The reaction of nitrogen dioxide (NO2) to nitrous acid (HONO) on suspended soot aerosol particles was investigated using NO2 labeled with 13N (a radioactive isotope with a half-life of 10.0 min) at low concentrations of 2--115 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 sunrise, producing OH, the most important oxidant in the troposphere. On soot, NO2 was rapidly reduced to HONO, presumably by a reactive surface site on the soot particle surface. No HNO3 was formed as a reaction product, indicating that a disproportionation of NO2 to HONO and HNO3 with surface-adsorbed water is not the dominant process on soot. The reaction rate is drastically reduced after the first few seconds because of consumption of the reactive surface sites giving a maximum of ~1¿1015 HONO molecules cm-2 particle surface area. For a reaction time of 20 s the amount of HONO increased with increasing relative humidity up to 30%, showing that H2O is necessary for the reaction. Above 40% relative humidity the HONO production decreased again because of competition of H2O adsorbing on the particle surface. In aging experiments, O3 oxidized the same particle surface sites as NO2, but simultaneous mixing of O3, NO2, and the aerosol showed that the O3 oxidation is slower than the fast reaction of NO2 to HONO. It is concluded that the NO2 to HONO reaction on soot rapidly saturates and is not the main source of HONO in the polluted boundary layer. ¿ 1999 American Geophysical Union