Long-term observation of real-world road traffic emission factors on a motorway in Switzerland

Long-term air quality measurements from a roadside monitoring site of the Swiss national air pollution-monitoring network (NABEL) are analysed for average real-world road traffic emission factors (EFs) by evaluation of concentration differences under upwind and downwind conditions. Time series of relative EFs for NO x , NO, NO 2, CO, SO 2, number of fine particles, and elemental carbon (EC) are estimated for the total vehicle fleet and converted into absolute EFs by scaling them with emission factors from an emission model [HBEFA, 2004. Handbuch für Emissionsfaktoren des Strassenverkehrs (handbook of emission factors for road traffic). Umweltbundesamt Berlin, Bundesamt für Umwelt, Wald und Landschaft Bern, Infras AG, Bern (published on CD-ROM, see also www.hbefa.net] for a specific reference year. For NO x , NO, NO 2, CO, and SO 2 the EFs of the total vehicle fleet decreased during the time period from 1992 to 2004 by 44%, 51%, 5%, 71%, and 55%, respectively. The much smaller negative trend of NO 2 compared to NO x indicates that the NO 2/NO x road traffic emissions ratio increased during this period (from 14% in 1992 to 23% in 2004). It is shown that this trend is caused by increasing primary NO 2 road traffic emissions rather than by increasing secondary NO 2 formation via reaction of NO with O 3. Data for total particle number concentrations and EC are only available since the year 2004. For 2004, the estimated average emission of total particles with diameter larger than 7 nm is 7.9×10 14 km −1 veh −1, for EC an average emission factor of 21 mg km −1 veh −1 is obtained.