the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Underestimation of brown carbon absorption based on the methanol extraction method and its impacts on source analysis
- 1Collaborative Innovation Center of Atmospheric Environment and Equipment Technology, Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control, School of Environmental Science and Engineering, Nanjing University of Information Science & Technology, 219 Ningliu Road, Nanjing 210044, China
- 2School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA 30332, United States
- 1Collaborative Innovation Center of Atmospheric Environment and Equipment Technology, Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control, School of Environmental Science and Engineering, Nanjing University of Information Science & Technology, 219 Ningliu Road, Nanjing 210044, China
- 2School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA 30332, United States
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 underestimated BrC absorption. In this work, water, methanol (MeOH), MeOH/dichloromethane (MeOH/DCM, 1:1, v/v), MeOH/DCM (1:2, v/v), tetrahydrofuran (THF), and N,N-dimethylformamide (DMF) were tested for extraction efficiencies of ambient OC, and the light absorption of individual solvent extracts was determined. Among the five solvents and solvent mixtures, DMF dissolved the highest fractions of ambient OC (up to ~95 %), followed by MeOH and MeOH/DCM mixtures (< 90 %), and the DMF extracts had significant (p < 0.05) higher light absorption than other solvent extracts. This is because the OC fractions evaporating at higher temperatures (> 280 °C) are less soluble in MeOH (~80 %) than in DMF (~90 %) and contain stronger light-absorbing chromophores. Moreover, the light absorption of DMF and MeOH extracts of collocated aerosol samples in Nanjing showed distinct time series. Source apportionment results indicated that the MeOH insoluble OC mainly came from unburned fossil fuels and polymerization processes of aerosol organics. These results highlight the necessity of replacing MeOH with DMF for further investigations on structures and light absorption of low-volatile BrC.
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Zhenqi Xu et al.
Status: open (until 17 Aug 2022)
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CC1: 'Comment on acp-2022-460', Guofeng SHEN, 13 Jul 2022
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Xu et al. examined the influence of solvent selection on brown carbon (BrC) absorption measurements and source analysis for ambient aerosols. Water, methanol, methanol-DCM mixtures, THF, and DMF were tested. Measurement results showed that DMF exhibited the highest extraction efficiency of ambient organic carbon (OC), particularly for low-volatile OC, and DMF extracts also had significant higher light absorption than other solvent extracts. Moreover, the comparison of sources between DMF and methanol extract absorption is very interesting and indicates that the methanol-extraction method will underestimate BrC contributions from non-combustion sources.
The authors suggested that DMF can extract more BrC than commonly used solvents. DMF might be an important solvent for investigating low-volatile OC in the near future. This manuscript provides very useful information for further studies on radiative forcing and sources of organic aerosols, and I recommend the publication of this manuscript in ACP, though I’d like the authors to address some minor specific comments below.
1. In this work, several solvent extracts of ambient OC were measured for light absorption, would the authors consider changing the title to “The dependence of brown carbon absorption on solvent selection and its impacts on source analysis”, or something similar to highlight the differences in different solvent extraction methods?
2. Line 31. “However, undissolved OC fractions will lead to underestimated BrC absorption.” What is the magnitude of this underestimation? Also, what about the mass? If the undissolved fraction has low light absorption, the underestimation might not be large, right?
3. Lines 41-42, “the light absorption of DMF and MeOH extracts of collocated aerosol samples in Nanjing showed distinct time series. Specifically, what is the difference, and do they have any common temporal patterns?
4. Lines 58-60, “The radiative forcing (RF) of the light-absorbing organic carbon, also termed “brown carbon” (BrC), is not well quantified due to the lack of its emission data and large uncertainties in in situ BrC measurements” The secondary formation will also add complexity on RF estimation of BrC. Please mention it.
5. Lines 261-262, “THF based on the two methods for rOC measurements (section 2.2) are compared in Figures S1 and S2.” Would the authors consider putting these two figures in the main text? They provide very useful information.
6. Section 3.1.2. Is the difference across solvent extraction methods related to the physicochemical properties of OC? If it is true, please state which factors have a substantial influence.
7. Page 13, lines 298–299. “This is because the light absorption of DMF extracts depends less on wavelengths than other solvent extracts (Å ~4.5, Table 2).”
Page 14, lines 339–341. “In comparison to Åm (6.81± 1.64; Table 3), the lower average Åd (5.25 ± 0.64, p < 0.01) supports that more-absorbing BrC had less spectral dependence than less-absorbing BrC.”
