On uncertainty of determination of particle optical thickness in atmospheric environment

Details On uncertainty of determination of particle optical thickness in atmospheric environment On uncertainty of determination of particle optical thickness in atmospheric environment

Apr 2002

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002coska..32....5k&link_type=abstract

Contributions of the Astronomical Observatory Skalnate Pleso, vol. 32, no. 1, p. 5-24.

Astronomy and Astrophysics

Astronomy

Particle Optical Thickness, Diffuse Radiation, Extinction, Optical Air Mass

Scientific paper

The uncertainty of particle optical thickness retrieval in the Earth's atmosphere is estimated. There are some specific factors, which are notoriously not evaluated i) the intensity distribution on the solar disk, ii) changes of the optical air mass along the solar disk, and iii) diffuse radiation in the solar aureola, which represents an addition to the measured intensity because of the finite instrument field-of-view. It is shown that the first two factors, in general, reduce the final value of the aerosol optical thickness tau_a, so that the corrected value tau_a^C looks to be less than the approximate value tau_a^A obtained by a simple retrieval mechanism. However, diffuse radiation in the aureola region causes an increasing of tau_a^C by a factor Q. The corrected value of tau_a^C can finally be about 2-3% larger than tau_a^A. This difference varies with modal radius of the aerosol size distribution, due to sensitive dependence of the scattering pattern (in the near-forward scattering region) on particle size. An error level (2-3%) cannot be ignored, because it is comparable to the standard systematic/random measurement error approx. 4-5%. It is shown that such a small uncertainty in the aerosol optical thickness can produce large changes in the solution of the inverse problem yielding the size distribution of the aerosol particles f(r). This influences data processing and it is, for instance, a source of new open questions when interpreting the measured brightness of F-corona (to retrieve the number density of interplanetary dust particles). The gained range of the possible solutions of f(r) then has a direct impact on the calculation of radiation fluxes in the atmosphere. The uncertainty in estimation of the radiative balance may change the view of chemical and physical processes in the planetary atmosphere, and may be a reason for partially inaccurate or inadequate physical conclusions.

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