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 NOx, NO, NO2, CO, SO2, 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 NOx, NO, NO2, CO, and SO2 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 NO2 compared to NOx indicates that the NO2/NOx 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 NO2 road traffic emissions rather than by increasing secondary NO2 formation via reaction of NO with O3.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 7.9×1014 km−1 veh−1, for EC an average emission factor of 21 mg km−1 veh−1 is obtained.