Soot particles were collected from a diesel engine using a procedure that realistically mimics exhaust gas conditions in tailpipes and during dilution at room temperature. After being sampled, the particles were exposed to NO2 concentrations and relative humidity in ranges relevant for the troposphere using 13N as tracer. Gas-phase nitrous acid(HONO) and irreversibly bound (i.e., chemisorbed) species were the main reaction products with initial yields of 80-90% and about 10%, respectively. Neither NO nor HNO3 were detectable. The HONO formation increased with increasing engine load (i.e., with a decreasing air to fuel ratio, lambda). The reaction rates of HONO and chemisorbed NO2 increased with increasing NO2 concentration and did not depend on relative humidity. At the beginning of reaction, the uptake coefficient averaged over 3 min ranged from 5 x 10(-6) to 10(-5) for NO2 concentrations between 2 and 40 ppb. The HONO formation rates decreased with time, indicating consumption of reactive surface species, while the chemisorption rates remained almost constant. The total HONO formation potential of the particles was estimated to about 1.3 x 10(17) molecules/mg of diesel soot or to about 4.7 mg/kg of diesel fuel, indicating that the reaction between NO2 and diesel soot particles does not provide a significant secondary HONO source in the atmosphere. A Langmuir-Hinshelwood type reaction mechanism was proposed that adequately describes the observed results and also allows discussing important general features of reactions on soot.

The atmospheric origin of nitrous acid (HONO) is largely unknown despite its estimated importance as an OH source during daytime due to its rapid photolysis. Recently, primary HONO contained in automobile exhaust as well as secondary HONO formation on soot particles have been invoked as possible HONO sources, but none of them is able to account for the observed HONO to NOx ratios of up to 0.04 in the atmosphere. In this paper, we show that semivolatile and/or water-soluble species contained in diesel exhaust are significantly involved in secondary HONO formation. These species are not associated with soot when the exhaust exits the tailpipe. To quantify these species and to assess the reaction kinetics leading to HONO, experiments were performed in which filtered but hot diesel exhaust gas interacted with a glass surface as well as a water film mimicking dry and wet surfaces to which exhaust might be exposed. A fraction of 0.023 of the NOx emitted was heterogeneously converted to HONO, which is at least three times more than the primary HONO emissions by diesel engines and a fraction of 50 larger than HONO formed on diesel soot particles that do not contain the semivolatile organics.

Abstract. The reaction of nitrogen dioxide (NO2) to nitrous acid (HONO) on suspended soot aerosol particles was investigated using NO 2 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, NO 2 was rapidly reduced to HONO, presumably by a reactive surface site on the soot particle surface. No HNO 3 was formed as a reaction product, indicating that a disproportionation of NO 2 to HONO and HNO 3 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 x 10 ls 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 H20 is necessary for the reaction. Above 40% relative humidity the HONO production decreased again because of competition of H20 adsorbing on the particle surface. In aging experiments, 0 3 oxidized the same particle surface sites as NO2, but simultaneous mixing of 03, NO2, and the aerosol showed that the 03 oxidation is slower than the fast reaction of NO 2 to HONO. It is concluded that the NO 2 to HONO reaction on soot rapidly saturates and is not the main source of HONO in the polluted boundary layer.