| Particle size distribution (size-range 3-900nm) and PM10 was measured
simultaneously at an urban background station in Copenhagen, a near-city
background and a rural location during a period in September-November 2002. The
study investigates the contribution from urban versus regional sources of
particle number and mass concentration.
The total particle number (ToN) and NOx are well correlated at the urban and
near-city level and show a distinct diurnal variation, indicating the common
traffic source. The average ToN at the three stations differs by a factor of
3. The observed concentrations are 2500#cm , 4500#cm
and 7700#cm at rural, near-city and urban level, respectively.
PM10 and total particle volume (ToV) are well correlated between the three
different stations and show similar concentration levels, in average within
30% relative difference, indicating a common source from long-range
transport that dominates the concentrations at all locations.
Measures to reduce the local urban emissions of NOx and ToN are likely to
affect both the street level and urban background concentrations, while for
PM10 and ToV only measurable effects at the street level are probable.
Taking into account the supposed stronger health effects of ultrafine
particles reduction measures should address particle number emissions.
The traffic source contributes strongest in the 10-200nm particle size
range. The maximum of the size distribution shifts from about 20-30nm at
kerbside to 50-60nm at rural level. Particle formation events were observed
in the 3-20nm size range at rural location in the afternoon hours, mainly
under conditions with low concentrations of pre-existing aerosol particles.
The maximum in the size distribution of the "traffic contribution" seems to
be shifted to about 28nm in the urban location compared to 22nm at kerbside.
Assuming NOx as an inert tracer on urban scale allows to estimate that ToN
at urban level is reduced by 15-30% compared to kerbside. Particle
removal processes, e.g. deposition and coagulation, which are most efficient
for smallest particle sizes ( 20nm) and condensational growth are likely
mechanisms for the loss of particle number and the shift in particle size. |