11th Conference on Atmospheric Radiation and the 11th Conference on Cloud Physics

Monday, 3 June 2002: 9:00 AM
Overview of ACE-Asia Spring 2001 investigations on aerosol-radiation interactions
P. B. Russell, NASA/Ames Research Center, Moffett Field, CA; and P. J. Flatau, F. P. J. Valero, T. Nakajima, B. Holben, P. Pilewskie, M. Bergin, B. Schmid, R. W. Bergstrom, A. Vogelmann, B. Bush, J. Redemann, S. Pope, J. Livingston, S. Leitner, N. C. Hsu, J. Wang, J. Seinfeld, D. Hegg, P. Quinn, and D. Covert
Poster PDF (724.4 kB)
In spring 2001 the Asia-Pacific Regional Aerosol Characterization Experiment (ACE-Asia) made extensive measurements from land, ocean, air and space platforms. A primary objective was to quantify the interactions between aerosols and radiation. This talk presents illustrative results from each type of platform, with initial assessments of regional aerosol radiative forcing obtained by combining satellite and suborbital results.

Early results include:

AERONET sun-sky radiometry at 13 stations in the region yielded time series of multiwavelength aerosol optical depth (AOD), Angstrom exponent, single-scattering albedo (SSA) at 440 and 870 nm, and size distribution. Results from Beijing and the Kosan, Korea supersite show marked anticorrelation between AOD and Angstrom exponent (caused by transient dust events), dust volume-size modes peaking at diameters near 8 microns, and dust SSA exceeding pollution SSA.

Measurements of photosynthetically active radiation in Yulin, China, combined with simultaneous aerosol measurements and radiative transfer calculations, yielded a 24-h average downward surface radiative forcing efficiency of -27 W m-2 (400-700 nm) per 500-nm AOD.

The R/V Brown cruise from Honolulu to Sea of Japan sampled an AOD gradient, with AOD(500 nm) varying between 0.1 and 1.1. In situ measurements of scattering and absorption showed that adding dust to pollution increased SSA(550 nm), typically from ~0.91 to ~0.97 for RH=55%. FTIR measurements of downwelling longwave radiance revealed aerosol effects in the 8-12 micron window, including signatures during the major dust event of early April. Longwave forcing estimated from radiance measurements was W m -2.

Vertical profiles of aerosol extinction from airborne sunphotometry and total-direct and diffuse spectral fluxes showed wavelength dependence that often varied strongly with height, in accord with the frequent layering of dust-dominated over pollution-dominated aerosols. Comparisons between sunphotometric extinction profiles and those from in situ measurements (size distribution and size-resolved composition, or scattering and absorption) showed good agreement in aerosol layer heights, but some differences in extinction and AOD, which are still being studied.

Solar spectral flux radiometry from different altitudes yielded absorption spectra for intervening atmospheric layers. Combining these with AOD spectra yields best-fit aerosol single scattering albedo spectra.

Visible, near-IR, and total solar fluxes combined with AOD give radiative forcing efficiencies at surface and aloft.

An advanced SeaWiFS retrieval (4 wavelengths, 440 to 860 nm) produces AOD values over water that agree with airborne sunphotometer measurements to less than ~0.04, a considerable improvement over the standard 1-wavelength SeaWiFS algorithm.

Combining monthly-average satellite AOD fields with aerosol intensive properties from suborbital measurements yields estimated aerosol forcing maps for the ACE-Asia region. For example, April 2001 standard SeaWiFS AODs combined with a model of dust over pollution yield a plume of surface shortwave radiative forcing east of China with monthly-average values exceeding 20 W m-2 for cloud-free skies.

Primary paths toward improved radiative forcing estimates include better characterizing dust, pollution, and mixed aerosol longwave and shortwave properties, validating and using more advanced satellite products (e.g., from EOS Terra), extending retrievals and calculations over land, improving surface albedo spectra, and accounting for clouds.

Supplementary URL: http://