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| Titel |
Spectral dependence of aerosol light absorption over the Amazon Basin |
| VerfasserIn |
L. V. Rizzo, A. L. Correia, P. Artaxo, A. S. Procópio, M. O. Andreae |
| 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. 17 ; Nr. 11, no. 17 (2011-09-01), S.8899-8912 |
| Datensatznummer |
250010044
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| Publikation (Nr.) |
 copernicus.org/acp-11-8899-2011.pdf |
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| Zusammenfassung |
| In this study, we examine the spectral dependence of aerosol absorption at
different sites and seasons in the Amazon Basin. The analysis is based on
measurements performed during three intensive field experiments at a pasture
site (Fazenda Nossa Senhora, Rondônia) and at a primary forest site
(Cuieiras Reserve, Amazonas), from 1999 to 2004. Aerosol absorption spectra
were measured using two Aethalometers: a 7-wavelength Aethalometer (AE30)
that covers the visible (VIS) to near-infrared (NIR) spectral range, and a
2-wavelength Aethalometer (AE20) that measures absorption in the UV and in
the NIR. As a consequence of biomass burning emissions, about 10 times
greater absorption values were observed in the dry season in comparison to
the wet season. Power law expressions were fitted to the measurements in
order to derive the absorption Ångström exponent, defined as the
negative slope of absorption versus wavelength in a log-log plot. At the
pasture site, about 70 % of the absorption Ångström exponents fell
between 1.5 and 2.5 during the dry season, indicating that biomass burning
aerosols have a stronger spectral dependence than soot carbon particles.
Ångström exponents decreased from the dry to the wet season, in
agreement with the shift from biomass burning aerosols, predominant in the
fine mode, to biogenic and dust aerosols, predominant in the coarse mode.
The lowest absorption Ångström exponents (90 % of data below 1.5)
were observed at the forest site during the dry season. Also, results
indicate that low absorption coefficients were associated with low
Ångström exponents. This finding suggests that biogenic aerosols
from Amazonia have a weaker spectral dependence for absorption than biomass
burning aerosols, contradicting our expectations of biogenic particles
behaving as brown carbon. In a first order assessment, results indicate a
small (<1 %) effect of variations in absorption Ångström
exponents on 24-h aerosol forcings, at least in the spectral range of
450–880 nm. Further studies should be taken to assess the corresponding
impact in the UV spectral range. The assumption that soot spectral
properties represent all ambient light absorbing particles may cause a
misjudgment of absorption towards the UV, especially in remote areas.
Therefore, it is recommended to measure aerosol absorption at several
wavelengths to accurately assess the impact of non-soot aerosols on climate
and on photochemical atmospheric processes. |
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