Photoenhanced Heterogeneous Uptake of NO2 and HONO Formation on Authentic Winter Time Urban Grime

Heterogeneous reactions of NO2 on surfaces seem to be an important sink for NO2 and a source for HONO in the atmosphere. Although heterogeneous reactions of NO2 have been extensively investigated in laboratory studies, kinetic data about this reaction on real ambient surfaces are limited. Therefore, in the present study, we investigated the heterogeneous reaction of NO2 on urban grime, representative of surface film in urban areas, under various experimental conditions using a Teflon box system. The role that light played in these reactions was also explored. It was found that the uptake coefficient (.) of NO2 and HONO yield both were significantly enhanced under UV conditions, which was consistent with the literature results. The. of NO2 on urban grime increased linearly with the increasing light intensity, whereas the formation of HONO seemed to be independent of the light intensity. In addition to the intensity of light, heterogeneous reactions of NO2 on urban grime were also significantly affected by the initial concentration of NO2 and relative humidity. On the basis of the data measured, the flux of HONO production from photoenhanced NO2 uptake on urban grime was calculated to be (3.8-9.9) x 10(9) molecules cm(-2) s(-1). This result suggests an important role of urban grime in the daytime NO2-to-HONO conversion and could be helpful to explain unknown daytime HONO sources in polluted urban areas.