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Particle size distribution (size-range 3-900nm) and PM10 was measuredsimultaneously at an urban background station in Copenhagen, a near-citybackground and a rural location during a period in September-November 2002. Thestudy investigates the contribution from urban versus regional sources ofparticle number and mass concentration.The total particle number (ToN) and NOx are well correlated at the urban andnear-city level and show a distinct diurnal variation, indicating the commontraffic source. The average ToN at the three stations differs by a factor of3. The observed concentrations are 2500#cm<IMG WIDTH="20" HEIGHT="18" ALIGN="BOTTOM" BORDER="0" src="acp-4-281-img1.gif" ALT="$^{-3}$">, 4500#cm<IMG WIDTH="20" HEIGHT="18" ALIGN="BOTTOM" BORDER="0" src="acp-4-281-img1.gif" ALT="$^{-3}$">and 7700#cm<IMG WIDTH="20" HEIGHT="18" ALIGN="BOTTOM" BORDER="0" src="acp-4-281-img1.gif" ALT="$^{-3}$"> at rural, near-city and urban level, respectively.PM10 and total particle volume (ToV) are well correlated between the threedifferent stations and show similar concentration levels, in average within30% relative difference, indicating a common source from long-rangetransport that dominates the concentrations at all locations.Measures to reduce the local urban emissions of NOx and ToN are likely toaffect both the street level and urban background concentrations, while forPM10 and ToV only measurable effects at the street level are probable.Taking into account the supposed stronger health effects of ultrafineparticles reduction measures should address particle number emissions.The traffic source contributes strongest in the 10-200nm particle sizerange. The maximum of the size distribution shifts from about 20-30nm atkerbside to 50-60nm at rural level. Particle formation events were observedin the 3-20nm size range at rural location in the afternoon hours, mainlyunder conditions with low concentrations of pre-existing aerosol particles.The maximum in the size distribution of the "traffic contribution" seems tobe 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 ToNat urban level is reduced by 15-30% compared to kerbside. Particleremoval processes, e.g. deposition and coagulation, which are most efficientfor smallest particle sizes (<IMG WIDTH="15" HEIGHT="29" ALIGN="MIDDLE" BORDER="0" src="acp-4-281-img2.gif" ALT="$<$">20nm) and condensational growth are likelymechanisms for the loss of particle number and the shift in particle size

Particle size distribution and particle mass measurements at urban,near-city and rural level in the Copenhagen area and Southern Sweden