Air Pollution as a Climate Forcing: A Workshop
Day 2 Presentations
Global Aerosol Observations from MODIS on Terra Satellite
Yoram Kaufman*, Lorraine Remer*, Didier Tanré+ and Mian Chin*
* NASA Goddard Space Flight Center, Greenbelt, MD, U.S.A.
+ Lab. d'Optique Atmos., CNRS, Univ. de Sciences et Techniques de Lille, France
Satellite observations can measure daily some key properties of the global aerosol system. In the last few years we saw the launch of spaceborne instruments that are at least partially designed for aerosol measurements. In the talk I show one year of aerosol data from the MODIS instrument. MODIS derives the aerosol optical thickness, a measure of the column aerosol concentration and distinguishes between fine submicron particles and coarse micron size particles. Instruments that are better suited for aerosol measurements are being considered for future flights.
|Figure 1: Global aerosol on April 16, 2001. East Asia dust transported to North America. View from MODIS (right) and GOCART model (left).||Figure 2: Smoke is observed only in the visible wavelengths. Dust is observed across the whole solar spectrum.|
Two instruments on the Terra satellite, launched by NASA in Dec. 1999, measure global aerosol concentrations and properties. MODIS is one of them. It is sensitive to the aerosol spectral signature across the solar range (0.47 to 2.1 µm). New techniques are used to derive aerosol information over land and ocean.
Over the dark ocean, MODIS uses the spectral signature of aerosol reflection of sunlight to distinguish fine aerosols associated with smoke from vegetation fires and urban/industrial pollution aerosol, from coarse sea-salt and dust. Fine particles (0.3-0.5 µm diameter) reflect sunlight in the visible and UV parts of the spectrum but are transparent at longer wavelengths. Coarse particles (2-10 µm diameter) reflect about equally across the solar spectrum.
Over the land, MODIS takes advantage of the transparency of smoke and urban pollution at 2.1 µm to observe the surface cover and estimate surface reflectances in the visible part of the solar spectrum. The aerosol AOT is then derived from the excess reflectance at the top of the atmosphere.
MODIS measures the optical thickness (AOT) of the fine and coarse aerosols. The AOT measures the attenuation of sunlight transmitted through the aerosol layer and is proportional to the aerosol concentration in the atmospheric column. Typically oceanic aerosol has total AOT of 0.07 at 0.55 µm, heavy pollution in US or Europe has AOT of 0.5-1.0, and regional smoke in the Amazon of 1.0 to 2.0. The estimated error of AOT over the ocean is ±0.03 AOT, and ±0.05 AOT over the land.
MODIS ability to distinguish fine from coarse particles can help to separate among the different aerosol types. Figure 3 shows an example of an image over East Asia. A dust plume over regional pollution is observed by the color composite on the left, and detected by MODIS on the right.
The data are used to observe the aerosol dynamics (a movie of one year of data was shown in the talk) and to improve aerosol simulations. Figure 4 shows an example of comparison between MODIS monthly data and model simulation.