Investigations of emissions and heterogeneous formation of HONO in a road traffic tunnel

Simultaneous measurements of nitrous acid (HONO) and nitrogen dioxide (NO2) using a differential optical absorption spectroscopy system, nitrogen oxide (NO)by an in situ chemiluminescence analyser and carbon dioxide (CO2) by a gas chromatographic technique were carried out in the Wuppertal Kiesbergtunnel. At high traffic density HONO concentrations of up to 45 ppbV were observed. However, at low traffic density unexpectedly high MONO concentrations of up to 10 ppbV were measured caused by heterogeneous HONO formation on the tunnel walls. In addition to the tunnel campaigns, emission measurements of MONO, NO2, NO and CO2 from different single vehicles (a truck, a diesel and a gasoline passenger car) were also performed. For the correction of the MONO emission data, the heterogeneous MONO formation on the tunnel walls was quantified by two different approaches (a) in different NO2 emission experiments in the tunnel without traffic and (b) on tunnel wall residue in the laboratory. The MONO concentration corrected for heterogeneous formation on the tunnel walls, in relation to the CO2 concentration can be used to estimate the amount of MONO, which is directly emitted from the vehicle fleet. From the measured data, emission ratios (e.g. MONO/NOx) and emission indices (e.g. mg HONOkg(-1) fuel) were calculated. The calculated emission index of 88 +/- 18 mg HONOkg(-1) fuel allows an estimation of the MONO emission rates from traffic into the atmosphere. Furthermore, the heterogeneous formation of HONO from NO2 on freshly emitted exhaust particles is discussed. (C) 2001 Elsevier Science Ltd. All rights reserved.