Path-integrated measurements of carbon dioxide in the urban canopy layer

Continuous CO2 concentration measurements have been recorded within the city center of Essen, Germany, using a path-integrated measuring system above a typical urban area over the course of nine months (February–October 2010). Mean monthly urban CO2 concentrations were 396 and 446 ppm in summer and winter, respectively, which were 8.5 % in average higher than at a nearby suburban measuring site. Urban-suburban differences mainly occur due to increased CO2 emissions from traffic and industry within the urban area, as well as domestic heating in winter. the analyzed meteorological variables, low wind velocities increased CO2 concentrations as well as high atmospheric stability within the urban boundary layer, respectively. The influence of wind direction reflects the heterogeneous distribution of local CO2 sources at the recording sites, particularly industrial point sources. Other point sources in the vicinity of the urban site strongly influence the additional point measurements but show no significant effect on the measured CO2 concentrations by the path-integrated measuring system. Within an eight-day case study, a significant positive correlation between CO2 concentration and traffic count (R = 0.26; p < 0.05) was found on weekdays using partial correlation analysis after excluding the influence of meteorological variables. This correlation diminishes when combined with the meteorological components, and mixing layer height shows the strongest negative correlation with measured urban CO2 (R = −0.59). The path-integrated system provides CO2 concentrations on a greater temporal and spatial scale than common point measurements, which can be influenced by strong adjacent local CO2 sources.