Heterogeneous Photochemistry of Oxalic Acid on Mauritanian Sand and Icelandic Volcanic Ash

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• 作者：Sarah A. Styler ; D. J. Donaldson
• 刊名：Environmental Science & Technology (ES&T)
• 出版年：2012
• 出版时间：August 21, 2012
• 年：2012
• 卷：46
• 期：16
• 页码：8756-8763
• 全文大小：377K
• 年卷期：v.46,no.16(August 21, 2012)
• ISSN：1520-5851

文摘

Teragram quantities of crustal and volcanic aerosol are released into the atmosphere on an annual basis. Although these substrates contain photoactive metal oxides, little is known about the role that they may play in catalyzing the heterogeneous phototransformation of semivolatile organic species. In the present study, we have investigated oxalic acid photochemistry at the surface of Fe₂O₃, TiO₂, Mauritanian sand, and Icelandic volcanic ash in the presence and absence of oxygen using a photochemical Knudsen cell reactor. Illumination of all sample types resulted in the production of gas-phase CO₂. In the case of Mauritanian sand, the production of gas-phase CO₂ scaled with the loss of surface oxalic acid. In the absence of oxygen, the production of CO₂ by the sand and ash films scaled with the absorption spectrum of iron oxalate, which suggests that the reaction is at least in part iron-mediated. The presence of oxygen suppressed CO₂ production at the Fe₂O₃ surface, enhanced CO₂ production at the Mauritanian sand surface, and did not have a net effect upon CO₂ production at the Icelandic ash surface. These different oxygen dependencies imply that oxalic acid photochemistry at the authentic surfaces under study was not solely iron-mediated. Experiments at the TiO₂ surface, which showed enhanced CO₂ production from oxalic acid in the presence of oxygen, suggest that Ti-mediated photochemistry played an important role. In summary, these results provide evidence that solid-phase aerosol photochemistry may influence the atmospheric lifetime of oxalic acid in arid regions, where its removal via wet deposition is insignificant.