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| Titel |
Long-term changes of aerosol optical and radiative properties and their role in global dimming and brightening |
| VerfasserIn |
N. Hatzianastassiou, C. D. Papadimas, C. Matsoukas, K. Pavlakis, A. Fotiadi, M. Wild, I. Vardavas |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250029942
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| Zusammenfassung |
| Global dimming and brightening (GDB) have profound effects on the Earth’s environment.
For example, GDB counteracts or supplements greenhouse warming. Atmospheric aerosols,
through their interaction with solar radiation (direct, indirect and semi-direct effects) can
affect GDB. Changes in aerosol burden or other physical and optical properties can modify
tendencies of GDB. For example, satellite observations of aerosol amounts, available since
the early 1980s, but only over the oceans, indicate a downward trend since about 1990,
consistent with the observed brightening during this period. There is a need, however, to
investigate similar trends, but also over land, and to relate them with contemporary
GDB.
The seasonal and inter-annual variability of the natural, but also anthropogenic aerosol
direct radiative effect on solar radiation at the Earth’s surface (DREsurf) and the
contribution of aerosols to global dimming and brightening (GDB) is estimated
over the period 1984-2001. This is achieved by using a spectral radiative transfer
model together with Total Ozone Mapping Spectrometer (TOMS) aerosol optical
thickness (AOT) and other satellite (International Satellite Cloud Climatology Project,
ISCCP-D2), NCEP/NCAR reanalysis and Global Aerosol Data Set (GADS) data
for surface and atmospheric parameters. The major findings are mostly related to
natural and less to anthropogenic aerosols because of limitations of the TOMS
observational technique. The model results indicate that aerosols exert a strong surface
cooling over the globe by reducing locally the incoming surface solar radiation by up
to 70 W m-2. This direct radiative effect averaged over the globe for the period
1984-2001, is equivalent to 5 W m-2, associated with 6.5 and 3.5 W m-2, for the
Northern and Southern Hemispheres, respectively. However, this aerosol DREsurf
effect shows an important inter-annual variability as large as 200%. A strong solar
brightening, or decreased aerosol DREsurf, by as much as 13 W m-2, from 1984 to
2001 is found in the northern tropical and sub-tropical latitudes. This is due to
decreased AOT levels in areas with strong production and transport of dust by trade
winds, and in the northern mid-to-high latitudes, e.g. in South Europe and East
USA due to clean-air legislation and decreased emissions. In contrast, significant
increases in AOT (by more than 100%) and DREsurf (by more than 17 W m-2)
are found in areas like East Asia (China) or the southern Amazonian basin, where
increasing emissions of anthropogenic aerosols and biomass burning activities have
persisted from 1984 to 2001. On a global basis (60Ë S-60Ë N) the AOT has slightly
increased (by about 4%) over the two-decadal study period, mainly in the Southern
Hemisphere. Consequently, the magnitude of aerosol DREsurf has also increased
by 0.38±0.1 W m-2 (or by 6%) indicating thus an aerosol solar dimming from
1984 to 2001. The aerosol-induced dimming in the 1980s is in accord with the
model-estimated overall solar dimming during the same period, whereas the aerosol
dimming during the 1990s and up until 2001 is not in accord with the estimated
solar brightening in the same period. Although on a global basis the contribution of
aerosols to GDB can be exceeded by the effects of other radiative components such
as clouds, aerosols are found to significantly contribute to GDB at the regional
scale, being responsible for almost 100% of the solar brightening over large areas
extending from the Arabian Sea through North Africa and the tropical Atlantic
Ocean, which is characterized by transport of African dust over to the coast of
South America. Aerosols are also responsible for solar dimming over areas such
as East China or the South Amazonian basin. The surface cooling due to aerosol
dimming, and especially its spatial and temporal variability, can have significant
effects on the hydrological cycle, surface energy budget and atmospheric dynamics. |
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