Heterogeneous reactions of SO2 and NO2 on carbonaceous surfaces

Four different types of carbonaceous material (active carbon, graphitized carbon black, source samples from a power plant and from an urban heating boiler) were exposed to synthetic gas mixtures of SO₂ and SO₂ + NO₂ in sub-ppm ranges at two different relative humidities (60 and 90% r.h.). The interaction of SO₂ and NO₂ with the carbonaceous materials led to sulfate and to three other products: NO, HONO and NO₂^?. Formation of NO and NO₂^? was observed only on active carbon while the other three materials yielded HONO. It was also found that NO₂ enhances the deposition of SO₂ only on the source sample from the power plant at 60% r.h., whereas the material from the urban heating boiler had no effect both at low and high r.h. The results obtained show that no general statements on the effect of NO₂ on the heterogeneous reaction of SO₂ on these carbonaceous materials can be made and also that NO₂ reactions in the presence of SO₂ on carbon-containing surfaces should be considered as a source for the production of HONO in the urban environment.