Heterogeneous formation of nitrous acid (HONO) on soot aerosol particles

The reaction of nitrogen dioxide (NO2) to nitrous acid (HONO) on suspended soot aerosol particles was investigated using NO2 labeled with N-13 (a radi oactive isotope with a half-life of 10.0 min) at low concentrations of 2-11 5 ppb. HONO is thought to be an important compound in the troposphere since it is accumulating during the night and photolyzed in the morning after su nrise, producing OH, the most important oxidant in the troposphere. On soot , NO2 was rapidly reduced to MONO, presumably by a reactive surface site on the soot particle surface. No HNO3 was formed as a reaction product, indic ating that a disproportionation of NO2 to HONO and HNO3 with surface-adsorb ed water is not the dominant process on soot. The reaction rate is drastica lly reduced after the first few seconds because of consumption of the react ive surface sites giving a maximum of similar to 1 x 10(15) HONO molecules cm(-2) particle surface area. For a reaction time of 20 s the amount of HON O increased with increasing relative humidity up to 30%, showing that H2O i s necessary for the reaction. Above 40% relative humidity the HONO producti on decreased again because of competition of H2O adsorbing on the particle surface. In aging experiments, O-3 oxidized the same particle surface sites as NO2, but simultaneous mixing of O-3, NO2 and the aerosol showed that th e O-3 oxidation is slower than the fast reaction of NO2 to MONO. It is conc luded that the NO2 to HONO reaction on soot rapidly saturates and is not th e main source of HONO in the polluted boundary layer.