Photonitration of pyrene adsorbed on silica gel with NO2

Abstract

To examine the heterogeneous photonitration of pyrene with NO2 (approximately 0.2 ppm) on a heavy-traffic road, we studied the photonitration of pyrene adsorbed (pyreneads) on silica gel, which was used as SiO2 in particulate matter (PM), with NO2 (10.2, 2.0, and 0.20 ppm) under the atmospheric concentration ratio of pyreneads to NO2 and compared the results with those obtained in the dark. The effects of irradiation, wavelength, and oxygen concentration in a NO2 diluent on the photonitration were examined using a fluidized-bed column irradiated with simulated or real sunlight. Under the UV-light absorption of pyrene, the concentration of pyrene decreased exponentially in accordance with a pseudo-first-order reaction, while in the dark, it decreased sigmoidally in accordance with a H+-autocatalyzed reaction. The distribution and the yields of formed nitration products and their photooxidation products were affected by the light intensity, concentrations of NO2, and oxygen in the NO2 diluent. In the photonitration experiments using a high-pressure mercury lamp, formed 1-nitropyrene and minor dinitropyrenes were decreased by the transformation into their photooxidation products. Under 8-h exposure of pyrene to 10.2-ppm NO2, the yield of 1-nitropyrene was 42% in N2 and 28% in air. The oxygen inhibitory effect can be explained by the energy transfer from 1pyrene* to oxygen. Radical cation intermediate (pyrene•+-NO2−) was proposed for 1-nitropyrene formation. Under 24-h exposure of pyrene to 2.0-ppm NO2, the yields of 1-nitropyrene and the photooxidation products were 21.6% and 8.0%, respectively, in N2 and 4.9% and 3.8%, respectively, in air. Under 24-h exposure of pyrene to 0.20-ppm NO2, which is two times the 1-h NO2 standard in the USA and China, the yields of 1-nitropyrene and the photooxidation products were 2.3% and 3.4%, respectively, in N2 and 2.1% and 0.9%, respectively, in air. The significant decrease in the yields of 1-nitropyrene and the photooxidation products under the concentration of 0.20-ppm NO2 can be explained by their easy photodecomposition with the increase in the photolysis of pyrene. Under the concentration of 0.20-ppm NO2 in air, which is approximately the concentration on heavy-traffic roads, the decay rate of pyrene by the photonitration was increased by own photolysis, although the photonitration was inhibited by oxygen in air.