Aerosol optical depth measurements by airborne sun photometer in SOLVE II: Comparisons to SAGE III, POAM III and airborne spectrometer measurements
1NASA Ames Research Center, MS 245-5, Moffett Field, USA
2SRI International, Menlo Park, USA
3Bay Area Environmental Research Institute, Sonoma, USA
4Applied Physics Laboratory, Johns Hopkins University, Laurel, USA
5National Center for Atmospheric Research, Boulder, USA
6NASA Langley Research Center, Hampton, USA
7SAIC, NASA Langley Research Center, Hampton, USA
8Computational Physics, Inc., Springfield, USA
9Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, USA
10Naval Research Laboratory, Washington, DC 20375-5351, USA
Abstract. The 14-channel NASA Ames Airborne Tracking Sunphotometer (AATS-14) measured solar-beam transmission on the NASA DC-8 during the Second SAGE III Ozone Loss and Validation Experiment (SOLVE II). This paper presents AATS-14 results for multiwavelength aerosol optical depth (AOD), including its spatial structure and comparisons to results from two satellite sensors and another DC-8 instrument. These are the Stratospheric Aerosol and Gas Experiment III (SAGE III), the Polar Ozone and Aerosol Measurement III (POAM III) and the Direct-beam Irradiance Airborne Spectrometer (DIAS). AATS-14 provides aerosol results at 13 wavelengths λ spanning the full range of SAGE III and POAM III aerosol wavelengths. Because most AATS measurements were made at solar zenith angles (SZA) near 90°, retrieved AODs are strongly affected by uncertainties in the relative optical airmass of the aerosols and other constituents along the refracted line of sight (LOS) between instrument and sun. To reduce dependence of the AATS-satellite comparisons on airmass, we perform the comparisons in line-of-sight (LOS) transmission and LOS optical thickness (OT) as well as in vertical OT (i.e., optical depth, OD). We also use a new airmass algorithm that validates the algorithm we previously used to within 2% for SZA<90°, and in addition provides results for SZA≥90°.
For 6 DC-8 flights, 19 January–2 February 2003, AATS and DIAS results for LOS aerosol optical thickness (AOT) at λ=400 nm agree to ≤12% of the AATS value. Mean and root-mean-square (RMS) differences, (DIAS-AATS)/AATS, are –2.3% and 7.7%, respectively.
For DC-8 altitudes, AATS-satellite comparisons are possible only for λ>440 nm, because of signal depletion for shorter λ on the satellite full-limb LOS. For the 4 AATS-SAGE and 4 AATS-POAM near-coincidences conducted 19–31 January 2003, AATS-satellite AOD differences were ≤0.0041 for all λ>440 nm. RMS differences were ≤0.0022 for SAGE-AATS and ≤0.0026 for POAM-AATS. RMS percentage differences in AOD ([SAGE-AATS]/AATS) were ≤33% for λ<~755 nm, but grew to 59% for 1020 nm and 66% at 1545 nm. For λ>~755 nm, AATS-POAM differences were less than AATS-SAGE differences, and RMS percentage differences in AOD ([AATS-POAM]/AATS) were ≤31% for all λ between 440 and 1020 nm. Unexplained differences that remain are associated with transmission differences, rather than differences in gas subtraction or conversion from LOS to vertical quantities. The very small stratospheric AOD values that occurred during SOLVE II added to the challenge of the comparisons, but do not explain all the differences.