Photoenhanced Uptake of NO₂ by Pyrene Solid Films

We report uptake kinetics measurements of the heterogeneous reaction of gas phase NO2 with solid films of pyrene. By using a coated flow tube equipped with several near-ultraviolet (UV) emitting lamps (range 300−420 nm), we examined the effect of actinic radiation on the heterogeneous loss kinetics of nitrogen dioxide. With atmospherically relevant concentrations of NO2, (20−119 ppbv), the uptake ranged from below 10−7 in the dark to 3.5 × 10−6 under near-UV irradiation. Under illuminated conditions, the uptake coefficient decreased markedly with increasing gas-phase concentration, suggestive of a Langmuir−Hinshelwood-type surface reaction mechanism. The NO2 reactivity was not a function of deposited Pyrene mass or of the relative humidity (in the range 10−89%) and depended linearly on the intensity of illumination. Gas-phase product analysis indicated that approximately 50% of the NO2 loss could be accounted for by HONO and NO release. These experimental results are discussed along with a possible nitration mechanism