In Tables 2 and 3, there seems to be a negative relationship between the MAE and Å values. To illustrate that strong BrC chromophores had less spectral dependence than weak ones, I would suggest showing the relationship visually by plotting MAE vs. Å.
8. Figures 2 and 3. I would suggest the authors to put Abs365, MAE365, and Å on the y-axis.
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RC1: 'Comment on acp-2022-460', Guofeng SHEN, 13 Jul 2022
reply
Xu et al. examined the influence of solvent selection on brown carbon (BrC) absorption measurements and source analysis for ambient aerosols. Water, methanol, methanol-DCM mixtures, THF, and DMF were tested. Measurement results showed that DMF exhibited the highest extraction efficiency of ambient organic carbon (OC), particularly for low-volatile OC, and DMF extracts also had significant higher light absorption than other solvent extracts. Moreover, the comparison of sources between DMF and methanol extract absorption is very interesting and indicates that the methanol-extraction method will underestimate BrC contributions from non-combustion sources.
The authors suggested that DMF can extract more BrC than commonly used solvents. DMF might be an important solvent for investigating low-volatile OC in the near future. This manuscript provides very useful information for further studies on radiative forcing and sources of organic aerosols, and I recommend the publication of this manuscript in ACP, though I’d like the authors to address some minor specific comments below.
1. In this work, several solvent extracts of ambient OC were measured for light absorption, would the authors consider changing the title to “The dependence of brown carbon absorption on solvent selection and its impacts on source analysis”, or something similar to highlight the differences in different solvent extraction methods?
2. Line 31. “However, undissolved OC fractions will lead to underestimated BrC absorption.” What is the magnitude of this underestimation? Also, what about the mass? If the undissolved fraction has low light absorption, the underestimation might not be large, right?
3. Lines 41-42, “the light absorption of DMF and MeOH extracts of collocated aerosol samples in Nanjing showed distinct time series. Specifically, what is the difference, and do they have any common temporal patterns?
4. Lines 58-60, “The radiative forcing (RF) of the light-absorbing organic carbon, also termed “brown carbon” (BrC), is not well quantified due to the lack of its emission data and large uncertainties in in situ BrC measurements” The secondary formation will also add complexity on RF estimation of BrC. Please mention it.
5. Lines 261-262, “THF based on the two methods for rOC measurements (section 2.2) are compared in Figures S1 and S2.” Would the authors consider putting these two figures in the main text? They provide very useful information.
6. Section 3.1.2. Is the difference across solvent extraction methods related to the physicochemical properties of OC? If it is true, please state which factors have a substantial influence.
7. Page 13, lines 298–299. “This is because the light absorption of DMF extracts depends less on wavelengths than other solvent extracts (Å ~4.5, Table 2).”
Page 14, lines 339–341. “In comparison to Åm (6.81± 1.64; Table 3), the lower average Åd (5.25 ± 0.64, p < 0.01) supports that more-absorbing BrC had less spectral dependence than less-absorbing BrC.”
In Tables 2 and 3, there seems to be a negative relationship between the MAE and Å values. To illustrate that strong BrC chromophores had less spectral dependence than weak ones, I would suggest showing the relationship visually by plotting MAE vs. Å.
8. Figures 2 and 3. I would suggest the authors to put Abs365, MAE365, and Å on the y-axis.
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CC2: 'Comment on acp-2022-460', Xiao He, 20 Jul 2022
reply
Publisher’s note: this comment was reposted as referee comment RC2.
This study compares the extraction of ambient PM2.5 samples applying different solvents and the subsequent light absorption and determination of brown carbon (BrC). Authors find that the traditional approaches using MeOH or water extraction underestimate BrC absorption due to the insolubility of OC possessing larger chromophores and DMF exhibits the highest extraction efficiency among all the tested solvents. They suggest that using DMF instead of MeOH for BrC extraction and incorporate the results into receptor model will generate distinct source apportionment results. After PMF analysis, they conclude that the contributions of BrC from unburned fossil fuels and polymerization of aerosol organics are underestimated particularly. I do appreciate the interesting work and the information provides new insights into the radiative forcing of BrC. The work is well drafted, and I recommend publication in ACP before a few comments to be addressed as below.
