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| Titel |
Large radiative forcing efficiency of atmospheric aerosols over the Himalaya |
| VerfasserIn |
Daniele Gasbarra, Alcide di Sarra, Daniela Meloni, Paolo Bonasoni, Claudia Di Biagio, Gian Paolo Gobbi, Angela Marinoni, Gian Pietro Verza, Elisa Vuillermoz |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250097189
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| Publikation (Nr.) |
 EGU/EGU2014-12741.pdf |
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| Zusammenfassung |
| This study is based on measurements made at the Nepal Climate Observatory at Pyramid
(NCO-P, 27.95 N, 86.82 E), located at 5079 m altitude in the Sagamartha National Park,
Eastern Nepal Himalaya. We analised seasonal variations of solar downward irradiance (SW),
columnar water vapour content (wv), aerosol optical depth at 500 nm (Ï) and surface albedo
(A) in the period between 2007 and 2010, in order to obtain the radiative perturbations
produced by aerosols in the SW. SW measurements are carried out by a CMP21 pyranometer,
while A is derived from a CNR1 radiometer. Values of wv and Ï are retrieved from the
measurements by the EVK2-CNR Cimel sunphotometer operating within the AERONET
network.
Ï was found to be lower than 0.1 in 98% of the cases. However, during the pre-monsoon
season, especially in the months of April and May, cases with Ï reaching 0.27 were
recorded.
The aerosol surface shortwave radiative effect in cloud-free periods was estimated during
the elevated aerosol optical depth cases using different methods. The “hybrid method” was
applied using experimental measurements of solar downward irradiance and simulations
made with the MODTRAN (MODerate resolution atmospheric TRANsmission) model. The
dependency of SW on A and wv was determined from MODTRAN simulations, and was
used to correct experimental measurements for albedo and water vapour changes. The
radiative perturbation produced by aerosol was thus obtained as the difference between
the measured irradiances and the modelled values for aerosol-free conditions and
the same water vapour and albedo values, and at the same solar zenith angle. The
aerosol radiative effect was also derived by comparing elevated and low aerosol
optical depth cases, at similar values of solar zenith angle, albedo, and column
water vapour. In addition the direct method, relating SW to changes in Ï, was also
used.
These three methods produce consistent results. Although the overall aerosol radiative
perturbation is small, it becomes relatively large during elevated aerosol cases. The radiative
forcing efficiency (radiative effect produced by a unit aerosol optical depth) is significantly
larger than at other sites worldwide, reaching values above 360 W/m2 at about 50Ë
solar zenith angle. The maximum radiative effect is about -90±18 Wm-2 (for
Ï=0.25), corresponding to a reduction by more than 10% of the solar radiation at the
surface.
During these elevated aerosol events high concentrations of pollutants were measured:
PM10 and PM 2.5 showed concentrations higher than 50 ng m-3, while the black carbon
concentration reached 3000 ng m-3. The backtrajectory analysis for the elevated aerosol
cases shows that the polluted airmasses observed at NCO-P come from Indo-Gangetic plain
and Punjab, regions characterized by the highest industrial and demographic concentration of
the Indian subcontinent. |
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