MODIS observation of aerosols and estimation of aerosol radiative forcing over southern Africa during SAFARI 2000

[1] MODIS provides almost complete global coverage daily. Aerosol optical thickness (AOT or τaλ) and other aerosol parameters are retrieved over land and ocean at a spatial scale of 10 km (level 2), then aggregated to a global grid of 1° spatial resolution on daily, weekly, and monthly time scales (level 3). The SAFARI 2000 ground-based measurements of AOT from the Aerosol Robotic Network (AERONET) group of Sun photometers were used to compare MODIS over-land AOT based on a spatiotemporal statistical technique. At low aerosol loading (when τa470 < 0.6), MODIS τaλ values agree with those of AERONET at the blue wavelength (λ = 440, 470 nm). But, at higher wavelengths and with increasing aerosol loading, MODIS AOT values underestimate those of AERONET. There are also regional variations in validation accuracy. This AOT underestimation by MODIS during SAFARI 2000 is attributed to the application of a constant single-scattering-albedo (SSA or ω0) value of 0.90 globally for smoke aerosol retrieval. Recent studies based on long-term observations with AERONET Sun photometers suggest that lower ω0 values of 0.88 and 0.84 at 440 nm and 670 nm wavelengths respectively would be more applicable over southern Africa (particularly over Zambia, where most of the measurements were based). A column climate model, with MODIS aerosol information as input, is used to calculate aerosol radiative forcing over the (southern Atlantic) Ocean part of the SAFARI 2000 region. For September 2000, the results show a forcing of −10 W/m2 at the top of the atmosphere and approximately −26 W/m2 at the terrestrial surface.

[1]  R. Swap,et al.  Southern African Regional Science Initiative (SAFARI 2000): Summary of science plan , 2002 .

[2]  Alexander Ignatov Sensitivity and information content of aerosol retrievals from the Advanced Very High Resolution Radiometer: radiometric factors. , 2002, Applied optics.

[3]  T. Eck,et al.  Variability of Absorption and Optical Properties of Key Aerosol Types Observed in Worldwide Locations , 2002 .

[4]  B. Holben,et al.  Validation of MODIS aerosol optical depth retrieval over land , 2002 .

[5]  Lorraine Remer,et al.  The MODIS 2.1-μm channel-correlation with visible reflectance for use in remote sensing of aerosol , 1997, IEEE Trans. Geosci. Remote. Sens..

[6]  Larry L. Stowe,et al.  Characterization of tropospheric aerosols over the oceans with the NOAA advanced very high resolution radiometer optical thickness operational product , 1997 .

[7]  Model assessment of the ability of MODIS to measure top-of-atmosphere direct radiative forcing from smoke aerosols , 2002 .

[8]  C. Justice,et al.  Satellite remote sensing of fires during the SAFARI campaign using NOAA Advanced Very High Resolution Radiometer data , 1996 .

[9]  J. Fishman,et al.  Identification of Widespread Pollution in the Southern Hemisphere Deduced from Satellite Analyses , 1991, Science.

[10]  Yoram J. Kaufman,et al.  Evaluation of the Moderate‐Resolution Imaging Spectroradiometer (MODIS) retrievals of dust aerosol over the ocean during PRIDE , 2003 .

[11]  Mark R. Schoeberl,et al.  Where did tropospheric ozone over southern Africa and the tropical Atlantic come from in October 1992? Insights from TOMS, GTE TRACE A, and SAFARI 1992 , 1996 .

[12]  A. Lacis,et al.  Aerosol retrievals over the ocean by use of channels 1 and 2 AVHRR data: sensitivity analysis and preliminary results. , 1999, Applied optics.

[13]  H. Gordon Atmospheric correction of ocean color imagery in the Earth Observing System era , 1997 .

[14]  A. Smirnov,et al.  AERONET-a federated instrument network and data archive for aerosol Characterization , 1998 .

[15]  B. Holben,et al.  Validation of MODIS aerosol retrieval over ocean , 2002 .

[16]  B. Holben,et al.  Remote sensing of smoke from MODIS airborne simulator during the SCAR‐B experiment , 1998 .

