Use of a Variety of Aerosols Transported off the US Northeast Coast in ICARTT 2004 for Multi-Grid-Box Validation of Satellite Optical Depth Retrievals

Transport of a variety of aerosol types off the US Northeast coast during INTEX/ITCT/ICARTT in Summer 2004 produced a wide range of aerosol optical depth (AOD) values, as well as many cases of horizontal gradients in AOD over the Gulf of Maine. The aerosol types included biomass smoke transported from wildfires in Alaska and Western Canada as well as particles in urban and power plant plumes transported from nearby sources on the New England coast and more distant sources in the Ohio River Valley. In these conditions we flew a sunphotometer on a Jetstream 31 (J31) aircraft across many adjacent aerosol retrieval grid boxes of the satellite radiometers MODIS-Terra, MODIS-Aqua, and MISR, in order to test and inter-compare their AOD retrievals, including their ability to capture the AOD gradients. Characterization of the retrieval environment was aided by using spectral flux radiometers on the J31, to measure water surface albedo spectra (which can contribute the largest uncertainty to satellite aerosol retrievals when AOD is small). Additional characterization came from vertical profiles by the J31 (showing aerosol vertical structure) and, on occasion, shipboard measurements of AOD, scattering, and absorption, and DC-8 measurements of aerosol scattering, absorption, size, composition, and lidar backscatter. The J31 sunphotometer was the 14-channel NASA Ames Airborne Tracking Sunphotometer (AATS-14), which provides AOD at thirteen discrete wavelengths, 354-2138 nm, spanning the range of aerosol retrieval wavelengths for MODIS over ocean (470-2130 nm) and MISR (446-866 nm). The J31 flux radiometers were the Solar Spectral Flux Radiometers (SSFR), which provide spectra of upwelling and downwelling irradiance at resolution 8-12 nm from 350 to 1670 nm. We show results from 11 J31 flights made 12 July-8 August 2004. These include comparisons of AATS AOD spectra to spaceborne retrievals for more than 88 grid boxes of MODIS-Terra, MODIS-Aqua, and MISR that were either along the J31 flight path or adjacent to such grid boxes. Midvisible AOD ranged from 0.05 to 1.2, with a wide range of AOD gradients. Results are summarized in terms of scatter plots, and reasons for agreement and disagreement are explored. Included is a case where smoke over cloud impaired the MODIS-Aqua cloud mask, resulting in retrieved AODs that significantly exceeded the AATS values.