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| Titel |
Combined retrievals of boreal forest fire aerosol properties with a polarimeter and lidar |
| VerfasserIn |
K. Knobelspiesse, B. Cairns, M. Ottaviani, R. Ferrare, J. Hair, C. Hostetler, M. Obland, R. Rogers, J. Redemann, Y. Shinozuka, A. Clarke, S. Freitag, S. Howell, V. Kapustin, C. McNaughton |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 11, no. 14 ; Nr. 11, no. 14 (2011-07-20), S.7045-7067 |
| Datensatznummer |
250009931
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| Publikation (Nr.) |
 copernicus.org/acp-11-7045-2011.pdf |
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| Zusammenfassung |
| Absorbing aerosols play an important, but uncertain, role in the global
climate. Much of this uncertainty is due to a lack of adequate aerosol
measurements. While great strides have been made in observational capability
in the previous years and decades, it has become increasingly apparent that
this development must continue. Scanning polarimeters have been designed to
help resolve this issue by making accurate, multi-spectral, multi-angle
polarized observations. This work involves the use of the Research Scanning
Polarimeter (RSP). The RSP was designed as the airborne prototype for the
Aerosol Polarimetery Sensor (APS), which was due to be launched as part of
the (ultimately failed) NASA Glory mission. Field observations with the RSP,
however, have established that simultaneous retrievals of aerosol absorption
and vertical distribution over bright land surfaces are quite uncertain. We
test a merger of RSP and High Spectral Resolution Lidar (HSRL) data with
observations of boreal forest fire smoke, collected during the Arctic
Research of the Composition of the Troposphere from Aircraft and Satellites
(ARCTAS). During ARCTAS, the RSP and HSRL instruments were mounted on the
same aircraft, and validation data were provided by instruments on an
aircraft flying a coordinated flight pattern. We found that the lidar data
did indeed improve aerosol retrievals using an optimal estimation method,
although not primarily because of the contraints imposed on the aerosol
vertical distribution. The more useful piece of information from the HSRL was
the total column aerosol optical depth, which was used to select the initial
value (optimization starting point) of the aerosol number concentration. When
ground based sun photometer network climatologies of number concentration
were used as an initial value, we found that roughly half of the retrievals
had unrealistic sizes and imaginary indices, even though the retrieved
spectral optical depths agreed within uncertainties to independent
observations. The convergence to an unrealistic local minimum by the optimal
estimator is related to the relatively low sensitivity to particles smaller
than 0.1 (μm) at large optical thicknesses. Thus, optimization
algorithms used for operational aerosol retrievals of the fine mode size
distribution, when the total optical depth is large, will require initial
values generated from table look-ups that exclude unrealistic size/complex
index mixtures. External constraints from lidar on initial values used in the
optimal estimation methods will also be valuable in reducing the likelihood
of obtaining spurious retrievals. |
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