If SSA at 1020nm value is much less than other wavelengths, i.e. 400, 500, 680, 870, derived through radiance inversion technique; this variation is observed throughout the year, only the value changes slightly. In other words from 400nm to 870nm there is a modest increase in tendency in SSA values thereafter it reduces sharply in 1020nm wavelength. Please also provide some relevant reference if available.
The determination of optical parameters (absorption coefficient, scattering coefficient, extinction coeffitient, SSA) as a function of wavelength has been used as a way to distinguish between various aerosol types. To investigate this wavelength dependence, it is calculated the Angstrom exponent, which is defined as the negative slope of the logarithm of the optical paramenter as a function of wavelength
The scattering and absorption Angstrom exponents are used as qualitative indicators of particle size and chemical composition. Smaller scattering Angstrom exponents are associated with larger particles and higher scattering Angstrom exponents are associated with smaller particles. Also, specific values of the absorption Angstrom exponents are characteristic of the chemical specie. For example, 1 for urban aerosol, higer than 1 for carbonaceous particles resulting from coal combustion, etc...
The wavelength dependence of SSA is also influenced by various aerosol species. For example, for urban-industrial aerosols and biomass burning, SSA decreases with increasing wavelength, whereas for desert dust, SSA increases with increasing wavelength. Some dust particles are strong absorbers, and the increase of absorption together with the decrease of scattering can also contribute to the change of the slope of SSA from positive to negative.
You can see, for example:
Mogo, S., Cachorro, V. E., Lopez, J. F., Montilla, E., Torres, B., Rodríguez, E., Bennouna, Y., and de Frutos, A. M.: In situ measurements of aerosol optical properties and number size distributions in a coastal region of Norway during the summer of 2008, Atmos. Chem. Phys., 12, 5841-5857, doi:10.5194/acp-12-5841-2012, 2012.
Virkkula, A., Backman, J., Aalto, P. P., Hulkkonen, M., Riuttanen, L., Nieminen, T., dal Maso, M., Sogacheva, L., de Leeuw, G., and Kulmala, M.: Seasonal cycle, size dependencies, and source analyses of aerosol optical properties at the SMEAR II measurement station in Hyytiälä, Finland, Atmos. Chem. Phys., 11, 4445-4468, doi:10.5194/acp-11-4445-2011, 2011.
Dubovik, O., Holben, B., Eck, T., Smirnov, A., Kaufman, Y., King, M., Tanré, D., and Slutsker, I.: Variability of absorption and optical properties of key aerosol types observed in worldwide locations, J. Atmos. Sci., 59, 590–608, 2002.
Thanks for the response. However, similar variation is reported by some other observations over India and China but reasons for such variation is not elaborated. the references also do not speak of such a variation. Apparently spectral nephelometer measurements may help to know more, so are the particle size and chemical analysis. The said spectral variation of SSA i.e. generally increasing from 400 to 870 and decreasing more or less sharply at 1020nm is a perennial feature over a muti-mining region with good amount of forest cover. Dubovik et al talks about variations from 400 to 1020 but variations similar to ours is not seen at the AERONET stations I looked at. May be an algorithm filtering difference?
I'm working mainly with in situ data in the UV and visible, so I don't know about that! However I'm also interested in know that answer.
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