Particle and trace gas emission factors under urban driving conditions in Copenhagen based on street and roof-level observations

Simultaneous measurements of particle size distribution (size/range 10–700 nm) inside an urban street canyon and a nearby urban background location in Copenhagen in May–November 2001 were used to separate the traffic source contribution in the street canyon from the background levels. The background concentrations are highly variable due to changing contributions from long-range transport and local sources showing a diurnal pattern with a shift to smaller particle sizes during midday hours. The average ratio background/street concentration is 0.26 for NOx and 0.35, 0.42, 0.60, 0.64, respectively, for CO, total particle number (ToN), surface and volume. The particle size distribution of the traffic source shows during daytime and evening hours (6–24) a maximum at particle sizes of 20–30 nm independent of the changing heavy-duty vehicle share during the same time interval. The particle number concentration highly correlated (R>0.83) with NOx through a wide range of particle sizes. The method of inverse modelling was applied to estimate average fleet emission factors typical of urban conditions in Denmark. Emission factors per average vehicle were estimated as (2.8±0.5)×1014 particles/km, (1.3±0.2) g NOx/(veh km) and (11±2) g CO/(veh km). We observe two types of ‘nanoparticle events’ (a) in background, probably due to photochemistry and (b) in the night hours when traffic is dominated by diesel taxis. During night hours (0–5), the maximum in the emitted particle size distribution is shifted to smaller sizes of about 15–18 nm. This shift to smaller particle sizes is related to an increase in the average NOx and ToN emission per vehicle by a factor of 2–3 and a reduced CO emission also by a factor of 2–3.