the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 19 Feb 2024
Water-insoluble organic carbon in PM2.5 over China: light-absorbing properties, potential sources, radiative forcing effects and possible light-absorbing continuum
Abstract. Water-insoluble carbon (WIOC) constitutes a substantial portion of organic carbon (OC) and contributes significantly to light absorption by brown carbon (BrC), playing pivotal roles in climate forcing and human health. China as hotspots regions with high level of OC and BrC, information regarding the sources and light-absorbing properties of WIOC on national scale remains scarce. Here, we investigated the light-absorbing properties and sources of WIOC in ten representative urban cities across China. On average, WIOC accounted for 33.4 ± 7.66 % and 40.5 ± 9.73 % of the concentrations and light-absorbing efficiency at 365 nm (Abs365) of extractable OC (EX-OC, comprising relatively hydrophobic OC [WIOC and humic-like substances: HULIS-C], and hydrophilic OC [non-humic-like substances: non-HULIS-C]). The mass absorption efficiency of WIOC at 365 nm (MAE365) was (1.59 ± 0.55 m2/gC) comparable to that of HULIS (1.54 ± 0.57 m2/gC) but significantly higher than non-HULIS (0.71 ± 0.28 m2/gC), indicating that hydrophobic OC possesses a stronger light-absorbing capacity than hydrophilic OC. Biomass burning (31.0 %) and coal combustion (31.1 %) were the dominant sources of WIOC, with coal combustion sources exhibited the strongest light-absorbing capacity. Moreover, employing the simple forcing efficiency (SFE300–700nm) method, we observed that WIOC exhibited the highest SFE300–700nm (6.57 ± 5.37 W/g) among the EX-OC fractions. The radiative forcing of EX-OC was predominantly contributed by hydrophobic OC (WIOC: 39.4 ± 15.5 % and HULIS: 39.5 ± 12.1 %). Considering the aromaticity, sources, and atmospheric processes of different carbonaceous components, we propose a light-absorbing carbonaceous continuum, revealing that components enriched with fossil sources tend to possess stronger light-absorbing capacity, higher aromatic levels, increased molecular weights, and greater recalcitrance in the atmosphere. Reducing fossil fuel emissions emerges as an effective means of mitigating both gaseous (CO2) and particulate light-absorbing carbonaceous warming components.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2024-130', Anonymous Referee #1, 18 Mar 2024
The manuscript provided a detailed study on the light-absorbing characteristics, potential sources, and radiative forcing effects of WIOC across ten cities in China. The authors compared the characteristics of extractable OC (EX-OC) with different polarities (WIOC, HULIS, and non-HULIS). They discovered that biomass burning and coal combustion are the primary sources of WIOC in China and radiative forcing of EX-OC is mainly due to WIOC and HULIS. Furthermore, this study proposed a hypothesis of a light-absorbing carbonaceous continuum, demonstrating that carbonaceous components from fossil sources tend to have a stronger light-absorbing capacity, higher aromatic levels, and greater recalcitrance in the atmosphere. Overall, the paper is well-structured and well-written, but there are still some areas that require improvement. Please see my comments below.
Major Comments:
- In this study, the authors divide China into areas with and without central heating, which is a meaningful approach. However, the division based on a simple line in Figure 1 lacks convincing evidence. Are there data to support this division? Moreover, the sources and light absorption properties of WIOC are expected to differ significantly between areas with and without central heating. However, the article does not adequately discuss the spatial differences in WIOC sources and light absorption capabilities. The authors should address this aspect.
- The method for extracting WIOC involves methanol extraction, concentrated, and analysis using an OC/EC analyzer, while WSOC is determined using a liquid TOC analyzer. The analysis mechanism of OC/EC analyzers and TOC analyzers differ, including the different catalysts and detectors they employ. Comparing measurement results from these two methods may be challenging. Additionally, the authors did not provide the blank value for OC/EC determination of WIOC, which is crucial given methanol's high propensity for extracting atmospheric organic matter.
- The authors primarily utilize the PMF model to quantify the sources of WIOC. The authors suggest that the increase in coal combustion led to an increase in Abs365 in overall EX-OC (line 407-411), which seems plausible. However, correlating the quantitative results from coal combustion predicted by PMF with the Abs365 of overall EX-OC (Figure S3c) is not appropriate. The authors quantified the sources of WSOC using dual carbon isotopes (δ13C and Δ14C) in their previous study. Comparing the PMF model with source apportionment results based on dual carbon isotopes presents challenges. While 14C can accurately distinguish fossil sources from non-fossil sources, without chemical markers, it's difficult to quantitatively analyze sources related to atmospheric processes (such as secondary sources). On the other hand, although PMF can theoretically incorporate various chemical markers and include secondary sources in quantitative analysis, its results involve more human interpretation factors and may not be as precise as 14C. Thus, caution is warranted when comparing source apportionment results obtained from PMF and dual carbon isotopes.