[17]  T. Eck,et al.  An emerging ground-based aerosol climatology: Aerosol optical depth from AERONET , 2001 .

[18]  Alexander Smirnov,et al.  Characterization of the optical properties of biomass burning aerosols in Zambia during the 1997 ZIBBEE field campaign , 2001 .

[19]  Thomas F. Eck,et al.  Variability of biomass burning aerosol optical characteristics in southern Africa during the SAFARI , 2003 .

[20]  Sundar A. Christopher,et al.  Shortwave Aerosol Radiative Forcing from MODIS and CERES observations over the oceans , 2002 .

[21]  B. Holben,et al.  Baseline maritime aerosol: Methodology to Derive the optical thickness and scattering properties , 2001 .

[22]  S. Piketh,et al.  Lower tropospheric aerosol loadings over South Africa: The relative contribution of aeolian dust, industrial emissions, and biomass burning , 1999 .

[23]  M. Andreae,et al.  The Southern Tropical Atlantic Region Experiment (STARE): Transport and Atmospheric Chemistry near the Equator‐Atlantic (TRACE A) and Southern African Fire‐Atmosphere Research Initiative (SAFARI): An introduction , 1996 .

[24]  D. Tanré,et al.  Remote sensing of aerosol properties over oceans using the MODIS/EOS spectral radiances , 1997 .

[25]  A. Thompson,et al.  A New Method of Deriving Time-Averaged Tropospheric Column Ozone over the Tropics Using Total Ozone Mapping Spectrometer (TOMS) Radiances: Intercomparison and Analysis Using TRACE A Data , 1996 .

[26]  Alexander Ignatov,et al.  Development, validation, and potential enhancements to the second‐generation operational aerosol product at the National Environmental Satellite, Data, and Information Service of the National Oceanic and Atmospheric Administration , 1997 .

[27]  D. Blake,et al.  Physical, chemical, and optical properties of regional hazes dominated by smoke in Brazil , 1998 .

[28]  William B. Grant,et al.  Tropospheric ozone derived from TOMS/SBUV measurements during TRACE A , 1996 .

[29]  E. Vermote,et al.  Operational remote sensing of tropospheric aerosol over land from EOS moderate resolution imaging spectroradiometer , 1997 .

[30]  C. Justice,et al.  The quantity of biomass burned in southern Africa , 1996 .

[31]  Alexander Smirnov,et al.  Development of a Global Validation Package for Satellite Oceanic Aerosol Optical Thickness Retrieval Based on AERONET Observations and Its Application to NOAA/NESDIS Operational Aerosol Retrievals. , 2002 .

[32]  M. Garstang,et al.  International geosphere-biosphere programme/international global atmospheric chemistry SAFARI-92 field experiment: Background and overview , 1996 .

[33]  D. Tanré,et al.  Retrieval of aerosol optical thickness and size distribution over ocean from the MODIS airborne simulator during TARFOX , 1999 .

[34]  Yoram J. Kaufman,et al.  Evaluation of the MODIS Retrievals of Dust Aerosol over the Ocean during PRIDE , 2002 .

[35]  M. Garstang,et al.  Horizontal and vertical transport of air over southern Africa , 1996 .

[36]  Paul Ginoux,et al.  A Long-Term Record of Aerosol Optical Depth from TOMS Observations and Comparison to AERONET Measurements , 2002 .

[37]  S. K. Satheesh,et al.  Large differences in tropical aerosol forcing at the top of the atmosphere and Earth's surface , 2000, Nature.

[38]  M. Garstang,et al.  The long‐range transport of southern African aerosols to the tropical South Atlantic , 1996 .

[39]  David J. Diner,et al.  MISR aerosol optical depth retrievals over southern Africa during the SAFARI‐2000 Dry Season Campaign , 2001 .

[40]  V. Ramanathan,et al.  Direct observations of clear-sky aerosol radiative forcing from space during the Indian Ocean Experiment , 2001 .

[41]  B. Holben,et al.  A spatio‐temporal approach for global validation and analysis of MODIS aerosol products , 2002 .