Multi-grid-cell validation of satellite aerosol property retrievals in INTEX/ITCT/ICARTT 2004

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Read Full Article (file size: 15927730 bytes) Cited by JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112, D12S09, doi:10.1029/2006JD007606, 2007 Multi-grid-cell validation of satellite aerosol property retrievals in INTEX/ITCT/ICARTT 2004 P. B. Russell NASA Ames Research Center, Moffett Field, California, USA J. M. Livingston SRI International, Menlo Park, California, USA J. Redemann Bay Area Environmental Research Institute, Sonoma, California, USA B. Schmid Bay Area Environmental Research Institute, Sonoma, California, USA S. A. Ramirez Bay Area Environmental Research Institute, Sonoma, California, USA J. Eilers NASA Ames Research Center, Moffett Field, California, USA R. Kahn Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA D. A. Chu Laboratory for Atmospheres, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA L. Remer Laboratory for Atmospheres, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA P. K. Quinn Pacific Marine Environmental Laboratory, NOAA, Seattle, Washington, USA M. J. Rood Department of Civil and Environmental Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, USA W. Wang Department of Civil and Environmental Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, USA Abstract Aerosol transport off the US Northeast coast during the Summer 2004 International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) Intercontinental Chemical Transport Experiment (INTEX) and Intercontinental Transport and Chemical Transformation (ITCT) experiments produced a wide range of aerosol types and aerosol optical depth (AOD) values, often with strong horizontal AOD gradients. In these conditions we flew the 14-channel NASA Ames Airborne Tracking Sun photometer (AATS) on a Jetstream 31 (J31) aircraft. Legs flown at low altitude (usually ≤100 m ASL) provided comparisons of AATS AOD spectra to retrievals for 90 grid cells of the satellite radiometers MODIS-Terra, MODIS-Aqua, and MISR, all over the ocean. Characterization of the retrieval environment was aided by using vertical profiles by the J31 (showing aerosol vertical structure) and, on occasion, shipboard measurements of light scattering and absorption. AATS provides AOD at 13 wavelengths λ from 354 to 2138 nm, spanning the range of aerosol retrieval wavelengths for MODIS over ocean (466–2119 nm) and MISR (446–866 nm). Midvisible AOD on low-altitude J31 legs in satellite grid cells ranged from 0.05 to 0.9, with horizontal gradients often in the range 0.05 to 0.13 per 10 km. When possible, we used ship measurements of humidified aerosol scattering and absorption to estimate AOD below the J31. In these cases, which had J31 altitudes 60–110 m ASL (typical of J31 low-altitude transects), estimated midvisible AOD below the J31 ranged from 0.003 to 0.013, with mean 0.009 and standard deviation 0.003. These values averaged 6% of AOD above the J31. MISR-AATS comparisons on 29 July 2004 in 8 grid cells (each ∼17.6 km × 17.6 km) show that MISR versions 15 and 16 captured the AATS-measured AOD gradient (correlation coefficient R² = 0.87 to 0.92), but the MISR gradient was somewhat weaker than the AATS gradient. The large AOD (midvisible values up to ∼0.9) and differing gradients in this case produced root-mean-square (RMS) MISR-AATS AOD differences of 0.03 to 0.21 (9 to 31%). MISR V15 Ångstrom exponent α ( = −dlnAOD/dlnλ) was closer to AATS than was MISR V16. MODIS-AATS AOD comparisons on 8 overpasses using 61 grid cells (each nominally 10 km × 10 km) had R² ∼ 0.97, with RMS AOD difference ∼0.03 (∼20%). About 87% of the MODIS AOD retrievals differed from AATS values by less than the predicted MODIS over-ocean uncertainty, Δτ = ±0.03 ± 0.05τ. In contrast to the small MODIS-AATS differences in AOD, MODIS-AATS differences in Ångstrom exponent α were large: RMS differences for α(553, 855 nm) were 0.28 for MODIS-Terra and 0.64 for MODIS-Aqua; RMS differences for α(855, 2119 nm) were larger still, 0.61 for MODIS-Terra and 1.14 for MODIS-Aqua. The largest MODIS-AATS Ångstrom exponent differences were associated with small AOD values, for which MODIS AOD relative uncertainty is large. Excluding cases with AOD(855 nm) < 0.1 reduced MODIS-AATS α differences substantially. In one grid cell on 21 July 2004, smoke over cloud appeared to impair the MODIS-Aqua cloud mask, resulting in retrieved AODs that significantly exceeded AATS values. Experiments with extending MODIS retrievals into the glint mask yielded MODIS AODs consistently less than AATS AODs, especially at long wavelength, indicating that the current MODIS glint mask limits should not be reduced to the extent tried here. The sign of the AOD differences within the glint mask (MODIS AOD < AATS AOD) is consistent with ship-measured wind speeds there. Received 31 May 2006; accepted 5 December 2006; published 8 May 2007. Keywords: aerosol; satellite; validation. Index Terms: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801, 4906); 0360 Atmospheric Composition and Structure: Radiation: transmission and scattering; 0394 Atmospheric Composition and Structure: Instruments and techniques. Read Full Article (file size: 15927730 bytes) Cited by Citation: Russell, P. B., et al. (2007), Multi-grid-cell validation of satellite aerosol property retrievals in INTEX/ITCT/ICARTT 2004, J. Geophys. Res., 112, D12S09, doi:10.1029/2006JD007606. Copyright 2007 by the American Geophysical Union.

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