Relative Contribution to Light Absorption by Methanol-soluble and Methanol-insoluble Brown Carbon Emitted from Biomass Burning
Khairallah Atwi, Zezhen Cheng, Omar El Hajj, Charles Perrie, RAWAD SALEH,
University of Georgia Abstract Number: 615
Working Group: Biomass Combustion: Outdoor/Indoor Transport and Indoor Air Quality
AbstractMethanol is often used to extract biomass-burning aerosol emissions in order to retrieve brown carbon (BrC) light-absorption properties. However, recent studies have reported that methanol-extraction leads to underestimating BrC absorption due to the existence of BrC that is insoluble in methanol. Here, we produced carbonaceous aerosol particles from the combustion of three biomass fuels (pine needles, hickory twigs, and oak foliage) in an environmental chamber. We performed a combination of online and offline measurements and optical calculations to estimate the mass fractions and contribution to light absorption by methanol-soluble BrC (MSBrC), methanol-insoluble BrC (MIBrC), and elemental carbon (EC). Averaged over all experiments, MSBrC accounted for 90% ± 5% of the carbonaceous aerosol species, while MIBrC and EC accounted for 9% ± 5% and 1% ± 0.5%, respectively. The BrC produced in all experiments was categorized as moderately absorbing BrC, with an imaginary part of the refractive index (k) at 532 nm ranging between 0.01 and 0.05. However, k at 532 nm of MIBrC (0.211 ± 0.113) was two orders of magnitude larger than that of MSBrC (0.004 ± 0.002), with MSBrC categorized as weakly absorbing BrC and MIBrC as strongly absorbing BrC. Consequently, even though MSBrC dominated the aerosol mass (90% ± 5%), MIBrC dominated light absorption at 532 nm (72% ± 11%). The findings presented in this poster provide support for previous studies that reported the existence of strongly absorbing BrC species that are insoluble in organic solvents. Furthermore, our results indicate that relying on methanol extraction to characterize BrC in biomass-burning emissions would severely underestimate its absorption.