RETRIEVAL OF AEROSOL PROPERTIES OVER LAND AND WATER USING (A)ATSR DATA

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1 RETRIEVAL OF AEROSOL PROPERTIES OVER LAND AND WATER USING (A)ATSR DATA ABSTRACT/RESUME Gerrit de Leeuw and Robin Schoemaker TNO, P.O. Box 96864, 2509 JG The Hague, The Netherlands The retrieval of aerosol properties using multi-spectral data provided by satellite based radiometers, has been explored by TNO since Aerosol properties that are currently retrieved using (A)ATSR data are aerosol optical depth (AOD), the variation of AOD with the wavelength expressed by the Ångström coefficient, and the dominant aerosol types. Algorithms have been developed for instruments such as AVHRR, GOME, SCHIAMACHY, OMI and (A)ATSR. A major challenge is to discriminate between the aerosol reflectances at the top of the atmosphere (TOA) and those by other atmospheric constituents and, in particular, the surface reflectance. Over water the surface is usually dark and the TOA reflectance is dominated by contributions from atmospheric constituents. Hence only one single viewing angle suffices for the retrieval of aerosol properties over water. The accuracy obtained for AOD retrieval over water using ATSR-2 data is 0.04, based on comparison with independent ground-based measurements, usually AERONET sun photometer data. Coastal waters often cause a problem because of sub-surface reflectances by suspended matter and the retrieval is often less reliable when this occurs. Also the presence of algae blooms (Chlorophyll) may render less accurate results. This causes discontinuities in the AOD across land-sea boundaries. Land surfaces are usually brighter and often the contribution of the surface to the reflectance at the TOA is significant and needs to be accounted for to accurately determine the aerosol properties. The surface contribution to the TOA reflectance can be eliminated by using multiple viewing angles such as provided by (A)ATSR. (A)ATSR provides one viewing angle in nadir and one in the forward direction. This feature has been applied over land in various regions around the world and comparisons with AERONET data provide a means to evaluate the results, showing an excellent accuracy of over most land surfaces. The TNO single and dual view algorithms are scientific algorithms, which have been used to develop a quasi-operational algorithm that has been demonstrated to work well over Europe. Data over Europe are available as a service through GSE PROMOTE and TEMIS. The application for 2003 is in progress. This quasi-operational algorithm will be expanded to other areas such as Asia and Africa, including deserts, based on scientific work in progress. The presentation will focus on the application of the quasi-operational algorithm over land, and scientific work on its extension to other areas than Europe. 1. INTRODUCTION Aerosol information over water has been available from AVHRR aboard NOAA satellites since TOMS has provided a long time series of the Aerosol Index (AI) over both water and land, starting in AI is a measure of the wavelength-dependent reduction of Rayleigh scattered radiance by aerosol absorption relative to a pure Rayleigh atmosphere [1]. A near UV algorithm to retrieve AOD and SSA has been applied to the TOMS record from 1979 to 2000 and several papers discussing the algorithm, AOD validation analyses and the climatological AOD record have been published [2-5]. Other instruments have been used to provide such information, such as OCTS, ATSR-2, and SeaWIFS (cf. [6]). However, none of the mentioned instruments were designed for the retrieval of aerosol properties, which over water could only be obtained because of the dark surface. Over brighter surfaces, and in particular over most land surfaces, satellite remote sensing of aerosols was not possible. This situation has significantly improved during the last decade, when methods were developed to retrieve aerosol properties over land, utilizing multiple views, polarization and multi-spectral information from instruments that were designed for the retrieval of aerosol properties, such as POLDER, MODIS, MISR, and other instruments that offered such features, in particular ATSR-2. ATSR-2 combines a nadir and forward view, which can be used to eliminate the effect of the land surface reflection, with four spectral bands that are suitable for determining the aerosol properties. Algorithms have been developed that apply over land.

