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page last modified:
October 2005

Case studies of anthropogenic NO₂ pollution

Introduction A number of case studies are presented in the following demonstrating the usage of the information provided on this website for the interpretation of high NO2 levels observed by the satellite sensors. The examples demonstrate the use of the auxiliary information from back trajectories, meteorological information such as cloud cover, lightning events, and ground based measurements. High NO2 tropospheric columns can be due either to local sources or due to transport from remote sources or due to lightning. In addition to the cases presented here, further example can be found on the web page of the POLPO-ESA project. Lightning (31st July 2003) Figure 1 show the NO2 tropospheric column measured by SCIAMACHY on the 31st July 2003. It is clear a very high spot over the Ligurian Sea with values up to 30x10¹⁵molec/cm²).

Figure 1: NO2 tropospheric colum measured by SCIAMACHY on the 31st July 2003.	Figure 2: Lightning events on the 31st July 2003.

Figure 3: NO2 tropospheric colum measured by GASCOD on the 31st July 2003.
The geographic location of the hot spot let us conclude that the NO2 is not due to local production due to human activity. The lightning map (Figure 2) show instead a strong activity for the same day in the hot spot area. Also the ground based column measurements with DOAS instruments (Figure 3) reported high NO2 values in Bologna mainly during the late afternoon when the lightning activity reached also the area of Bologna(about 80x10¹⁵molec/cm² 6-7 PM).
Off shore transport (13 October 2003) On the 13th October 2003 SCIAMACHY observations (Figure 4) reported high tropospheric column of NO2 in the Po valley area and off shore along the Adriatic sea from Trieste down to Ravenna. The wind direction at 850 mb (see Figure 5) show a transport of airmasses from the Venice-Mestre area eastward that could justify the NO2 presence over the Adriatic Sea. The BOLAM analysis (Figure 6) indicates the presence of clouds all over the observed area giving thus two possible interpretation to the observed NO2 columns: 1) The NO2 is layerd above the clouds. 2) The NO2 is still present between the ground and the cloud layer but the real column amount is much more higher since clouds hide to satellite measurements the great part of the NO2 amount. Actually the real situation could have been a mix of the two hypothesis.

Figure 4: NO2 tropospheric colum measured by SCIAMACHY on the 13th October 2003.	Figure 5: PT and wind @ 850 mb at 10 LT on the 13th October 2003.

Figure 6: Cloud cover at 10 LT on the 13th October 2003.	Figure 7: Lightning events on the 13th October 2003.
Identification of pollution sources (23rd June 2003) The 23rd of June 2003 was a cloud free day (Figure 8) at the SCIAMACHY overpass in the our region of interest and it was possible to identifiy the emissions of NO2 from several cities or industrialized areas (see Figure 9). Only the area of Genoa and Zurich was partially covered with clouds. This could explain the relative low NO2 column over Zurich and Genoa.

Figure 8: Cloud cover from the BOLAM analysis on the 23rd of June 2003.	Figure 9: NO2 tropospheric colum measured by SCIAMACHY on the 23th of June 2003.
Comparison with ground-based DOAS measurements (26th March 2003) Both SCIAMACHY and GASCOD gave a very high NO2 column in the lower troposphere for the 26th of March 2003. When converted to total column i.e. diveded by the Air Mass Factor (equal to 1.25 at 10.30 LT) the NO2 column measured at the ground turns to be comparable with satellite observation (30x10¹⁵ molec/cm² versus 32x10¹⁵ molec/cm²). Such agreement let us conclude that horizontal distribution of NO2 was homogeneus in the Bologna area and this seems to be conformed also by the satellite picture since high similar values of NO2 tropospheric column are reported also in the surrounding of Bologna (Ferrara and Ravenna areas).

Figure 10: NO2 tropospheric colum measured by SCIAMACHY on the 26th of March 2003.	Figure 11:DOAS measurements of NO2 column in the PBL on the 26th of March 2003.