Assessment of two aerosol optical thickness retrieval algorithms applied to MODIS Aqua and Terra measurements in Europe

P Glantz,M Tesche

doi:10.5194/amt-5-1727-2012

Abstract

Abstract. The aim of the present study is to validate AOT (aerosol optical thickness) and Ångström exponent (α), obtained from MODIS (MODerate resolution Imaging Spectroradiometer) Aqua and Terra calibrated level 1 data (1 km horizontal resolution at ground) with the SAER (Satellite AErosol Retrieval) algorithm and with MODIS Collection 5 (c005) standard product retrievals (10 km horizontal resolution), against AERONET (AErosol RObotic NETwork) sun photometer observations over land surfaces in Europe. An inter-comparison of AOT at 0.469 nm obtained with the two algorithms has also been performed. The time periods investigated were chosen to enable a validation of the findings of the two algorithms for a maximal possible variation in sun elevation. The satellite retrievals were also performed with a significant variation in the satellite-viewing geometry, since Aqua and Terra passed the investigation area twice a day for several of the cases analyzed. The validation with AERONET shows that the AOT at 0.469 and 0.555 nm obtained with MODIS c005 is within the expected uncertainty of one standard deviation of the MODIS c005 retrievals (ΔAOT = ± 0.05 ± 0.15 · AOT). The AOT at 0.443 nm retrieved with SAER, but with a much finer spatial resolution, also agreed reasonably well with AERONET measurements. The majority of the SAER AOT values are within the MODIS c005 expected uncertainty range, although somewhat larger average absolute deviation occurs compared to the results obtained with the MODIS c005 algorithm. The discrepancy between AOT from SAER and AERONET is, however, substantially larger for the wavelength 488 nm. This means that the values are, to a larger extent, outside of the expected MODIS uncertainty range. In addition, both satellite retrieval algorithms are unable to estimate α accurately, although the MODIS c005 algorithm performs better. Based on the inter-comparison of the SAER and MODIS c005 algorithms, it was found that SAER on the whole is able to obtain results within the expected uncertainty range of MODIS Aqua and Terra observations.

Highlights

Beside an increase in greenhouse gases, human activities have lead to a perturbation of the atmospheric content of aerosol particles (IPCC, 2007)
Periods 1 and 3 were both influenced by high pressure systems, estimated by the European Centre for Medium-Range Weather Forecasts (ECMWF), with cores mainly located over Finland and eastern Ukraine/southern Russia, respectively
Findings of Aerosol optical thickness (AOT) and α from satellite retrievals with the MODIS c005 and Satellite AErosol Retrieval (SAER) algorithms, applied to data collected from the MODIS Aqua and Terra sensors over Europe, were inter-compared and validated against AERONET measurements

Summary

Introduction

Beside an increase in greenhouse gases, human activities have lead to a perturbation of the atmospheric content of aerosol particles (IPCC, 2007). Studies of the effects of aerosols on climate and measures for environmental control require an accurate identification of aerosol sources, their strength, and the released aerosol type. Such information can be retrieved from space-borne observations, and several contemporary satellite-borne radiometers have produced data for aerosol investigations during the last decade. Satellite retrieval is no straightforward task, since the radiance at the top-of-atmosphere (TOA) as detected by the nadir viewing sensors is, beside aerosols, affected by gases and surface reflection. Beside the development of retrieval algorithms, high priority should be given to the validation of aerosol optical properties from satellite observations against ground-based data.

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