Comparison of Airborne Sunphotometer to MODIS and MISR Retrievals of Aerosol Optical Depth during MILAGRO/INTEX-B

The 14-channel Ames Airborne Tracking Sunphotometer (AATS-14) was operated on a Jetstream 31 (J31) aircraft based in Veracruz, Mexico in March 2006 during MILAGRO/INTEX-B (Megacity Initiative-Local And Global Research Observations /Phase B of the Intercontinental Chemical Transport Experiment). AATS measured aerosol optical depth (AOD) at 13 wavelengths (354-2139 nm) and water vapor column content in 13 flights that sampled clean and polluted airmasses over the Gulf of Mexico and Mexico City. J31 flights were coordinated with overflights of several satellites, including Aqua, Aura, Terra, and Parasol. In this paper we will focus on comparing AATS retrievals of AOD with corresponding AOD values retrieved from spatially and temporally coincident or near-coincident measurements acquired by the satellite sensors MODIS (Aqua and Terra), and MISR (Terra). Our preliminary analyses of 37 coincident observations by AATS and MODIS-Terra and 18 coincident observations between AATS and MODIS-Aqua indicate notable differences between MODIS Collection 004 and Collection 005, the latter representing a reprocessing of the entire MODIS data set completed during 2006. For example, our analyses show that for MODIS Collection 004, 81.9% of spectral MODIS-Terra AOD retrievals fall within the estimated uncertainty range of +- 0.03 +- 0.05 AOD , while 90.5% of MODIS-Aqua AOD retrievals fall within this uncertainty range. By contrast, in collection 005, these values drop to 50.5% and 78.6%, respectively. Reasons for these differences are currently under investigation. In this paper, we will summarize the comparisons between AATS-14 and both MODIS collections regarding AOD, Angstrom exponent and FMF. Collocated measurements by AATS, MODIS-Terra and MISR within three MISR retrieval grid cells over the Gulf of Mexico on March 10 have been analyzed. Mid-visible (at 446 and 558 nm) AOD retrievals produced by the MISR standard operational algorithm Version 19 compare well both with AATS and with MODIS-Terra observations in two of the three MISR grid cells, while the MISR AOD retrievals in the third cell exceed those obtained from AATS or MODIS. MISR near-IR (866nm) AOD retrievals agree with corresponding AATS and MODIS-Terra retrievals within measurement uncertainty.