The AMF maps offer a general overview of what can happen in the presence of aerosol. However, as it was already demonstrated in the previous chapter, to completely understand what the source of the variation is, it becomes important to consider several aspects. Not only is essential to look into the optical properties of the aerosol, but also consider the vertical distribution of NO2 and aerosol, and, particularly for this case, the ash. Therefore, various locations across Europe (several in Germany) were selected for a more detailed analysis (see Figure 4.15). This selection was carried out according to several factors, attempting coverage of different possible profiles for trace gas and aerosol, as well as assuring the presence and absence of ash on the different days. An example is Cabauw that, unlike the remaining ones, is not a large city but a remote area affected by the transport of pollution from other Dutch cities. In addition, lidar and AERONET stations are located at this site which allows for a comparison of aerosol scenario definitions.
Figure 4.9 NO2 AMF for the 16th of April 2010, plotted for overpass time of GOME-2. AMFs were determined with EURAD model output and calculated for 3 scenarios: (A) no aerosol, (B) total aerosol, (C) all aerosol types except for ash.
Figure 4.10 NO2 AMF for the 17th of April 2010, plotted for overpass time of GOME-2. AMFs were determined with EURAD model output and calculated for 3 scenarios: (A) no aerosol, (B) total aerosol, (C) all aerosol types except for ash.
Figure 4.11 NO2 AMF for the 18th of April 2010, plotted for overpass time of GOME-2. AMFs were determined with EURAD model output and calculated for 3 scenarios: (A) no aerosol, (B) total aerosol, (C) all aerosol types except for ash.
Figure 4.12 Ratio of NO2 AMFs of calculations with no aerosol and total aerosol, for the days 16 to 18 of April 2010, determined with EURAD model output.
Figure 4.13 Ratio of NO2 AMFs of calculations with no aerosol and all aerosol except ash, for the days 16 to 18 of April 2010, determined with EURAD model output.
Figure 4.14 Ratio of NO2 AMFs of calculations with no ash and total aerosol, for the days 16 to 18 of April 2010, determined with EURAD model output. (Note the different colour scale from the previous figures.)
The trace gas and aerosol profiles were extracted from the SCIATRAN input files, and the AODs and SSA values calculated. These variables, the AMFs for the different scenarios and their ratios are presented in Table 4.4, Table 4.5 and Table 4.6, for the days 16 to 18 of April, respectively.
Figure 4.15 Selected locations for a detailed analysis of the effect of ash on the NO2 AMFs.
The different scenarios presented in Figure 4.16, Figure 4.17 and Figure 4.18 provide an overview on various possible situations in terms of aerosol and trace gas distribution. For the majority of the cases, the NO2 was concentrated close to the surface, within the lowest kilometre. On most of the days and locations, the high volume mixing ratios were fairly constant up to a certain height when a sharp decrease was observed, indicating well mixed boundary layers. Two different profiles are presented for the aerosol, in function of total extinction of the plume and extinction of the volcanic ash. For the selected locations, the great fraction of the ash was located above the NO2. However, on the 18th it was possible to see that the ash aerosol presented an almost homogeneous distribution from surface to 5 km (or even higher), for the sites of Cabauw, London and Paris. It is also interesting to observe that the profile of total extinction (corresponding to all aerosol types) was similar to the NO2 vertical distribution with exception of those layers where ash was present. As mentioned above, the main focus of this study was not to analyse the impact of an aerosol plume (i.e., total mixture) on the satellite observations, but mainly the influence by volcanic ash. Nevertheless, it is important to note that those profiles might have contributed to an enhancement of the NO2 close to the surface. This was clearly what occurred, for example, on the 18th of April in Bremen, Hamburg and Cabauw, where the values of the ratios between scenario A and B were smaller than 1 (see Table 4.6). In addition, the shielding effect of ash standing above the NO2 can, in other occasions, be compensated by the multiple scattering closer to the surface. It has been observed before (see results of scenario L in
previous chapter) that the combination of the two effects often results in an overall modest change of the tropospheric NO2 AMF.
From the three days considered, the most interesting, for this study, is certainly the 16th of April when the amount of ash was highest above Europe. Several conclusions can be drawn when analysing the profiles and results presented in Table 4.4. The most significant impact on the NO2 AMF was detected on the 16th of April for Cabauw, Bremen and Hamburg, where the volcanic ash caused a decrease of the AMFs of 7.9%, 5.0% and 2.1%, respectively. For Hamburg, it is also important to point out that the magnitude of the AMF change induced by ash was similar to that caused by the remaining aerosol types. Yet, the optical depth of ash alone was only 0.03 compared to the 0.05 of all other aerosol types. This factor highlights once again the importance of the different characteristics of aerosol in the radiative transfer calculations. The situation observed for Düsseldorf is also a good example for this case, where the AMFs obtained were comparable for the three scenarios. Still, the total aerosol load (0.12) was higher than what was predicted for Hamburg (0.07). The potential shielding caused by the ash layer standing above the NO2 (with AOD of 0.04 and a peak at ~6.5 km) may have been partly cancelled by enhancement of the measurement sensitivity due to a large amount of aerosol mixed with the trace gas (AOD of ~0.03 in the first 500 m). The results obtained on all sites considered, for the days 17 and 18 of April, do not show a significant variation of the AMFs related to the volcanic ash, with the exception of London and Paris. Focusing on the last mentioned day, the results obtained for London are quite exceptional. If on the one hand, a 3.7% reduction of the AMF was observed because of the ash, on the other hand, when comparing the scenarios A (no aerosol) and C (no ash), the AMF increased from 1.018 to 1.034. This is once more a good example of how the interaction of the various aerosol types with the radiation can derive different consequences. In Paris, for both days, the ash has reduced the AMFs by ~2%. However, while on the 17th this reduction was higher than the effect of the other aerosol types, on the 18th the opposite was verified.
As it was demonstrated in the previous chapter, also the absorbing properties of the aerosol are influencing the AMF values. A column averaged SSA was obtained by the following equation:
, ,
( )
l.
ext ll ext l
l
SSA SSA k
k
(4.2),where SSAl and kext,l are, respectively, the determined single scattering albedo and extinction coefficients for a layer l in the profile. As it is possible to see from the values in the tables below, the aerosol mixture for these locations was generally highly absorbing. The lowest SSA for the total mixture (0.70) obtained for Paris on the 17th, and the highest scattering aerosol plume (0.90) observed on the 18th in the atmosphere above Bremen and Hamburg. Such values are in accordance to values reported from previous studies, as for example SSA values for China, derived from ground and
satellite measurements, presented by Lee et al. (2007). Unfortunately, AERONET inversion products that might provide SSA values were not available for the relevant time period of this event. In addition, it is interesting to see that the presence of ash did not affect much this property.
Figure 4.16 NO2 and aerosol (total aerosol (open circles) and ash (filled squares)) vertical profiles for selected European locations, on the 16th of April 2010.
Figure 4.17 NO2 and aerosol (total aerosol (open circles) and ash (filled squares)) vertical profiles for selected European locations, on the 17th of April 2010.
Figure 4.18 NO2 and aerosol (total aerosol (open circles) and ash (filled squares)) vertical profiles for selected European locations, on the 18th of April 2010.