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| Titel |
Estimation of the mass absorption cross section of the organic carbon component of aerosols in the Mexico City Metropolitan Area |
| VerfasserIn |
J. C. Barnard, R. Volkamer, E. I. Kassianov |
| 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 ; 8, no. 22 ; Nr. 8, no. 22 (2008-11-19), S.6665-6679 |
| Datensatznummer |
250006456
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| Publikation (Nr.) |
 copernicus.org/acp-8-6665-2008.pdf |
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| Zusammenfassung |
| Data taken from the MCMA-2003 and the 2006 MILAGRO field campaigns are used
to examine the absorption of solar radiation by the organic component of
aerosols. Using irradiance data from a Multi-Filter Rotating Shadowband
Radiometer (MFRSR) and an actinic flux spectroradiometer (SR), we derive
aerosol single scattering albedo, ϖ0, λ, as a function of
wavelength, λ. We find that in the near-UV spectral range (250 to
400 nm) ϖ0, λ is much lower compared to ϖ0, λ
at 500 nm indicating enhanced absorption in the near-UV range.
Absorption by elemental carbon, dust, or gas cannot account for this enhanced
absorption leaving the organic carbon component of the aerosol (OA) as the
most likely absorber. We use data from a surface deployed Aerodyne Aerosol
Mass Spectrometer (AMS) along with the inferred ϖ0, λ to
estimate the Mass Absorption Cross section (MAC) for the organic aerosol. We
find that the MAC is about 10.5 m2/g at 300 nm and falls close to zero
at about 500 nm; values that are roughly consistent with other estimates of
organic aerosol MAC. These MAC values can be considered as "radiatively
correct", because when used in radiative transfer calculations, the
calculated irradiances/actinic fluxes match those measured at the wavelengths
considered here. For an illustrative case study described here, we estimate
that the light absorption by the "brown" (organic) carbonaceous aerosol can
add about 40% to the light absorption of black carbon in Mexico City. This
contribution will vary depending on the relative abundance of organic aerosol
relative to black carbon. Furthermore, our analysis indicates that organic
aerosol would slow down photochemistry by selectively scavenging the light
reaching the ground at those wavelengths that drive photochemical
reactions. Finally, satellite retrievals of trace gases that are used
to infer emissions currently assume that the MAC of organic carbon is zero.
For trace gases that are retrieved using wavelengths shorter then 420 nm
(i.e. SO2, HCHO, halogenoxides, NO2), the assumption of
non-zero MAC values will induce an upward correction to the inferred
emissions. This assumption will be particularly relevant in polluted urban
atmospheres and areas of biomass burning where organic aerosols are
particularly abundant. |
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