Light absorption by organic carbon from wood combustion University of Illinois, Champaign-Urbana, IL, USA
Received: 04 September 2009 – Published in Atmos. Chem. Phys. Discuss.: 30 September 2009 Abstract. Carbonaceous aerosols affect the radiative balance of the Earth by absorbing
and scattering light. While black carbon (BC) is highly absorbing, some
organic carbon (OC) also has significant absorption, especially at
near-ultraviolet and blue wavelengths. To the extent that OC absorbs visible
light, it may be a non-negligible contributor to positive direct aerosol
radiative forcing. Quantification of that absorption is necessary so that
radiative-transfer models can evaluate the net radiative effect of OC.
Revised: 20 January 2010 – Accepted: 26 January 2010 – Published: 16 February 2010
In this work, we examine absorption by primary OC emitted from solid fuel
pyrolysis. We provide absorption spectra of this material, which can be
related to the imaginary refractive index. This material has polar character
but is not fully water-soluble: more than 92% was extractable by methanol
or acetone, compared with 73% for water and 52% for hexane.
Water-soluble OC contributes to light absorption at both ultraviolet and
visible wavelengths. However, a larger portion of the absorption comes from
OC that is extractable only by methanol. Absorption spectra of water-soluble
OC are similar to literature reports. We compare spectra for material
generated with different wood type, wood size and pyrolysis temperature.
Higher wood temperature is the main factor creating OC with higher
absorption; changing wood temperature from a devolatilizing state of 210 °C
to a near-flaming state of 360 °C causes about a factor of four
increase in mass-normalized absorption at visible wavelengths. A clear-sky
radiative transfer model suggests that, despite the absorption, both
high-temperature and low-temperature OC result in negative top-of-atmosphere
radiative forcing over a surface with an albedo of 0.19 and positive
radiative forcing over bright surfaces. Unless absorption by real ambient
aerosol is higher than that measured here, it probably affects global
average clear-sky forcing very little, but could be important in energy
balances over bright surfaces.
Citation: Chen, Y. and Bond, T. C.: Light absorption by organic carbon from wood combustion, Atmos. Chem. Phys., 10, 1773-1787, doi:10.5194/acp-10-1773-2010, 2010.