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| Titel |
Size-resolved measurements of brown carbon in water and methanol extracts and estimates of their contribution to ambient fine-particle light absorption |
| VerfasserIn |
J. Liu, M. Bergin, H. Guo, L. King, N. Kotra, E. Edgerton, R. J. Weber |
| 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 ; 13, no. 24 ; Nr. 13, no. 24 (2013-12-19), S.12389-12404 |
| Datensatznummer |
250085892
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| Publikation (Nr.) |
 copernicus.org/acp-13-12389-2013.pdf |
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| Zusammenfassung |
| Light absorbing organic carbon, often called brown carbon, has the potential
to significantly contribute to the visible light-absorption budget,
particularly at shorter wavelengths. Currently, the relative contributions
of particulate brown carbon to light absorption, as well as the sources of
brown carbon, are poorly understood. With this in mind size-resolved direct
measurements of brown carbon were made at both urban (Atlanta), and rural
(Yorkville) sites in Georgia. Measurements in Atlanta were made at both a
representative urban site and a road-side site adjacent to a main highway.
Fine particle absorption was measured with a multi-angle absorption
photometer (MAAP) and seven-wavelength Aethalometer, and brown carbon
absorption was estimated based on Mie calculations using direct size-resolved
measurements of chromophores in solvents. Size-resolved samples were
collected using a cascade impactor and analyzed for water-soluble organic
carbon (WSOC), organic and elemental carbon (OC and EC), and solution light-absorption spectra of water and methanol extracts. Methanol extracts were
more light-absorbing than water extracts for all size ranges and
wavelengths. Absorption refractive indices of the organic extracts were
calculated from solution measurements for a range of wavelengths and used
with Mie theory to predict the light absorption by fine particles comprised
of these components, under the assumption that brown carbon and other
aerosol components were externally mixed. For all three sites, chromophores
were predominately in the accumulation mode with an aerodynamic mean
diameter of 0.5 μm, an optically effective size range resulting in
predicted particle light absorption being a factor of 2 higher than bulk
solution absorption. Mie-predicted brown carbon absorption at 350 nm
contributed a significant fraction (20 to 40%) relative to total light
absorption, with the highest contributions at the rural site where organic to
elemental carbon ratios were highest. Brown carbon absorption, however, was
highest by the roadside site due to vehicle emissions. The direct
size-resolved measurement of brown carbon in solution definitively shows
that it is present and optically important in the near-UV range in both a
rural and urban environment during the summer when biomass burning emissions are
low. These results allow estimates of brown carbon aerosol absorption from
direct measurements of chromophores in aerosol extracts. |
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