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J. Phys. Chem. A 2011, 115, 490–499
Heterogeneous Photochemistry of Trace Atmospheric Gases with Components of Mineral Dust Aerosol Haihan Chen,† Juan G. Navea,‡,§ Mark A. Young,*,‡ and Vicki H. Grassian*,†,‡ Departments of Chemical and Biochemical Engineering and Chemistry, The UniVersity of Iowa, Iowa City, Iowa 52242, United States ReceiVed: October 23, 2010; ReVised Manuscript ReceiVed: December 1, 2010
Mineral dust aerosol is known to provide a reactive surface in the troposphere for heterogeneous chemistry to occur. Certain components of mineral dust aerosol, such as semiconductor metal oxides, can act as chromophores that initiate chemical reactions, while adsorbed organic and inorganic species may also be photoactive. However, relatively little is known about the impact of heterogeneous photochemistry of mineral dust aerosol in the atmosphere. In this study, we investigate the heterogeneous photochemistry of trace atmospheric gases including HNO3 and O3 with components of mineral dust aerosol using an environmental aerosol chamber that incorporates a solar simulator. For reaction of HNO3 with aluminum oxide, broadband irradiation initiates photoreactions to form gaseous NO and NO2. A complex dynamic balance between surface adsorbed nitrate and gaseous nitrogen oxide products including NO and NO2 is observed. For heterogeneous photoreactions of O3, iron oxide shows catalytic decompositions toward O3 while aluminum oxide is deactivated by ozone exposure. Furthermore, the role of relative humidity, and, thus, adsorbed water, on heterogeneous photochemistry has been explored. The atmospheric implications of these results are discussed. Introduction Of the estimated annual emission of 3000-5000 Tg of particulate matter in the form of mineral dust, sea salt, sulfate aerosols, organic compounds, and soot,1 components of mineral dust aerosol are particularly reactive with trace atmospheric gases.2 Convincing evidence from field, laboratory, and modeling studies has shown that heterogeneous reactions with mineral dust alter the chemical composition of the troposphere. Atmospheric processing though heterogeneous reactions can alter the physiochemical properties of mineral dust aerosol, itself, as well.2,3 It is also well established that mineral dust aerosol impacts solar radiation and can influence radiative transfer by absorption and scattering of solar radiation (direct effect) and by changing the size distribution, optical properties, nucleation and lifetime of clouds (indirect effects).1,2,4 While mineral dust can contain various photoactive semiconductor metal oxides, such as TiO2, ZnO, and Fe2O3, as well as light absorbing coatings associated with the dust particle surface, little is known about heterogeneous chemical processes activated by light and surface photocatalysis. Although heterogeneous reactions of trace atmospheric gases, such as SO2, HNO3, and O3, with mineral dust have been previously well established, possible heterogeneous photochemistry of mineral dust has not yet been as widely considered. Relevant questions, then, are what is the role of heterogeneous photochemistry in the troposphere and how does the daytime chemistry of mineral dust aerosol differ from nighttime chemistry? Short wavelength (
