Angstrom exponent

The common assumption the ai = a2 = a can lead to errors for the urban, maritime, and rural aerosol profiles. The Angstrom exponents are determined as a function of aerosol type and relative humidity (cf. Appendix B of Gueymard.)7... 

Angstrom exponent scattering efficiency of substance X (m g ) upscatter fraction axial angle (rad) solar zenith angle (rad) wavelength (nm)... 

The symbol a is widely used to represent either the mass scattering efficiency or the Angstrom exponent, so care must be exercised to avoid confusion. 

Another important aerosol optical property is column aerosol extinction, the vertical integral of the aerosol extinction coefficient, again a function of wavelength. This quantity is often also denoted aerosol optical thickness (AOT) or aerosol optical depth. The wavelength dependence of AOT is also often expressed as an Angstrom exponent. 

Figure 17 Time series of AOT (top) and Angstrom exponent (bottom) at the Department of Energy Atmospheric Radiation Measurement site in North Central Oklahoma over the period 1993-1999. Measurements are by sun-photometry. The points represent daily averages the curves are smooth fits of the data to guide the eye (Michalsky et aL, 2001) (reproduced by permission of American Geophysical Union from /. Geophys. Res. 2001,106, 12099-12107).

Figure 17 also shows the Angstrom exponent (cf. Section 4.04.2) evaluated from the dependence of aerosol optical thickness, r p, on wavelength A SiS a = — d log Tgp/d log A. A greater Angstrom... 

Figure 18 Global distribution of AOT t at 865 nm (upper panel) and dependence on wavelength A as the Angstrom exponent, a= — d In r/d In A (lower panel) for June 1997. The distributions were derived from radiance measurements at 18-km resolution by the POLDER (POLarization and Directionality of the Earth s Reflectance) radiometer aboard the ADEOS (ADvanced Earth Observing Satellite) platform. Retrieval is limited to the atmosphere above water surfaces. Courtesy of Laboratoire d Optique Atmospherique, Lille, France Laboratoire des Sciences du Climat et de TEnvironnement, Gif sur Yvette, France Centre National d etudes Spatiales, Toulouse, France and National Space Development Agency, Japan. For further information see http //earth-sciences.cnes.fr 8060/polder/...

I Angstrom Exponent It has proved to be useful in some cases to represent the wavelength dependence of the aerosol extinction coefficient fcext, by beM X . The exponent, d, is called the Angstrom exponent. The Angstrom exponent is calculated from measured values of the extinction coefficient as a function of... 

FIGURE 15.9 Angstrom exponent for a lognormally distributed water aerosol (ag = 2.0) with refractive index m — 1.33 — Oi in the wavelength range X. =- 550 to 700nm. (Courtesy of J. A. Ogren.)... 

CO Block The coordinates are stored as integer values. The actual units used (angstrom, au, relative screen units, etc.) are specified in the starting record, while the exponent to convert the integers to the true floating point values is given in the first data record. 

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