On the NO2 plus soot reaction in the atmosphere

Recent modeling studies have suggested that soot is a key component of tropospheric chemistry in remote regions, acting to reduce HNO3 to NO2 and possibly NO2 to NO. It may be expected then that soot also affects the chemistry of rural and urban areas, where soot concentrations are typically several orders of magnitude higher than in the remote, troposphere. In order to test this assumption, a modeling study was conducted for typical urban and rural areas, with the same HNO3/NO2/soot chemistry proposed in the previous modeling studies of the remote troposphere. Unreasonable results were found (e.g., nearly total suppression of urban ozone, in contradiction to common observations), suggesting that the NO2/soot reaction was considerably overestimated in previous modeling studies. Therefore the NO2/soot chemistry was reconsidered. A new preliminary mechanism is suggested, based on recent laboratory studies of this reaction. Results show that the NO2/soot reaction does not notably affect the Ox-NOx-HOx chemistry of the lower continental troposphere, except maybe during nighttime in urban environments. A potential contribution of the NO2/soot interaction to HONO production is noted.