The Angstrom exponent denotes the spectral slope of columnar aerosol optical depths. It is well known that high Angstrom exponent (> 1) correspond to fine-mode aerosol dominance and vice versa. What does the negative values of Angstrom exponent correspond to?
To my understanding, those AE values (negative) close to zero still indicate the presence of (desert) dust. For example:
See: Fig. 8 (h) of Holben et al. (2001) JGR
See: Fig. 2 of Prasad & Singh (2007) JGR - [Kanpur]
{There are other relevant literature too}.
Negative AE occurs due to higher AOD in the longer wavelengths. For example, in monsoon because of the rain atmosphere used to very clean and moisture content is very high, which cause the higher attenuation in longer wavelengths (700 nm -900nm). Sometimes, AE has been seen in the range of -1 to -6, depends upon the region.
They are valid and indicate coarse mode dominated aerosol distribution. Basic assumption in deriving Angstrom exponent from a pair of spectral AOD is that the aerosol distribution is unimodal. In reality it is bimodal ( - usually in the optically active wavelength range) or multimodal (in general).
Some key publications:
Article Bimodal size distribution influences on the variation of Ang...
https://doi.org/10.1029/2005JD006328
Also, high AE (usually higher than 1.0) combined with high AOD, usually indicate dust events. However, in general view point, AE show the dominant presence of the coarse aerosol, which implies not only dust, but also other types of coarse aerosol.
The negative value of AE mainly represent the presence of coarse mode aerosol that may be be due to dust in summer and also due to hygroscopic growth of particles.
Hello Barman, Mandija and Shani,
Can you please provide the literature references for your interpretations?
Look at these articles (and references herein), to use AE for discrimination of aerosol types:
https://www.esrl.noaa.gov/gmd/publications/annual_meetings/2014/slides/43-140331-C.pdf
http://www.aaqr.org/files/article/1376/22_AAQR-14-07-OA-0143_985-993.pdf
https://www.atmos-chem-phys.net/10/1155/2010/acp-10-1155-2010.pdf
It can also be an error. Because, when the absorption coefficients are very low, the error in calculating its exponent can be very high.
Hope it helps!
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