Light absorption by organic carbon from wood combustion

Abstract

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.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 massnormalized 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.

Spectral- and size-resolved mass absorption efficiency of mineral dust aerosols in the shortwave spectrum: a simulation chamber study

“…For black carbon, MAE values are in the range of 6.5-7.5 m 2 g −1 at 850 nm (Bond and Bergstrom, 2006;Massabò et al, 2016) and decrease in a linear way with the logarithm of the wavelength. For brown carbon, the reported MAE range between 2.3 and 7.0 m 2 g −1 at 350 nm (Chen and Bond, 2010;Kirchstetter et al, 2004;Massabò et al, 2016), 0.05 and 1.2 m 2 g −1 at 440 nm (Wang et al, 2016) and 0.08 and 0.72 m 2 g −1 at 550 nm (Chen and Bond, 2010). The analysis of Table 4 indicates that, at every wavelength, the MAE values in the PM 2.5 fraction are equal or higher than those for PM 10.6 .…”

Carbonaceous aerosols recorded in a southeastern Tibetan glacier: analysis of temporal variations and model estimates of sources and radiative forcing

“…Second, we treated all the water-insoluble OC from the ice-core measurements as lightabsorbing brown carbon in the forcing estimation, which also likely results in an overestimation of OC forcing if a significant fraction of OC is non-absorbing. However, the watersoluble part, accounting for about half of OC observed in the Manora peak and northwestern India (Ram et al, 2010;Rajput et al, 2014), can also contribute to some absorption of UV and visible light (Chen and Bond, 2010;Beine et al, 2011). Thus, the absorption by water-soluble OC that was not included in the forcing estimate may compensate for the high bias to some extent.…”

Near-Ultraviolet Absorption Cross Sections of Nitrophenols and Their Potential Influence on Tropospheric Oxidation Capacity

“…Little is known about aerosol properties in this spectral region, although several recent studies have suggested that particle absorption increases strongly at sub-visible wavelengths. [53][54][55] …”