Line 146-147. In the sampling setup, PUF is attached after two quartz filters to collect the gas phase polar and non-polar organic compounds. However, we do not see the subsequent treatment of the gas phase samples. Also, the absorption of vapors to quartz filter is substantial. In this regard, the sampling artifacts of this experimental design may be great concern and should be addressed.
Session 3.3 PMF analysis. Current discussion about the PMF is brief, and the following key information should be included, either in the main text or the SI. (1) the stability test of the final solution, as it indicates the robustness of the solution. A solution fails the robustness test is meaningless. (2) The change of the Qrobust/Qexp with factor numbers should be examined.
Figure S5 UV-VIS spectra of 4-nitrophenol and 4-nitrocatechol. There is a strong light absorption at around 450 nm using DMF, which is not observed in other samples. It looks that unknown reactions occur, and the products introduce the unexpected light absorption. Considering that 4-nitrophenol and 4-nitrocatechol are representative tracers for biomass burning, readers may concern that DMF extracts would cause significant bias when investigate the BB BrC.
Line 317-318. The authors propose that the low-volatility OC fractions are possibly featured with PAH skeleton and DMF has higher dissolubility for those compounds than MeOH. Nevertheless, no light absorbance difference is observed in Figure S5 g-l. What are the 25 PAHs in the mixture solution and can you give some example structures that DMF have higher solubility than MeOH.
Line 283-284. As the author put it, the lower capability of MeOH in dissolving low-volatility OC fractions (OC3 and OC4) would lead to an underestimation of BrC absorption. Can you give an estimation of the underestimation so that the readers have intuitive knowledge?
Line 132. There should be a space before and after multiple sign.
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RC2: 'Comment on acp-2022-460', Xiao He, 20 Jul 2022
reply
This study compares the extraction of ambient PM2.5 samples applying different solvents and the subsequent light absorption and determination of brown carbon (BrC). Authors find that the traditional approaches using MeOH or water extraction underestimate BrC absorption due to the insolubility of OC possessing larger chromophores and DMF exhibits the highest extraction efficiency among all the tested solvents. They suggest that using DMF instead of MeOH for BrC extraction and incorporate the results into receptor model will generate distinct source apportionment results. After PMF analysis, they conclude that the contributions of BrC from unburned fossil fuels and polymerization of aerosol organics are underestimated particularly. I do appreciate the interesting work and the information provides new insights into the radiative forcing of BrC. The work is well drafted, and I recommend publication in ACP before a few comments to be addressed as below.
Line 146-147. In the sampling setup, PUF is attached after two quartz filters to collect the gas phase polar and non-polar organic compounds. However, we do not see the subsequent treatment of the gas phase samples. Also, the absorption of vapors to quartz filter is substantial. In this regard, the sampling artifacts of this experimental design may be great concern and should be addressed.
Session 3.3 PMF analysis. Current discussion about the PMF is brief, and the following key information should be included, either in the main text or the SI. (1) the stability test of the final solution, as it indicates the robustness of the solution. A solution fails the robustness test is meaningless. (2) The change of the Qrobust/Qexp with factor numbers should be examined.
Figure S5 UV-VIS spectra of 4-nitrophenol and 4-nitrocatechol. There is a strong light absorption at around 450 nm using DMF, which is not observed in other samples. It looks that unknown reactions occur, and the products introduce the unexpected light absorption. Considering that 4-nitrophenol and 4-nitrocatechol are representative tracers for biomass burning, readers may concern that DMF extracts would cause significant bias when investigate the BB BrC.
Line 317-318. The authors propose that the low-volatility OC fractions are possibly featured with PAH skeleton and DMF has higher dissolubility for those compounds than MeOH. Nevertheless, no light absorbance difference is observed in Figure S5 g-l. What are the 25 PAHs in the mixture solution and can you give some example structures that DMF have higher solubility than MeOH.
Line 283-284. As the author put it, the lower capability of MeOH in dissolving low-volatility OC fractions (OC3 and OC4) would lead to an underestimation of BrC absorption. Can you give an estimation of the underestimation so that the readers have intuitive knowledge?
Line 132. There should be a space before and after multiple sign.
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RC3: 'Comment on acp-2022-460', Anonymous Referee #3, 29 Jul 2022
reply
The comment was uploaded in the form of a supplement: https://acp.copernicus.org/preprints/acp-2022-460/acp-2022-460-RC3-supplement.pdf
Zhenqi Xu et al.
Zhenqi Xu et al.
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