the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Enhanced behaviors of optical properties and the radiative effects of molecular-specific brown carbon from dung combustion in the Tibetan Plateau
- 1Key Laboratory of Northwest Resource, Environment and Ecology, MOE, Xi’an University of Architecture and Technology, Xi’an 710055, China
- 2Department of Environmental Science and Engineering, Xi’an Jiaotong University, Xi’an 710049, China
- 3School of Environmental Science and Engineering, Southern University of Science and Technology, Shenzhen, 518055, China
- 4Key Lab of Aerosol Chemistry & Physics, SKLLQG, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710061, China
- 1Key Laboratory of Northwest Resource, Environment and Ecology, MOE, Xi’an University of Architecture and Technology, Xi’an 710055, China
- 2Department of Environmental Science and Engineering, Xi’an Jiaotong University, Xi’an 710049, China
- 3School of Environmental Science and Engineering, Southern University of Science and Technology, Shenzhen, 518055, China
- 4Key Lab of Aerosol Chemistry & Physics, SKLLQG, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710061, China
Abstract. Traditional animal dung fuel use is a prominent source of brown carbon (BrC) in the Tibetan Plateau (TPL) region. Changes in burning conditions, fuel types, and uses of dung fuels in Plateau areas can lead to considerable uncertainties about molecular absorption properties and their radiative forcing influence on BrC. Here, the constituents of BrC’s chromophoric molecules emitted from residential heating and cooking scenarios using dung fuels were proposed using ultra-high performance liquid chromatography-quadrupole time-of-flight mass spectrometry in the TPL region, China. Our results show that dung fuels in this study can release abundant BrC emissions with substantial high BrC absorption when compared with those observed in bitumite. Particularly, the linkage between BrC molecules and their absorption properties was quantified by partial least squares regression. Among detected BrC’s molecular groups, above 70 % of N-containing compounds (CHON and CHONS) with low-oxygen-containing and unsaturated aromatic bonds and 20.9–27.5 % of CHO compounds measured from the dung combustion samples were lies in the potential BrC chromophores regions. Further, a significantly enhanced contribution of molecular absorption coefficient (Mbabs) to total Mbabs (up to 99.7 %) was observed in the presence of both CHO and CHON (N2+N4) with distinctive characteristics of long carbon chains and high levels of unsaturation. Interestingly, the identified CHONS markers were highly oxygenated with abundant unsaturated double-bonds and high Mbabs, but were rarely produced under insufficient oxygen conditions at the high-altitude plateau. Meanwhile, the incomplete combustion of dung produced high values of integrated simple forcing efficiency and thus destroyed the radiation balance over the TPL region.
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Qian Zhang et al.
Status: open (until 23 Feb 2023)
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RC1: 'Comment on acp-2022-801', Anonymous Referee #1, 12 Jan 2023
reply
This manuscript, prepared by Qian Zhang et al., describes an analysis of Brown Carbon (BrC) emitted from burning traditional solid fuels (yak dung, sheep dung, and bitumite) commonly used in the Qinghai Tibet Plateau (TPL) region. With the measurement of light absorptivity and molecular-level information, the authors attempted to identify major light-absorbing species and markers for such combustion. TPL is a climatically sensitive region, and addressing light-absorbing climate forcers, such as BrC, emitted in this region is of paramount importance. Particularly, emissions arising from traditional (but still widely practiced) solid fuels, including dung burning, are significantly understudied in the literature. In this regard, the research topic is important, timely, and fits the scope of ACP. In particular, I find the observation that these emissions are particularly high in BrC emission fascinating.
However, I cannot recommend publishing this manuscript in its current form. I have a number of major scientific concerns, as listed below, and I think parts of the discussion just seem to be incomplete in the current version. It may be potentially publishable after significant rewriting. Last but not least, the literary grade of the manuscript is not excellent, with countless grammatical errors. I would recommend the authors undergo additional editing and proofreading.
