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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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Measuring light absorption by freshly emitted organic aerosols: optical artifacts in traditional solvent-extraction-based methods
Abstract. Recent studies have shown that organic aerosol (OA) could have a nontrivialrole in atmospheric light absorption at shorter visible wavelengths. Goodestimates of OA light absorption are therefore necessary to better estimateradiative forcing due to these aerosols in climate models. One of the commontechniques used to measure OA light absorption is the solvent extractiontechnique from filter samples which involves the use of a spectrophotometerto measure bulk absorbance by the solvent-soluble organic fraction ofparticulate matter. Measured solvent-phase absorbance is subsequentlyconverted to particle-phase absorption coefficient using scaling factors.The conventional view is to apply a correction factor of 2 to absorptioncoefficients obtained from solvent-extracted OA based on Mie calculations.The appropriate scaling factors are a function of biases due to incompleteextraction of organic carbon (OC) by solvents and size-dependent absorption properties of OA.The range for these biases along with their potential dependence on burnconditions is an unexplored area of research. Here, we performed a comprehensive laboratory study involving three solvents(water, methanol, and acetone) to investigate the bias in absorptioncoefficients obtained from solvent-extraction-based photometry techniques ascompared to in situ particle-phase absorption for freshly emitted OA frombiomass burning. We correlated the bias with OC∕TC (total carbon) mass ratio and singlescattering albedo (SSA) and observed that the conventionally used correctionfactor of 2 for water and methanol-extracted OA might not be extensible toall systems, and we suggest caution while using such correction factors toestimate particle-phase OA absorption coefficients. Furthermore, a linearcorrelation between SSA and the OC∕TC ratio was also established. Finally, fromthe spectroscopic data, we analyzed the differences in absorptionÅngström exponents (AÅE) obtained from solution- andparticulate-phase measurements. We noted that AÅE fromsolvent-phase measurements could deviate significantly from their OAcounterparts.
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- Award ID(s):
- 1455215
- PAR ID:
- 10188239
- Date Published:
- Journal Name:
- Atmospheric Chemistry and Physics
- Volume:
- 19
- Issue:
- 13
- ISSN:
- 1680-7324
- Page Range / eLocation ID:
- 8817 to 8830
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.5194/acp-19-8817-2019
