Abstract
Abstract. Airborne sunphotometer measurements acquired by the NASA Ames Airborne Tracking Sunphotometer (AATS-14) aboard the NASA P-3 research aircraft are used to evaluate dark-target over-land retrievals of extinction aerosol optical depth (AOD) from spatially and temporally near-coincident measurements by the Moderate Resolution Imaging Spectroradiometer (MODIS) during the summer 2008 Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) field campaign. The new MODIS Collection 6 aerosol data set includes retrievals of AOD at both 10 km × 10 km and 3 km × 3 km (at nadir) resolution. In this paper we compare MODIS and AATS AOD at 553 nm in 58 10 km and 134 3 km retrieval grid cells. These AOD values were derived from data collected over Canada on four days during short time segments of five (four Aqua and one Terra) satellite overpasses of the P-3 during low-altitude P-3 flight tracks. Three of the five MODIS–AATS coincidence events were dominated by smoke: one included a P-3 transect of a well-defined smoke plume in clear sky, but two were confounded by the presence of scattered clouds above smoke. The clouds limited the number of MODIS retrievals available for comparison, and led to MODIS AOD retrievals that underestimated the corresponding AATS values. This happened because the MODIS aerosol cloud mask selectively removed 0.5 km pixels containing smoke and clouds before the aerosol retrieval. The other two coincidences (one Terra and one Aqua) occurred during one P-3 flight on the same day and in the same general area, in an atmosphere characterized by a relatively low AOD (< 0.3), spatially homogeneous regional haze from smoke outflow with no distinguishable plume. For the ensemble data set for MODIS AOD retrievals with the highest-quality flag, MODIS AOD agrees with AATS AOD within the expected MODIS over-land AOD uncertainty in 60% of the retrieval grid cells at 10 km resolution and 69% at 3 km resolution. These values improve to 65 % and 74%, respectively, when the cloud-affected case with the strongest plume is excluded. We find that the standard MODIS dark-target over-land retrieval algorithm fails to retrieve AOD for thick smoke, not only in cloud-contaminated regions but also in clear sky. We attribute this to deselection, by the cloud and/or bright surface masks, of 0.5 km resolution pixels that contain smoke.
Highlights
The 14-channel Ames Airborne Tracking Sunphotometer (AATS-14 or AATS) was operated on a NASA P-3 aircraft based in Cold Lake, Alberta, Canada, during the summer phase of the 2008 Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) field campaign (Jacob et al, 2010)
We compare AATS aerosol optical depth (AOD) from five short time segments of four P-3 flights with AOD retrieved by the Moderate Resolution Imaging Spectroradiometer (MODIS) dark-target land aerosol retrieval algorithm from measurements acquired over land during one Terra and four Aqua satellite overpasses that were spatially and temporally near-coincident with the AATS measurements
We compare AATS and MODIS AOD retrieved from measurements acquired over land during four P-3 flights that included five satellite overpasses on 30 June (Aqua), 2 July (Aqua), 3 July (Terra and Aqua), and 9 July (Aqua) 2008
Summary
The 14-channel Ames Airborne Tracking Sunphotometer (AATS-14 or AATS) was operated on a NASA P-3 aircraft based in Cold Lake, Alberta, Canada, during the summer phase of the 2008 Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) field campaign (Jacob et al, 2010). Shinozuka et al (2011) provided a detailed overview of AOD calculated from AATS measurements during the spring and summer phases of ARCTAS They focused on data obtained during aircraft vertical profiles, for which they compared AOD magnitude and spectral behavior, in addition to particle fine-mode fraction calculated from AATS data, with parameters derived from P-3 in situ sensor measurements. The five MODIS–AATS near-coincident measurement events analyzed in this paper represent the first comparison of airborne sunphotometer AOD with the MODIS C006 10 km and the new 3 km resolution over-land aerosol retrieval products, as well as the largest number of over-land intercomparisons from a single AATS field mission. Airborne studies, with their more precise collocations and case studies, can provide a deeper understanding of algorithmic features that cannot be discerned through the averaging procedures of the larger statistical data bases, especially when inhomogeneous conditions exist
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