Aerosol optical depth measurements by airborne sun photometer in SOLVE II: Comparisons to SAGE III, POAM III and airborne spectrometer measurements
[摘要] 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 ValidationExperiment (SOLVE II). This paper presents AATS-14 results for multiwavelength aerosoloptical depth (
AOD), including comparisons to results from two satellite sensors and anotherDC-8 instrument, namely the Stratospheric Aerosol and Gas Experiment III (SAGE III), thePolar Ozone and Aerosol Measurement III (POAM III) and the Direct-beam Irradiance AirborneSpectrometer (DIAS). AATS-14 provides aerosol results at 13 wavelengths λ spanning therange of SAGE III and POAM III aerosol wavelengths. Because most AATS measurements weremade at solar zenith angles (
SZA) near 90°, retrieved
AODs are strongly affected byuncertainties in the relative optical airmass of the aerosols and other constituents along the lineof sight (LOS) between instrument and sun. To reduce dependence of the AATS-satellitecomparisons on airmass, we perform the comparisons in LOS transmission and LOS opticalthickness (OT) as well as in vertical OT (i.e., optical depth,
OD). We also use a new airmassalgorithm that validates the algorithm we previously used to within 2% for
SZA<90°, and inaddition provides results for
SZA≥90°.
For 6 DC-8 flights, 19 January-2 February 2003, AATS and DIAS results for LOS aerosol OT atλ=400nm 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>440nm, because ofsignal depletion for shorter λ on the satellite full-limb LOS. For the 4 AATS-SAGE and4 AATS-POAM near-coincidences conducted 19-31 January 2003, AATS-satellite AODdifferences were ≤0.0041 for all λ>440nm. RMS differences were ≤0.0022 for SAGE-AATSand ≤0.0026 for POAM-AATS. RMS relative differences in AOD ([SAGE-AATS]/AATS) were≤33% for λ<~755nm, but grew to 59% for 1020nm and 66% at 1545nm. For λ>~755nm,AATS-POAM differences were less than AATS-SAGE differences, and RMS relativedifferences in AOD ([AATS-POAM]/AATS) were ≤31% for all λ between 440 and 1020nm.Unexplained differences that remain are associated with transmission differences, rather thandifferences in gas subtraction or conversion from LOS to vertical quantities. The very smallstratospheric AOD values that occurred during SOLVE II added to the challenge of thecomparisons, but do not explain all the differences.
