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The light-absorption properties of brown carbon (BrC) are often estimated using offline, solvent-extraction methods. However, recent studies have found evidence of insoluble BrC species that are unaccounted for in solvent extraction. In this work, we produced carbonaceous aerosol particles from the combustion of three biomass fuels (pine needles, hickory twigs, and oak foliage). We utilized 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, the majority of the carbonaceous aerosol species were attributed to MSBrC (90% ± 5%), while MIBrC and EC constituted 9% ± 5% and 1% ± 0.5%, respectively. The BrC produced in all experiments was moderately absorbing, with an imaginary component of the refractive index ( k ) at 532 nm ranging between 0.01 and 0.05. However, the k values at 532 nm of the MSBrC (0.004 ± 0.002) and MIBrC (0.211 ± 0.113) fractions were separated by two orders of magnitude, with MSBrC categorized as weakly absorbing BrC and MIBrC as strongly absorbing BrC. Consequently, even though MSBrC constituted the majority of the aerosol mass, MIBrC had a dominant contribution to light absorption at 532 nm (72% ± 11%). The findings presented in this paper provide support for previous reports of the existence of strongly absorbing, methanol-insoluble BrC species and indicate that relying on methanol extraction to characterize BrC in biomass-burning emissions would severely underestimate its absorption.
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Potential underestimation of ambient brown carbon absorption based on the methanol extraction method and its impacts on source analysis
Abstract. The methanol extraction method was widely applied to isolate organic carbon(OC) from ambient aerosols, followed by measurements of brown carbon (BrC)absorption. However, undissolved OC fractions will lead to underestimatedBrC absorption. In this work, water, methanol (MeOH), MeOH / dichloromethane(MeOH / DCM, 1:1, v/v), MeOH / DCM (1:2, v/v), tetrahydrofuran (THF), andN,N-dimethylformamide (DMF) were tested for extraction efficiencies ofambient OC, and the light absorption of individual solvent extracts wasdetermined. Among the five solvents and solvent mixtures, DMF dissolved thehighest fractions of ambient OC (up to ∼95 %), followed byMeOH and MeOH / DCM mixtures (<90 %), and the DMF extracts hadsignificantly (p<0.05) higher light absorption than other solventextracts. This is because the OC fractions evaporating at highertemperatures (>280∘) are less soluble in MeOH(∼80 %) than in DMF (∼90 %) and containstronger light-absorbing chromophores. Moreover, the light absorption of DMFand MeOH extracts of collocated aerosol samples in Nanjing showed consistenttemporal variations in winter when biomass burning dominated BrC absorption, while the average light absorption of DMF extracts was more than 2 timesgreater than the MeOH extracts in late spring and summer. The average lightabsorption coefficient at 365 nm of DMF extracts was 30.7 % higher (p<0.01) than that of MeOH extracts. Source apportionment resultsindicated that the MeOH solubility of BrC associated with biomass burning,lubricating oil combustion, and coal combustion is similar to their DMFsolubility. The BrC linked with unburned fossil fuels and polymerizationprocesses of aerosol organics was less soluble in MeOH than in DMF, whichwas likely the main reason for the large difference in time series betweenMeOH and DMF extract absorption. These results highlight the importance oftesting different solvents to investigate the structures and lightabsorption of BrC, particularly for the low-volatility fraction potentiallyoriginating from non-combustion sources.
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- Award ID(s):
- 1743401
- PAR ID:
- 10390644
- Date Published:
- Journal Name:
- Atmospheric Chemistry and Physics
- Volume:
- 22
- Issue:
- 20
- ISSN:
- 1680-7324
- Page Range / eLocation ID:
- 13739 to 13752
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.5194/acp-22-13739-2022
