VALIDATION OF AEROSOL OPTICAL THICKNESS RETRIEVED BY BAER (BEMEN AEROSOL RETRIEVAL) IN THE MEDITERRANEAN AREA Wolfgang von Hoyningen-Huene (1), Alexander Kokhanovsky (1), John P. Burrows (1), Maria Sfakianaki (2), and Maria Kanakidou (2) (1) University of Bremen, IUP, Otto-Hahn-Allee 1,D-28334 Bremen, Germany, (2) University of Crete, ECPL, GR-71409 Heraklion, Greece ABSTRACT/RESUME The pattern of aerosol loading over Europe can be affected by various source processes, like desert dust transports, forest fires, urban and industrial pollution as well as by the natural variability of the aerosol. The most sources are over land. Satellite remote sensing of aerosol properties over land and ocean surfaces yields the regional distribution of aerosol load in terms of aerosol optical thickness (AOT). For this purpose the BAER (Bremen AErosol Retrieval) approach is used, using multi-spectral separation techniques between spectral surface and atmospheric properties, c.f. von Hoyningen-Huene et al. 2003. ESA ENVISAT's MERIS provides sufficient spectral coverage to investigate the main atmospheric aerosol loading. MERIS-L1b data, giving top-of-atmosphere (TOA) reflectance of cloudless scenes are used for the determination of spectral AOT for the MERIS channels 1-7. The pattern of AOT in August 2003 shows the distribution of forest fire plumes from Iberian peninsula over almost whole Europe. The results of AOT for Europe are in close agreement with ground-based AOT measurements. 1 INTRODUCTION Retrievals of reliable aerosol properties as spectral AOT with over both, land and ocean surface, comparable results, is a difficult task. For this purpose, mainly for the determination of AOT over land surfaces, the BAER approach has been developed and adapted to the performance of the MERIS instrument onboard of ENVISAT. The general method has been described in von Hoyningen-Huene et al. 2003. For the general application of BAER within global processing schemes it has to be validated in different regions and also for a multi-instrument use. Therefore several validation activities have been undertaken within the framework of Accent network. BAER has been applied with SeaWiFS L1 data (1) in the vicinity of Ireland and (2) over Europe with the focus on the on the Mediterranean region. Validations ob BAER, using SeaWiFS data are presented by Kokhanovsky et al, 2004 and Lee et al. 2004 and Lee et al. 2005. The present contribution is focused on MERIS L1 data. The regional focus was on Europe to the Mediterranean. The specific task was the investigation of the effect of forest fires at the Iberian peninsula in August 2003 and its effect on optical thickness over Europe, covering a wide range of AOT. 2 BAER APPLICATION USING MERIS L1 DATA AND VALIDATION For validation of BAER AOT MERIS L1 reduced resolution data of August 2003 have been available. Large forest fires in Portugal and the Iberian peninsula have been the strongest aerosol source in August 2003. Thus the forest fire season have been chosen for validation activities. Retrievals of the AOT within this time can cover the AOT range from low to very high aerosol loadings. Examples of the retrievals of aerosol optical thickness from MERIS L1 scenes are presented in Fig. 1. Results of MERIS channel 2, 0.443 µm are presented. Fig. 1a: Pattern of AOT for MERIS channel 0.443 µm over western Europe for several days of August 2003, showing the distribution of increased AOT from Portuguese forest fires over almost whole Europe.
Fig. 1a: Pattern of AOT for MERIS channel 0.443 µm over western Europe for several days of August 2003, showing the distribution of increased AOT from Portuguese forest fires over almost whole Europe: (upperleft 05.Aug., upper right - 08.Aug., lower left - 09. Aug, lower right - 11. Aug).
Fig. 1b: Pattern of AOT for MERIS channel 0.443 µm over western Europe for several days of August 2003, showing the distribution of increased AOT from Portuguese forest fires over almost whole Europe (left: - 12. Aug., right 13. Aug.) The example of scenes show the general situation of aerosol optical thickness over Western Europe. Relatively high aerosol loadings have been transported from Iberian peninsula over France, UK until Northern Germany. HYSPLIT trajectories clearly indicate the origin in fire plumes in Portugal. In the figures clouds, snow and bright soil conditions are masked in black. The average range of aerosol optical thickness of channel 2 was in the range of 0.3 0.5. However also regions with larger pollution have been observed. The scenes show also the disadvantage of the strong sun-glint effects over Mediterranean in the east part of the scene. Fig. 2 Results of validation of AOT for MERIS channel 2 (0.443 µm) with the AERONET AOT for 0.440 µm. for August 2003.
For the validation ground based observation of AOT, provided by AERONET instruments(holben et al., 1998) in Europe (Spain, France, Belgium, Germany, Switzerland, Sweden, Italy, Greece and Poland) have been used. Additional data came from instruments outside of AERONET, contributed by University of Bremen, KNMI and DWD. The AOT data for the channel 0.400 µm of the AERONET data base with the closest time of the ENVISAT over-flights have been taken. From the BAER retrievals a 5 x 5 pixel matrix around the AERONET site is selected from the scene and average and standard deviation of non cloudy pixels are used for the comparison. The validation of retrieval results of August 2003 gave a quite good correspondence between the AOT retrieved from MERIS L1b data by BAER and ground based measurements obtained from AERONET instruments in Europe, c.f. Fig.2. The obtained correlation is: AOT GROUND (0.443 µm) = 0.795 AOT BAER (0.443 µm) + 0.092, with a correlation coefficient of r = 0.94 and a standard deviation σ = 0.045. For wavelength of channels 1-4 similar correlations are obtained. The comparisons showed an offset of the linear relationship between retrievals and ground based AOT. The reasons of this must be subject of further investigations: does it come from spectral surface reflectance model or from consideration of Rayleigh scattering. The offset can be of importance for relatively increased retrieval errors for lower AOT. For higher AOT it is of less importance. Fig. 3: HYSPLIT-trajectory plot for the 09. August 2003. The consideration of the whole aerosol situation over Europe for this time showed, that wide regions of Europe have been affected by this forest fire plumes. Trajectory calculations show the transport of forest fire plume over France, UK to Germany, affecting almost whole Europe. The estimation of forest fire contribution to the AOT over Europe is made from satellite retrievals in August 2003 and values of AOT for the region in summer 2002, using first MERIS data over Germany and France. Comparisons of histograms of AOT for the channel 0.443 µm with the conditions in 2002 showed an increased AOT of approximately 0.2 over almost whole Europe.
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