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| Titel |
Uncertainties of aerosol retrieval from neglecting non-sphericity of dust aerosols |
| VerfasserIn |
Chi Li, Yong Xue, Leiku Yang, Jie Guang |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250071834
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| Zusammenfassung |
| The Mie theory is conventionally applied to calculate aerosol optical properties in satellite
remote sensing applications, while dust aerosols cannot be well modeled by the Mie
calculation for their non-sphericity. It has been cited in Mishchenko et al. (1995; 1997) that
neglecting non-sphericity can severely influence aerosol optical depth (AOD, Ï) retrieval in
case of dust aerosols because of large difference of phase functions under spherical
and non-spherical assumptions, whereas this uncertainty has not been thoroughly
studied.
This paper aims at a better understanding of uncertainties on AOD retrieval caused by
aerosol non-sphericity. A dust aerosol model with known refractive index and size
distribution is generated from long-term AERONET observations since 1999 over China.
Then aerosol optical properties, such as the extinction, phase function, single scattering
albedo (SSA) are calculated respectively in the assumption of spherical and non-spherical
aerosols. Mie calculation is carried out for spherical assumption, meanwhile for
non-spherical aerosol modeling, we adopt the pre-calculated scattering kernels and software
package presented by Dubovik et al. (2002; 2006), which describes dust as a shape
mixture of randomly oriented polydisperse spheroids. Consequently we generate two
lookup tables (LUTspheric and LUTspheroid) from simulated satellite received
reflectance at top of atmosphere (TOA) under varieties of observing conditions
and aerosol loadings using Second Simulation of a Satellite Signal in the Solar
Spectrum – Vector (6SV) code. All the simulations are made at 550 nm, and for
simplicity the Lambertian surface is assumed. Using the obtained LUTs we examine the
differences of TOA reflectance (ΔÏTOA = Ïspheric - Ïspheroid) under different surface
reflectance and aerosol loadings. Afterwards AOD is retrieved using LUTspheric from the
simulated TOA reflectance by LUTspheroid in order to detect the retrieval errors
(ΔÏ = Ïretreived -Ïinput) induced by straightforwardly utilizing Mie theory in dust aerosol
retrieval.
As expected we find that the uncertainties mainly result from the obvious difference of
phase functions (Pspheric and Pspheroid). Errors may be positive or negative, depending on
the specific geometry. In scattering angle (Î) regions where Psphericis greater (30º~85º &
145º~180º), we generally get positive ΔÏTOA and negative ΔÏ, and vice versa (85º~145º).
For low aerosol loading (Ï ~0.25) and black surface, |ΔÏTOA| could be greater than 0.004
and 0.012 around Î ~120º and Î ~170º, with |ΔÏ| of ~0.04 and ~0.12 respectively. In
most back scattering cases (Î >100º), the magnitude of ΔÏ is about ten times that of
ΔÏTOA, while this ratio (|ΔÏ|/|ΔÏTOA|) significantly reduces to as low as ~0.5
for forward scattering, and can reach ~20 at Î ~145º. Moreover, this errors and
|ΔÏ|/|ΔÏTOA| can increase more than ten times as aerosol loading gets higher
and surface gets brighter. Therefore we conclude that the neglect of non-sphericity
introduces substantial errors on radiative transfer simulation and AOD retrieval.
As a result of this study, a representative aspheric aerosol model other than Mie
calculation is recommended for inversion algorithms related with dust-like non-spherical
aerosols.
References
Dubovik, O., Holben, B. N., Lapyonok, T., Sinyuk, A., Mishchenko, M. I., Yang, P., and
Slutsker, I. (2002). Non-spherical aerosol retrieval method employing light scattering by
spheroids. Geophyscal Research Letters, 29(10), 1415, doi:10.1029/2001GL014506.
Dubovik, O., Sinyuk, A., Lapyonok, T., Holben, B. N., Mishchenko, M., Yang, P., Eck, T.
F., Volten, H., Muñoz, O., Veihelmann, B., van der Zande, W. J., Leon, J.-F., Sorokin, M., and
Slutsker, I. (2006). Application of spheroid models to account for aerosol particle
nonsphericity in remote sensing of desert dust. Journal of Geophysical Research, 111,
D11208, doi:10.1029/2005JD006619.
Mishchenko, M. I., Lacis, A. A., Carlson, B. E., and Travis, L. D. (1995). Nonsphericity
of dust-like aerosols: Implications for aerosol remote sensing and climate modeling,
Geophyscal Research Letters, 22, 1077– 1080.
Mishchenko, M. I., Travis, L. D., Kahn, R. A., and West, R. A. (1997). Modeling phase
functions for dustlike tropospheric aerosols using a shape mixture of randomly oriented
polydisperse spheroids, Journal of Geophysical Research, 102, 16831– 16847. |
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