Major comment
My biggest concern is that the analytical method used, UHPLC-ToF-MS/MS, has limitations in both quantitation and identification of molecules. Although I acknowledge that the chemical analyses were done carefully, and with much appreciated chemical insights, the authors not only did not carefully discuss these limitations but seem to be unaware of them. Please see my breakdown below.
Completeness of the detected species
There are multiple reasons the method used here can detect only a fraction of BrC emitted. 1) methanol cannot extract all BrC compounds on the filter 2) The column cannot elute all the compounds in MSOC. 3) ESI(-) only detects compounds with an acidic proton, which is far from the entire spectrum of MSOC compounds.
Quantitation
ESI is not a very quantitative method, as the ionization efficiency of molecules varies drastically between compounds due to size and functional group.1 However, the authors employ relative intensity - which I don’t think is clearly defined (see minor comment) - as the sole indicator for the abundance and concentration of species in the sample.
Identification
Molecular identification using MS, even with the assistance of MS/MS, is challenging. How can the authors be confident about structure identification (Table 2) down to the isomer level? As the authors stated themselves, there are numerous isomers in this complex sample (Line 206). It seems that the authors have investigated a lot of work into a self-built library (Line 113), but it is not clear in the manuscript.
Combining compleeteness, quantitation, and identification above, many of the authors’ conclusions and implications are questionable and should be revised across the manuscript. Some of these include, but may not be limited to:
- The Mbabs values and the relative contribution of each compound class to the total absorption (basically everything in Figure 6).
- The fact that authors consider the observed BrC species are all that contribute to light absorbers. E.g., “The values confirm that CHON compounds were likely to be the dominant light absorbers in solid fuel combustion over the TPL region (Line 254)”.
- The conclusion “this work provides exhaustive molecular information” (Line 327)
Burning conditions/scenarios.
One of the conclusions from the paper is that burning type/scenario (i.e., cooking vs heating) significantly affects BrC emissions. The conclusion is valid and interesting, but the authors did not provide further information on how they are different. The readers can only see that they are different, their pictures (SI) and “heating has poor oxygen conditions than cooking” (Line 152), but without further support. I thought the authors measured OC/EC to gauge combustion efficiency, but I was surprised that none of the data had been presented in the manuscript.
Minor Comments
- I don’t think Mbabs is clearly defined in the main article (it is in the abstract). As such, the method to determine Mbabs is unclear. Is the PLSR method from line ~118 for Mbabs?
- ‘Relative intensity’ is not properly defined. Relative to what?
- Line 218 - what is atmospheric conditions? Is it burning conditions instead?
- In the paragraph starting Line 220, the discussion related to Figure 4 does not seem to fully reflect the observations on Figure 4. For example, ‘a wide DBE range from 0 to 20’ - it seems to be wider. And ‘CHON and CHONS compounds (beyond 70%) lies in the potential BrC chromophore range - I don’t think its beyond 70%, looking at the data.
Grammatical errors
There are numerous that I believe can be corrected by careful proofreading and editing.
Additionally, certain sentences appear too subjective, and I suggest the authors revise the tone of the writing.
E.g, :
“We all know that burning biofuels of yak and sheep dung is still the most traditional and popular way of heating and cooking over the TPL region. The total consumption of both yak and sheep dung can even possess up to nearly 70% of the total dung fuel consumptions during heating periods (Zhang et al., 2022a).”
‘We all know that’ , ‘still the most traditional and’ , ‘dung can even possess up to’ can all be considered subjective.
Reference
(1) Kenseth, C. M.; Hafeman, N. J.; Huang, Y.; Dalleska, N. F.; Stoltz, B. M.; Seinfeld, J. H. Synthesis of Carboxylic Acid and Dimer Ester Surrogates to Constrain the Abundance and Distribution of Molecular Products in α-Pinene and β-Pinene Secondary Organic Aerosol. Environ. Sci. Technol. 2020, 54 (20), 12829–12839. https://doi.org/10.1021/acs.est.0c01566.
Qian Zhang et al.
Qian Zhang et al.
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