Photoenhanced NO2 loss on simulated urban grime.

The present study focuses on the heterogeneous reaction between gaseous NO 2 and solid pyrene/KNO 3 films, used as a simplified proxy of urban grime. This reaction is investigated under simulated atmospheric conditions with respect to relative humidity, NO 2 concentration and irradiation in a coated-wall flow-tube reactor. The geometric steady-state uptake coefficients γ geo for pyrene/KNO 3 films exposed to 50 ppbv of NO 2 ranged from 1.12 × 10 -7 in the dark to 2.67 × 10 -6 under near-UV irradiation (300-420 nm) and decreased with increasing NO 2 concentration in the range 30-120 ppbv. NO 2 removal is linearly dependent on light intensity, with release of gas-phase NO and HONO. Analysis of the solid film by ion chromatography and GC-MS showed the formation of nitrite ions and traces of 1-nitropyrene. A light-induced reaction mechanism is proposed. The results discussed herein suggest that PAH-containing urban grime on windows and buildings may be a key player in urban air pollution.