2 2. AEROSOL RETRIEVAL OVER LAND USING ATSR DATA ATSR-2 onboard ERS-2 and AATSR onboard ENVISAT are dual view imaging spectrometers with seven wavelength bands, four in the visible and NIR (0.555, 0.659, 0.865, and 1.6 µm) and three in TIR (3.7, 11, and 12 µm). The resolution of these instruments is 1 x 1 km 2 at nadir view and the swath width is 512 km, resulting in a return time of three days at mid-latitudes. The nadir view and the forward view at 55º incident angle to the surface allow for nearsimultaneous observation of the same area on the Earth s surface through two different atmospheric columns within a time interval of approx. 2.5 minutes. Algorithms for the retrieval of the aerosol optical depth (AOD) and derived parameters from ATSR-2 measurements have been developed and applied by TNO, The Hague, The Netherlands, for a variety of locations that are representative for the occurrence of characteristic aerosol types [7-10]. The single view algorithm is applied over water surfaces and uses either the nadir or the forward view. The dual view algorithm is applied over land surfaces, which in general are brighter than water surfaces. In the dual view algorithm the two views are combined to eliminate the effect of the land surface reflectance and the total reflectance received by ATSR at the top of the atmosphere (TOA). The actual retrieval is similar for both algorithms once the surface contribution has been accounted for. A radiative transfer model is applied to calculate the TOA reflectance (Doubling-Adding method at KNMI (DAK) [11]. This is done for several aerosol models and the results are stored in look-up tables (LUTs). The calculated TOA reflectances are compared with the measured values and by selecting different LUTs the error function, describing the difference between model and measurement, is minimized to find the most suitable aerosol model. This procedure is applied for the ATSR-2 wavelengths of 0.55 µm, 0.67 µm, 0.87 µm (only over water) and 1.6 µm) and hence the optimization procedure determines the aerosol mixture that best fits the measurements over the applicable wavelength range. Thus in fact the parameters determined are the AOD at the suitable wavelengths, the Ångström parameter describing the wavelength dependence of the AOD. The aerosol type and mixing ratio, given by the most suitable aerosol model, are retained as well. The aerosol types considered over Europe are marine aerosol (r eff = 1 µm) [12] and anthropogenic aerosol (sulphate/nitrate water soluble, r eff = 0.05 µm) [13], which are externally mixed. The vertical structure is described by the Navy Oceanic Vertical Aerosol Model (NOVAM) [14]. This model appears to work well over Europe, as evidenced from comparison with sun photometer derived AOD values. Other aerosol models have been implemented for areas such as south-east Asia and the Indian Ocean, and Africa [9]. An important condition to retrieve aerosol properties from space-borne sensing is the absence of clouds. To accomplish this, three test are applied as described in [9], i.e. a 12 µm gross cloud test, a reflectance test for 0.67 µm, and a reflectance ratio test (0.67 µm / 0.87 µm). These procedures are based on [15]. Application of the DV algorithm relies on viewing the same area and atmospheric column in forward and nadir. This may not be achieved over elevated terrain because the standard co-registration of nadir and forward views of the ATSR- 2 sensor does not account for surface elevation. References [16, 17] conclude that such problems only occur for pixels with surface elevations higher than 1.5 km. The single and dual view algorithms and their initial application and validation are described in [7, 18-20]. Reference [9] extended the algorithms and automated them to allow for the processing of large data sets covering extended time periods and large areas such as Europe for August 1997 [10]. The SV and DV are scientific algorithms and their use is very time consuming. They were used as the basis for the TNO quasi-operational ATSR-2/AATSR aerosol retrieval algorithm which was developed as part of the ESA DUP project TEMIS. This quasi-operational algorithm requires as input the ATSR level 1 GBTR (ATS-TOA-1P) product provided by ESA and delivers an output in ASCII and HDF format that can be used for further level-3 post-processing. Cloud screening is fully automated [9] and the aerosol properties are retrieved for each (A)ATSR pixel. Default outputs are provided with spatial resolution of 1x1 km 2 and 10x10 km 2, other grid sizes can be made available on user request. In particular, the AOD over Europe for the year 2000, retrieved in the framework of TEMIS and the EU FP5 GMES targeted project CREATE, were regridded to a resolution of 1x1 o for comparison with transport models in the framework of AEROCOM ( As an example, Fig. 1 shows the AOD maps for August 2000 over Europe and part of North Africa, (20-80N; 20-40E) retrieved from ATSR-2 data. Retrieval is done over cloud-free scenes for the 1 x 1 km 2 sensor resolution and binned in

