Variability of particle number concentration and particle size dynamics in an urban street canyon under different meteorological conditions.

During a six-month study period, aerosol number size distributions, mean meteorological conditions and turbulent exchange were measured within an urban street canyon in Essen, Germany. The findings were compared to simultaneous measurements conducted at suburban sites within the study area. The effects of turbulent exchange and different canyon flow situations on aerosol number concentration variability within the street canyon were studied. In comparison to a suburban background site, the busy urban street canyon aerosol number concentration was significantly elevated in the size range below 70 nm throughout the daytime hours. During the morning rush hour, total number concentrations were a factor of 2.2 higher. On average, the total number concentration at the street canyon site roughly doubled the suburban background concentrations (by a factor of 1.9). The intensity of turbulent mixing within the street canyon was sensitive to the prevailing flow regime. The highest turbulent mixing during cross-canyon flow from directions downwind of the measurement spot was accompanied by the lowest number concentration of all flow regimes observed within the canyon. This behaviour was consistent for the different aerosol size classes considered in this study. The effects of meteorology and traffic intensity on total aerosol number concentrations were parameterised using a multiple linear regression analysis and indicated that turbulent mixing within the canyon, traffic intensity and NOx concentrations were the most significant parameters. The model is characterised by an average relative uncertainty of 29%. During situations with a total number concentration>7500 cm(-3), a relative uncertainty of the modelled data of ±25% emerges but displays a larger deviation for low particle concentrations.