While many studies of heterogeneous chemistry on Cr 2 O 3 surfaces have focused on its catalytic activity, less is known about chemistry on this surface under atmospheric conditions. We report here studies of the thermal and photochemical reactions of NO 2 on Cr 2 O 3 at one atm in air. In order to follow surface species, the interaction of 16-120 ppm NO 2 with a 15 nm Cr 2 O 3 thin film deposited on a germanium crystal was monitored in a flow system using attenuated total reflectance (ATR) coupled to a Fourier transform infrared (FTIR) spectrometer. Gas phase products were monitored in the effluent of an B285 ppm NO 2 -air mixture that had passed over Cr 2 O 3 powder in a flow system. A chemiluminescence NO y analyzer, a photometric O 3 analyzer and a long-path FTIR spectrometer were used to probe the gaseous products. In the absence of added water vapor, NO 2 formed nitrate (NO 3 À ) ions coordinated to Cr 3+ . These surface coordinated NO 3 À were reversibly solvated by water under humid conditions. In both dry and humid cases, nitrate ions decreased during irradiation of the surface at 302 nm, and NO and NO 2 were generated in the gas phase. Under dry conditions, NO was the major gaseous product while NO 2 was the dominant species in the presence of water vapor. Heating of the surface after exposure to NO 2 led to the generation of both NO 2 and NO under dry conditions, but only NO 2 in the presence of water vapor. Elemental chromium incorporated into metal alloys such as stainless steel is readily oxidized in contact with ambient air, forming a chromium-rich metal oxide surface layer. The results of these studies suggest that active photo-and thermal chemistry will occur when boundary layer materials containing chromium(III) or chromium oxide such as stainless steel, roofs, automobile bumpers etc. are exposed to NO 2 under tropospheric conditions.