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 oxid e (NO)by an in situ chemiluminescence analyser and carbon dioxide (CO2) by a gas chromatographic technique were carried out in the Wuppertal Kiesbergt unnel. At high traffic density HONO concentrations of up to 45 ppbV were ob served. However, at low traffic density unexpectedly high MONO concentratio ns of up to 10 ppbV were measured caused by heterogeneous HONO formation on the tunnel walls. In addition to the tunnel campaigns, emission measuremen ts of MONO, NO2, NO and CO2 from different single vehicles (a truck, a dies el and a gasoline passenger car) were also performed. For the correction of the MONO emission data, the heterogeneous MONO formation on the tunnel wal ls was quantified by two different approaches (a) in different NO2 emission experiments in the tunnel without traffic and (b) on tunnel wall residue i n the laboratory. The MONO concentration corrected for heterogeneous format ion on the tunnel walls, in relation to the CO2 concentration can be used t o estimate the amount of MONO, which is directly emitted from the vehicle f leet. 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 emis sion rates from traffic into the atmosphere. Furthermore, the heterogeneous formation of HONO from NO2 on freshly emitted exhaust particles is discuss ed. (C) 2001 Elsevier Science Ltd. All rights reserved.