The behaviors of NO₂in N₂and in air under the UV irradiation were studied at room temperature. NO₂in N₂decomposed quantitatively to NO and O₂under a high pressure mercury lamp irradiation. The decomposition rate was proportional to the irradiation energy and the concentration of NO₂. Under a low pressure mercury lamp irradiation NO₂decomposed to NO and O₂, and partially to N₂and O₂. The decomposition rate of NO₂to N₂was proportional to the concentration of NO₂and the smaller value of the 1 st order rate constant was obtained in later stage of the reaction than that in the early stage.
In NO₂-O₂-N₂mixture, NO₂decomposed to NO and O₂under a high pressure mercury lamp irradiation. The change between NO₂and NO was in equilibrium because of the oxidation of NO formed, and the equilibrium concentration of NO₂increased with the initial concentration of O₂While, under a low pressure mercury lamp irradiation, the oxidation of NO₂to higher oxides and the reduction of NO₂to NO occured simultaneously. NO formed was oxidized to higher oxides in later stage. The oxidation rate increased with the initial concentration of O₂and was retarded by NO formed.
No effect by H₂O was observed in the behavior of NO₂ in NO₂-O₂-N₂mixture under a high pressure mercury lamp irradiation. While, under a low pressure mercury lamp irradiation, NO did not form and the oxidation rate increased remarkably in the presence of H₂O, and the rate was proportional to the concentration of NO₂.

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