PNNL

Abstract

Aerosol transport off the US Northeast coast in Summer 2004 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 Sunphotometer (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 ? 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 x 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 rootmean-square (RMS) MISR-AATS AOD differences of 0.03 to 0.21 (9 to 31%). MISR V15 Ångstrom exponent a (=-dlnAOD/dln?) was closer to AATS than was MISR V16. MODIS-AATS AOD comparisons on 8 overpasses using 61 grid cells (each nominally 10 km x 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, ?t = ±0.03 ± 0.05t . In contrast to the small MODIS-AATS differences in AOD, 2006jd007606 Page 3 of 50 2:41 PM, 1/19/07 RUSSELL ET AL.: MULTI-GRID-CELL SATELLITE VALIDATION MODIS-AATS differences in Ångstrom exponent a were large: RMS differences for a(553, 855 nm) were 0.28 for MODIS-Terra and 0.64 for MODIS-Aqua; RMS differences for a(855, 2119 nm) were larger still: 0.61 for MODIS-Terra and 1.14 for MODIS-Aqua. The largest MODISAATS Ångstrom exponent differences were associated with small AOD values, for which MODIS AOD relative uncertainty is large. Excluding cases with AOD(855 nm)