3 pixels of 10x10 km 2 by means of an automated post-processing step. The final maps however cannot be considered as monthly averages; rather they are composites providing information on the spatial variation of aerosols, hot spots and other regions with high aerosol loading. The results are validated by comparison with collocated AERONET sun photometer AOD measurements [22], within 30 minutes of the satellite overpass. The sun photometer AOD are determined with accuracy of [23]. By comparison, the ATSR-2 AOD accuracy over land has been determined as Other results, including extensive validation with AERONET data, can be viewed at and Fig. 1. Composite showing the ATSR-2 derived AOD over Europe for the month of August AEROSOL RETRIEVAL OVER BRIGHT LAND SURFACES The ATSR algorithm has been applied over land in Europe [7-10], South Asia [9, 21] and Africa south of the equator [9]. In most of these areas the surface is vegetated and therefore relatively dark. To evaluate the ATSR dual view algorithm over very bright areas, TNO participated in the United Arab Emirates Unified Aerosol Experiment (UAE 2 ) field campaign in August/September 2004 (cf. AERONET provided 14 sun photometers for comparison with a number of satellites and ground based measurements. The available AATSR frames over the area have been used to retrieve the aerosol properties with the current algorithm and the results were compared with the UAE 2 AERONET sun photometer data obtained within 10 minutes from the time of overpass. Preliminary results are presented in Figs. 2 and 3. Fig. 2a shows the comparison for all water sites, i.e. all sites for which sun photometers were at an island. The AOD values retrieved from the AATSR and the sun photometers compare favourably, with a regression coefficient of 0.78 (22 data points). There seems to be a very small bias with the AATSR derived AOD slightly higher, but mostly within one standard deviation. For the retrieval over land (Fig. 2b) the comparison is less favourable, with a regression coefficient of 0.57 (53 data points) and a slight bias to underestimating the AATSR derived AOD. However, in view of the bright surface, and most AOD within 0.1 of the AERONET AOD, this result is may be considered quite satisfactory. Fig. 3 shows a preliminary result of the AOD at 0.67 µm over the UAE 2 area retrieved from AATSR data on 7 September Clearly visible are the aerosol plumes advected off the UAE coast and in the stagnant air over the Gulf. There seems to be a discontinuity in the AOD distribution over land along the coast, which has to be further investigated. However, apart from this, the AOD over land, away from the coast, and over water, are quite similar.

4 (a) (b) Figure 2. Comparison of AOD derived from AATSR data during UAE2 with AERONET sun photometer data: (a) over water with the sun photometers on islands (b) over land. Figure 3. AOD at 0.67 µm over part of the UAE 2 area retrieved from AATSR data on 7 September 2004 (Preliminary result). 4. CONCLUSION A quasi-operational aerosol retrieval algorithm has been developed for ATSR-2 and is currently applied to AATSR data. Products for Europe are delivered to PROMOTE core users who will evaluate them as regards their suitability for use in their work such as reporting the air quality over Europe and selected regions. Other uses are, e.g., comparison with model results and assimilation in chemistry transport models. Expectations are that the models are constrained and

