Atmospheric Chemistry and Physics
www.atmos-chem-phys.net
1680-7316
1680-7324
5
5
2005
10.5194/acp-5-1311-2005
http://www.atmos-chem-phys.net/5/1311/2005/
http://www.atmos-chem-phys.net/5/1311/2005/acp-5-1311-2005.html
http://www.atmos-chem-phys.net/5/1311/2005/acp-5-1311-2005.pdf
1311
1339
2005-06-01
Aerosol optical depth measurements by airborne sun photometer in SOLVE II: Comparisons to SAGE III, POAM III and airborne spectrometer measurements
P. Russell
J. Livingston
B. Schmid
J. Eilers
R. Kolyer
J. Redemann
S. Ramirez
J.-H. Yee
W. Swartz
R. Shetter
C. Trepte
A. Risley Jr.
B. Wenny
J. Zawodny
W. Chu
M. Pitts
J. Lumpe
M. Fromm
C. Randall
K. Hoppel
R. Bevilacqua
NASA Ames Research Center, MS 245-5, Moffett Field, CA 94035-1000, USA
SRI International, Menlo Park, CA 94025, USA
Bay Area Environmental Research Institute, Sonoma, CA 95476, USA
Applied Physics Laboratory, Johns Hopkins University, Laurel, MD 20723-6099, USA
National Center for Atmospheric Research, Boulder, CO 80307-3000, USA
NASA Langley Research Center, Hampton, VA 23681-2199, USA
SAIC, NASA Langley Research Center, Hampton, VA 23681-0001, USA
Computational Physics, Inc., Springfield, VA 22151, USA
Naval Research Laboratory, Washington, DC 20375-5351, USA76, USA
Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO 80309-0392, USA
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 (<I>AOD</I>), including comparisons to results from two satellite sensors and another
DC-8 instrument, namely 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
range of SAGE III and POAM III aerosol wavelengths. Because most AATS measurements were
made at solar zenith angles (<I>SZA</I>) near 90°, retrieved <I>AOD</I>s are strongly affected by
uncertainties in the relative optical airmass of the aerosols and other constituents along the line
of sight (LOS) between instrument and sun. To reduce dependence of the AATS-satellite
comparisons on airmass, we perform the comparisons in LOS transmission and LOS optical
thickness (OT) as well as in vertical OT (i.e., optical depth, <I>OD</I>). We also use a new airmass
algorithm that validates the algorithm we previously used to within 2% for <I>SZA</I><90°, and in
addition provides results for <I>SZA</I>≥90°.
<P style="line-height: 20px;">
For 6 DC-8 flights, 19 January-2 February 2003, AATS and DIAS results for LOS aerosol OT at
λ=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 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 <I>AOD</I>
differences were ≤0.0041 for all λ>440nm. RMS differences were ≤0.0022 for SAGE-AATS
and ≤0.0026 for POAM-AATS. RMS relative differences in <I>AOD</I> ([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 relative
differences in <I>AOD</I> ([AATS-POAM]/AATS) were ≤31% for all λ between 440 and 1020nm.
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 <I>AOD</I> values that occurred during SOLVE II added to the challenge of the
comparisons, but do not explain all the differences.