- In Section 3.4, the authors propose a concept of a light-absorbing carbonaceous aerosol continuum, which I find intriguing. It's important to note that in this study, WIOC, HULIS, and non-HULIS are well-defined based on their polarity. These components indeed exhibit significant differences in both sources and light absorption properties. However, regarding BC, I contend that BC is the strongest light-absorbing component. Although this assertion isn't reflected in Figure 6, the authors further subdivide BC into char and soot in this section without providing a clear definition. In reality, char and soot are defined differently across various environmental matrices (https://doi.org/10.1038/s43017-022-00316-6). For aerosols, biomass burning and coal combustion emit large amounts of large molecular weight soluble compounds, which may char and produce false char EC signals (artifacts). Additionally, there are overlaps between char and BrC. These issues warrant attention and clarification from the authors.
Minor Comments:
- Line 20: “water-insoluble carbon” change to “water-insoluble organic carbon”
- Line 145 -147, the same phrase appears twice.
- Line 148, the authors mention using SPE for HULIS extraction. Given the various methods available for HULIS isolation (as referenced in https://doi.org/10.1016/j.envpol.2013.05.055), it would be helpful to provide reasons for choosing SPE for HULIS isolation.
- Line 271-272, “BrC” changed to “extractable OC”. There is no accurate method for extracting BrC.
- Line 388, it should be "carbon mass contribution"
- Figure 6. I suggest emphasizing in the caption that WIOC tends to be OC soluble in methanol but not in water. The WIOC in this work is not real water-insoluble organic carbon.
Citation: https://doi.org/10.5194/egusphere-2024-130-RC1 - RC2: 'Comment on egusphere-2024-130', Anonymous Referee #2, 25 Mar 2024
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RC3: 'Comment on egusphere-2024-130', Anonymous Referee #3, 01 Apr 2024
Mo et al. investigated light absorption and the sources of extractable organic carbon (EX-OC), which encompasses water-insoluble carbon (WIOC), humic-like substances (HULIS-C), and hydrophilic OC (non-HULIS-C). The study revealed that WIOC constituted the majority of OC mass concentrations and light-absorbing efficiency at 365 nm. Additionally, the authors found that the radiative forcing effects of EX-OC were mainly contributed by relatively hydrophobic fractions. The authors also proposed a light-absorbing carbonaceous continuum, revealing that components enriched with fossil sources exhibit stronger light-absorbing capacity, higher aromatic levels, increased molecular weights, and greater atmospheric recalcitrance. This study is pivotal for comprehensively understanding the climate-forcing brown carbon and developing related mitigating strategies. However, before publication, the authors need to address the following concerns:
- Line 25–27: Too many brackets in this sentence. Please simplify this sentence to create a concise abstract.
- Line 51: Please review and remove any duplicate abbreviation definitions found throughout the manuscript, such as OC, WIOC, etc.
- Lines 65-91: This paragraph lacks clarity in addressing the main questions concerning WIOC. While the author discusses the method of measuring light-absorbing OC and compares the light-absorbing properties of WIOC and WSOC, the discussion on the health effects and atmospheric lifetime of WIOC seems disconnected. Additionally, the motivation for this study is relatively vague. Therefore, I would suggest that the authors restructure the paragraph to clearly emphasize current scientific inquiries and provide a more cohesive rationale for their research.
- Lines 145-147: Please review the two identical sentences and revise them accordingly.
- Line 251: Figure 1c should be Figure 2c?
- Lines 262-304: A more comprehensive discussion would involve comparing the light-absorbing properties of EX-OC between areas with and without central heating.
- Lines 269-272: Given that the light absorption of BrC, as measured by solvent extraction, appears to be underestimated compared to under ambient aerosol conditions, it is imperative to determine whether the authors considered this factor when comparing with "tar ball" and "unextractable dark BrC".
- Line 327: Please maintain consistency to ensure a unified description of E2/E3 in both the plot and text.
- Lines 333-335 and 341-342: The authors observed a robust correlation between WIOC and Abs365,WIOC (r = 0.97, p < 0.01), indicating similar sources and formation processes. However, they also concluded differences in sources and formation processes between WIOC and light-absorbing compounds during warm seasons. These findings may appear contradictory and confusing. Clarification is needed regarding the rationale behind analyzing the correlations between WIOC and Abs365,WIOC in individual warm and cold seasons.
- Line 375: Figure 3b should be Figure 4b?
- Line 382: Please clarify the methodology employed by the authors to determine the source contributions to Abs365,WIOC.
- Lines 390-392: Could you elaborate on why biomass burning, rather than secondary sources, contributes more to the light absorption of BrC in summer? What distinguishes the conclusion of your research from the previous studies mentioned?
- Lines 398-411: Was the correlation between biomass burning source contribution and the light absorption of WIOC discussed by the author? Was there a correlation similar to that observed with coal combustion?
- Line 423: Figure 1c should be Figure 2c?
- Line 504: Figures 2b and c should be Figures 3b and c?
Citation: https://doi.org/10.5194/egusphere-2024-130-RC3
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