This article presents the results of mobile measurements made along a 15 km long road profile running from north to south through the city of Essen (j=51°27', l=7°00'; 600,000 inhabitants; 300,000 vehicles), in the Ruhr area, Germany, with the sun at its zenith on clear days (0/8 cloud cover) with low wind velocity between 1998 and 2000. The aim of the measurements was to compare conditions in the city with those in the surrounding area to detect differences within the urban area with reference to four types of land use (industrial area, suburb, inner city, surrounding area) and roads with different traffic densities (between 31,000 and 52,000 vehicles per day). The average speed of the measuring van, equipped with a pyranometer (Thies CM11; 305 nm < l < 2800 nm) was 8 m/s, the average resolution of the measured values was 1 Hz and the duration of the test trips was 50 min. Factors which could have an adverse impact on measurement quality, such as shadows of roadside trees, bridges and buildings, daily and seasonal variations in the elevation of the sun, road gradient and vehicle speed, were eliminated by measurement and evaluation procedures. The data were compared with those recorded by a radiation measurement station located outside the city area. On average, the global radiation flux density in the city area was 5.3 % (s=1,8 %) lower than in the surrounding area; the reduction was 7.1 % in the spring and 4.0 % in the summer. The differences are significant with an error probability of 5 %. As regards the distribution patterns of global radiation flux density, all the test trips showed very good agreement. The variation between average values measured on sections and the values of the station in the surrounding area ranged from -11 % to +0.2 %. The reasons are explained by plausibility considerations (effects of different wind directions, busy motorway intersections, industrial areas in the vicinity, humidity etc.) and the data are compared with results found in other publications. In order to investigate the causes, correlations were also computed between the concentrations of atmospheric constituents (NO, NO2, O3, CO2) and the radiation attenuation measured at the same time. The partial correlation coefficients show that there is no significant relationship between the trace substances measured and the global radiation flux density.