Mobile measurements of carbon dioxide within the urban canopy layer of Essen, Germany

In the city of Essen (51° 28'N, 7° 0'E, North Rhine-Westphalia, Germany), 24 highly frequented spatial and temporal mobile measurements of carbon dioxide were taken during winter 2002/03 (December-February) and summer 2003 (June-August). Since it is a part of a German conurbation called the "Ruhrgebiet" with about 6 million inhabitants altogether, Essen (599.000 inhabitants, 210 km2) must be considered as a typical conurbation city concerning its structure of carbon dioxide emissions. The route of the taken measurements started in the southern part of the city and ended after 65 kilometres in the north of Essen, considering all different kinds of its land utilization. The aim of this investigation was to determine the allocation between carbon dioxide concentrations, being measured at 1.5 m agl, within the urban canopy layer regarding different conditions: anticyclonic und cyclonic weather situations, on weekdays and weekends, at different times of day and in different seasons (phases of vegetation, contribution to domestic fuel in winter). All of these conditions were related to the twelve different urban and suburban kinds of land utilization. The contribution of motor vehicles to the carbon dioxide concentration was being determined by calculating the relation of carbon dioxide to other harmful atmospheric substances, such as CO, NO, NO2 and O3. All atmospheric substances were captured at the same time and height with a measurement frequency of 1 Hz and a travel speed of 30 km/h (8m/s) in maximum. The results of each mobile measurement were condensed to average values for homogeneous route sections. This facilitated a comparison of the urban CO2 concentration, the CO2-data of a rural station and the results of a station maintained by the Federal Environmental Agency (UBA). Over and above that, wind-dependent data were calculated for the rural measuring station. With assistance of different statistical methods it was possible to show up the average carbon dioxide concentration for each kind of land utilization - referring to the width of the road - dependent of the average daily traffic density. Fundamentally, the results of this investigation have shown that it is neither generally possible to call the urban area a permanent source for CO2 nor to describe it as an urban CO2-dome as it is often mentioned in literature.