Heterogeneous conversion of NO2 on carbonaceous surfaces

In the present study the heterogeneous conversion of NO2 on commercial soot , freshly prepared flame soot and commercial soot treated with sulfuric aci d has been investigated. The experimental results indicate a complex reacti on mechanism in which the soot has to be treated as a reactant leading to d eactivation of active sites on the surface with increasing NO2 consumption. Mean uptake coefficients y for the first few minutes of the reaction of si milar to 10(-6) have been determined for a consumption of similar to 10(13) NO2 cm(-2). The y-values decrease to <10(-8) for a consumption of similar to 10(15) NO2 cm(-2), indicating that the influence of soot on the partitio ning of NOx in the atmosphere is negligible. Nitrous acid (HONO) and nitric oxide (NO) were found to be the major gas phase products. HONO yields up t o 80% are observed, clearly showing that the nature of the reaction is non- catalytic. The humidity dependence of the reaction system was studied and i ndicates that water is necessary for HONO formation. In the reaction of pur e HONO on soot surfaces, high yields of NO were observed, indicating a cons ecutive process: NO2 --> HONO --> NO. When soot was coated with sulfuric ac id the yield of HONO decreased while that of NO increased. This behaviour i s caused by an increasing reactivity of the intermediate HONO on the modifi ed surface. The experimental results indicate that without any recycling me chanism HONO formation on soot surfaces is not of major importance in the a tmosphere.