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

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 gasphase CO2. In the case of Mauritanian sand, the production of gasphase 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.