Direct measurement of CO2 and particle emissions from an urban area

Abstract

From July 9 th through September 24th, 2009, turbulent particle number fluxes and CO 2 fluxes were measured above the city area of Munster, north-west Germany. The goal was to characterize the respective vertical fluxes in the urban boundary area. The measurements were conducted at a height of 65 m a.g.l. on a military radio tower at 10 Hz temporal resolution. Fluxes were calculated applying the eddy covariance method. To determine the impact of traffic emissions on particle number fluxes and CO 2 fluxes, hourly traffic activities for 45° sectors, related to the tower, were calculated. Averaged diurnal and sectoral fluxes are consistently directed upward, implying that the urban area of Munster acts continuously as particle (number) and CO 2 source. Traffic activities vary in the course of the day and within the 45° sectors. The latter is attributable to differences in land use between the sectors. In the course of the day, two peaks are discernible, during the morning and the evening rush hours, respectively. Averaged diurnal particle (number) fluxes are correlated significantly to traffic activity. Accordingly, traffic related emissions are the main sources for urban particle (number) fluxes. Averaged sectoral CO 2 fluxes also correlate fairly well with sectoral traffic activities. In addition, daytime photosynthesis is a controlling variable for the CO 2 flux, leading to lower upward fluxes in daytime. The contribution of the photosynthetic activity of the vegetation in the urban area to the CO 2 flux is quantified. Further, the contribution of traffic related emissions to the CO 2 flux is computed by applying emission factors for carbon dioxide to the traffic activity. They contribute in daytime about 40 to 50 % to the CO 2 flux, whereby, nightly contributions are minimal.