The role of ambient temperature for particle number concentrations in a street canyon

Two and a half years (June 2002 to September 2004) of air pollution and meteorological parameters in a street canyon in Stockholm were analyzed. A distinctive dependence of particle number concentration, normalized by NOxNOx, with ambient temperature was found. By comparison with NOxNOx, it is concluded that this variation is not related to variability in the emissions nor due to changes in atmospheric dilution. The observed ratio between total particle number and NOxNOx increases from ca. [150±60150±60] particles per picogram of NOxNOx [pgNOx-1] at +15∘C to [380±50380±50] [pgNOx-1] at -15∘C. The increase in particle number concentration with decreasing temperatures is different for different particle sizes. For particle sizes smaller than 40 nm the variation is more pronounced while for particle sizes larger than 100 nm the change is not statistically significant. This reflects the impact of temperature dependent processes that affect mainly particles smaller than 100 nm in diameter. Modeling results indicate that condensation of volatile compounds onto pre-existing aerosols gives similar changes in the particle size distribution as those observed. Coagulation is shown to have only minor effects on the particle size distribution. Even though, with the present data set is not possible to quantify the impact of an increased nucleation rate with low temperatures, it was shown that nucleation mode particles are largely influenced by relative humidity with high concentrations during high relative humidity periods and therefore it is expected that for cold periods, when the relative humidity is higher, nucleation is enhanced by the water content of the air.