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1 LA - Laboratoire d-aérologie - LA 2 LOA - Laboratoire d-optique atmosphérique 3 CNRM-GAME - Groupe d-étude de l-atmosphère météorologique 4 LSCE - Laboratoire des Sciences du Climat et de l-Environnement Gif-sur-Yvette 5 GSFC - NASA Goddard Space Flight Center 6 National Observatory of Athens

Abstract : Aerosol absorption properties are of high importance to assess aerosol impact on regional climate. This study presents an analysis of aerosol absorption products obtained over the Mediterranean basin or land stations in the region from multi-year ground-based AERONET observations with a focus on the Absorbing Aerosol Optical Depth AAOD, Single Scattering Albedo SSA and their spectral dependence. The AAOD and Absorption Angström Exponent AAE dataset is composed of daily averaged AERONET level 2 data from a total of 22 Mediterranean stations having long time series, mainly under the influence of urban-industrial aerosols and-or soil dust. This dataset covers the 17-yr period 1996-2012 with most data being from 2003-2011 ~89% of level-2 AAOD data. Since AERONET level-2 absorption products require a high aerosol load AOD at 440 nm > 0.4, which is most often related to the presence of desert dust, we also consider level-1.5 SSA data, despite their higher uncertainty, and filter out data with an Angström exponent < 1.0 in order to study absorption by carbonaceous aerosols. The SSA dataset includes AERONET level-2 products. Sun-photometer observations show that values of AAOD at 440 nm vary between 0.024 ± 0.01 resp. 0.040 ± 0.01 and 0.050 ± 0.01 0.055 ± 0.01 for urban dusty sites. Analysis shows that the Mediterranean urban-industrial aerosols appear -moderately- absorbing with values of SSA close to ~0.94-0.95 ± 0.04 at 440 nm in most cases except over the large cities of Rome and Athens, where aerosol appears more absorbing SSA ~0.89-0.90 ± 0.04. The aerosol Absorption Angström Exponent AAE, estimated using 440 and 870 nm is found to be larger than 1 for most sites over the Mediterranean, a manifestation of mineral dust iron and-or brown carbon producing the observed absorption. AERONET level-2 sun-photometer data indicate a possible East-West gradient, with higher values over the eastern basin AAEEast = 1.39-AAEWest = 1.33. The North-South AAE gradient is more pronounced, especially over the western basin. Our additional analysis of AERONET level-1.5 data also shows that organic absorbing aerosols significantly affect some Mediterranean sites. These results indicate that current climate models treating organics as nonabsorbing over the Mediterranean certainly underestimate the warming effect due to carbonaceous aerosols.





Autor: M. Mallet - O. Dubovik - P. Nabat - F. Dulac - R. Kahn - J. Sciare - D. Paronis - J. F. Léon -

Fuente: https://hal.archives-ouvertes.fr/



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