃烧排放的。在模拟阳光下，DMB 形成三重态激发态，迅速将苯酚氧化形成低挥发性 SOA。这些反应的速率常数很快，并且随着 pH 值的降低和苯酚的甲氧基取代程度的增加而增加。对于所有三种苯酚，水相 SOA 的质量产率接近 100%。对于生物质燃烧地区典型的环境条件，(3)C对苯酚的水相氧化比羟基自由基快，尽管速率强烈依赖于 pH 值、氧化剂浓度和苯酚的种类。我们的结果表明，在受生物质燃烧影响的地区，(3)C可能是苯酚的主要水相氧化剂，这是形成 SOA 的重要途径。