5 thus their reliability and accuracy will improve. Satellite and models will work together to better interprete atmospheric data. Research issues are the use of satellite data to obtain direct information on air quality, in particular information on PM10 and/or PM2.5. The TNO quasi-operational algorithm has thus far mainly been applied over Europe. However, scientific results, mainly in connection with campaigns that provide in-situ data from comparison with the retrieval results, are of sufficient quality for implementation in the quasi-operational algorithm. Validation is an important issue and is routinely undertaken as part of the retrieval. Products from the TNO quasi-operational algorithm are the validated AOD and the Ångström parameter, as well as the dominant aerosol types and their mixing ratio. The latter need to be thoroughly validated by comparison with field campaign results. 5. REFERENCES 1. Herman, J. R. et al. Global distribution of UV-absorbing aerosols from Nimbus 7/TOMS data, J. Geophys. Res., 102, , Torres, O. et al. Derivation of aerosol properties from satellite measurements of backscattered ultraviolet radiation, Theoretical Basis, J. Geophys. Res.,103, , Torres, O. et al. Aerosol properties from EP-TOMS near UV observations, in press, Adv. Space Res., 2001a 4. Torres, O. et al. A long term record of aerosol optical thickness from TOMS observations and comparison to AERONET measurements, submitted to J. Atm. Sci., 2001b 5. Torres, O. et al. OMI Aerosol Retrieval Algorithm. In: P. Stammes and R. Noordhoek (eds.) OMI Algorithm Theoretical Basis Document Volume III: Clouds, Aerosols and Surface UV Irradiance, ATBD-OMI-03, pp , King, M.D. et al. Remote sensing of tropospheric aerosols from space: Past, present, and future, Bull. Am. Meteorol. Soc., 80, , Veefkind J. P. Aerosol satellite remote sensing, PhD thesis, University of Utrecht, Veefkind, J.P. et al. Regional Distribution of Aerosol over Land, Derived from ATSR-2 and GOME, Rem. Sens. Of the Env., 74, pp , Robles Gonzalez, C., Retrieval of Aerosol Properties using ATSR-2 Observations and their Interpretation, PhD thesis, University of Utrecht, Robles Gonzalez C. et al. Aerosol optical depth over Europe in August 1997 derived from ATSR-2 data, Geophysical Research Letters, 27, (No 7), , Stammes, P., Manual for the DAK program, Royal Netherlands Meteorological Institute, De Bilt, The Netherlands, Shettle, E.P. and R.W. Fenn, Models for the Aerosols of the Lower Atmosphere and the Effects of Humidity Variations on Their Optical Properties, AFGL-TR Environmental Research Papers, #676, published by the Air Force Geophysics Laboratory, Hanscom AFB, MA 01731, De Leeuw, G. et al. Modeling of aerosols in the marine mixed-layer. SPIE Proceedings Volume 1115, "Propagation Engineering," pp , Volz, F.E., Infrared refractive index of atmospheric aerosol substances, Applied Optics, 11, pp , Koelemeijer, R.B.A. et al. A fast method for retrieval of cloud parameters using oxygen-a band measurements from the Global Ozone Measurement Instrument, J. Geophys. Res., 106, pp , Holzer-Popp, T. et al. Retrieving aerosol optical depth and type in the boundary layer over land and ocean from simultaneous GOME spectrometer and ATSR-2 radiometer measurements, 1, Method description J. Geophys. Res., 107, D21, pp. AAC16-1 AAC16-17, Holzer-Popp, T. et al. Retrieving aerosol optical depth and type in the boundary layer over land and ocean from simultaneous GOME spectrometer and ATSR-2 radiometer measurements, 2, Case study application and validation, J. Geophys. Res., 107, D24, pp. AAC10-1 AAC10-8, Veefkind J. P. et al. Retrieval of Aerosol Optical Depth over Land using two angle view Satellite Radiometry during TARFOX, Geophysical Research Letters, 25, 3,135-3,138, Veefkind J. P. et al. Regional Distribution of Aerosol over land derived from ATSR-2 and GOME, Accepted for publication in Rem. Sens. of the Env, Veefkind J. P. et al. Aerosol optical depth retrieval using ATSR-2 data and AVHRR data during TARFOX, J. Geophys. Res., 104 (D2), , 1999.

6 21. Robles Gonzalez, C. et al. Aerosol properties over the INDOEX campaign area retrieved from ATSR-2. MS 2005JD006184, Submitted for publication in J. Geophys. Res. 22. Holben B. N., T. F. Eck, I. Slutsker, D. Tanre, J. P. Buis, A. Setzer, E. Vermote, J. A. Reagan, Y. J. Kaufman, T. Nakajima, F. Lavenu, I. Jankowiak, and A. Smirnov, AERONET- A Federated Instrument Network and Data Archive for Aerosol Characterisation, Remote Sensing of the Environment, 66, 1-16, Eck, T.F., B.N. Holben, J.S. Reid, O. Dubovik, A. Smirnov, N.T.O'Neill, I.Slutsker, and S.Kinne (1999), Wavelength dependence of the optical depth of biomass burning, urban and desert dust aerosols, J. Geophys. Res., 104, pp ACKNOWLEDGEMENTS Aerosol retrieval work at TNO is supported by national user support programs supervised by SRON and NIVR, by ESA projects such as the Data User Programme project TEMIS and the GMES Service Element PROMOTE, and projects supported by the European Commission in the framework of FP5 (CREATE and DAEDALUS) and FP6 (NoE ACCENT), by the Netherlands Ministry of Defence and by TNO internal funding.