Abstract: Airborne sunphotometer measurements of aerosol optical depth and water vapor in ACE-Asia and their comparisons to correlative measurements and calculations (11th Conference on Atmospheric Radiation and the 11th Conference on Cloud Physics)

Monday, 3 June 2002

Airborne sunphotometer measurements of aerosol optical depth and water vapor in ACE-Asia and their comparisons to correlative measurements and calculations

Jens Redemann, NASA/Ames Research Center, Moffett Field, CA; and B. Schmid, J. M. Livingston, P. B. Russell, J. A. Eilers, R. Kolyer, S. Ramirez, R. Kahn, N. C. Hsu, J. Wang, S. Masonis, T. Anderson, D. R. Collins, R. Flagan, J. H. Seinfeld, H. Jonsson, D. Hegg, D. S. Covert, A. Clark, S. Howell, and C. McNaughton

In the Spring 2001 phase of the Asian Pacific Regional Aerosol Characterization Experiment (ACE-Asia), the 6-channel NASA Ames Airborne Tracking Sunphotometer (AATS-6) operated on 15 of the 19 research flights of the NCAR C-130, while its 14-channel counterpart (AATS-14) flew successfully on all 18 research flights of the CIRPAS Twin Otter. ACE-Asia studied aerosol outflow from the Asian continent to the Pacific basin. It was designed to integrate suborbital and satellite measurements and models so as to reduce the uncertainty in calculations of the climate forcing due to aerosols.

AATS-6 and AATS-14 measured solar beam transmission at 6 and 14 wavelengths (380-1021 and 354-1558 nm, respectively), yielding aerosol optical depth (AOD) spectra and column water vapor (CWV). Vertical differentiation in profiles yielded aerosol extinction spectra and water vapor concentration.

In this paper we plan to present examples of the following preliminary findings that are based in part on our airborne sunphotometer measurements:

1. The wavelength dependence of sunphotometer-derived AOD and extinction indicates that supermicron dust was often a major component of the aerosol, frequently extending to high altitudes. In data flights analyzed to date the percentage of full-column AOD (525 nm) that lay above 3 km was typically 34±13%. In contrast, the analogous percentage of columnar water vapor was only 10±4%.

2. Initial comparison studies between AOD data obtained by AATS-6 and AATS-14 during coordinated low-level flight legs show agreement well within the instruments' error bars.

3. Aerosol extinction has been derived from airborne in situ measurements of scattering (nephelometers) and absorption (particle soot/ absorption photometer, PSAP) or calculated from particle size distribution measurements (mobility analyzers and optical particle counters). Comparison with corresponding extinction values derived from the Ames airborne sunphotometer measurements shows good agreement for the vertical distribution of aerosol layers. However, the level of agreement in absolute magnitude of the derived aerosol extinction/optical depth varied among the aerosol layers sampled, yet showed no clear dependence on the measurement technique used and is still being studied.

4. Initial comparisons of sunphotometer-derived AOD to preliminary MISR-derived AOD show that AODs from the unvalidated MISR algorithm exceed sunphotometer-derived AODs.

5. Initial comparisons of sunphotometer-derived AOD to preliminary SeaWiFS-derived AOD show very good agreement if the advanced 4-wavelength SeaWIFS algorithm of Hsu et al. is used, but poorer agreement if the standard algorithm is used.

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