the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 25 Nov 2024
Significant influence of oxygenated volatile organic compounds on atmospheric chemistry analysis: A case study in a typical industrial city in China
Abstract. Oxygenated volatile organic compounds (OVOCs), key components of volatile organic compounds (VOCs), are either directly emitted or secondary generated via photochemical processes, and play a crucial role in tropospheric photochemistry and act as important ozone (O3) precursors. However, due to measurement limitations, the influence of OVOCs on O3 formation has often been underestimated. In this study, 74 VOCs (including 18 OVOCs) were measured at five representative stations in Zibo, an industrial city in the North China Plain. The VOCs level in Zibo (44.6±20.9 ppb) is in the upper-middle range compared with previous studies, with OVOCs contributing 30.0 %~37.8 % to the total VOCs concentration. The overall O3 formation potential in Zibo is 410.4±197.2 µg m-3, with OVOCs being the dominant contributor (31.5 %~55.9 %). An observation-based model (OBM) was used to access the contributions of chemical production (RNetProd) and emissions/transport (REmis&Trans) to individual OVOC. Daytime RNetProd is the highest at the urban site (5.9 ppb h-1), while nighttime REmis&Trans is most significantly negative at the industrial site (0.76 ppb h-1). Simulations without OVOCs constraint overestimates OVOCs (42.1 %~126.5 %) and key free radicals (e.g., HO2 (5.3 %~20.4 %) and RO2 (6.6 %~35.1 %)), leading to a 1.8 %~11.9 % overestimation of O3. This overestimation causes an underestimation of OH (1.8 %~20.9 %) and atmospheric oxidizing capacity (3.5 %~12.5 %). These findings emphasize the importance of comprehensive OVOCs measurements to constrain numerical models, especially in regions with dense anthropogenic emissions, to better reproduce atmospheric photochemistry, and to formulate more effective air pollution control strategies.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2024-3201', Anonymous Referee #1, 06 Jan 2025
The article „Significant influence of oxygenated volatile organic compounds on atmospheric chemistry analysis: A case study in a typical industrial city in China” highlights OVOCs as critical players for air quality analysis. Dai et al. base their study on an experimental dataset of VOC, OVOC and other air quality and meteorological parameters at five monitoring sites within the city of Zibo. They present these data and analyse the role of OVOC on radical chemistry and ozone formation potential with an observation-based model (OBM). With this, the study fits well into the scope of ACP. The combined approach of experimental and theoretical analysis is interesting and valid for the study’s overall aim of comparing the different sites and the impact of including or excluding OVOC on a typical air quality analysis and conclusions for ozone production potential.
After reading the preprint, I have three major critiques, which I want to outline here and later on discuss in the specific comments in further detail.
Firstly, the study is based on the comparison of five experimental datasets from five different sites in the city of Zibo, which were monitored in parallel during August 8-12, 2021. VOC were measured with online GC-FID and OVOC were sampled on sorbent tubes and analysed offline with HPLC. However, I neither see a list of the measured VOC or OVOC nor information about calibrations and uncertainties. Most importantly, how do you know that the results of the five stations are comparable? Did you perform any comparative side-by-side measurements?
Secondly, the study is presented in three parts:
- Observations and comparison of air composition at the five sites within the city of Zibo
- OVOC sources and sinks
- Importance of OVOC in the OBM air quality analysis
While I can follow the presentation of results and argumentation of the discussion for parts 1 and 3 very well, I am not convinced by the approach, presentation and discussion of part 2. As far as I could understand, the budget of sinks and sources in the box model is used to describe the time-dependent rate of change of the OVOC as observed with the measurements. The budget of sinks and sources consists of the net production (R_NetProd), the deposition (R_dep), and emissions and transport (R_Emis&Trans). Here, I have several questions: Why were emissions and transport combined? Why is the chemical or photolytic loss of the OVOC not included in this budget? In order to understand the conclusions of part 2, I would need a much more precise description of the methods, chosen approach, and better references.
Thirdly, it seems like the model set-up and constrain were complex and based on several assumptions (e.g. as outlined in SI). I miss a thorough presentation of these in the manuscript and the discussion of the model uncertainties. When using the MCM mechanism for analysing the role of OVOC, the ROx chemistry and ozone formation potential (OFP), which draw-backs do you expect or which questions cannot be answered with your model approach? Where are the limitations?
Finally, as Dai et al. contribute to the current discussion of VOC and OVOC for urban air quality, I would like to recommend this article for publication, yet, with major revisions. Please, find in the following my specific comments that should be addressed before publication.
Abstract
In general, the abstract would benefit from a more precise and scientific wording. Please, consider to ask a native speaker to review the manuscript for assistance in improving the English. In the abstract, it would be great to highlight the findings of each of the three results-parts more clearly.
Line 12-14: I stumbled upon the formulation in the first sentence as it seems rather bulky and unprecise. Please rephrase. For example, what do you mean with the statement that OVOC are “key components” of VOC? Do you want to highlight them as a sub-group of VOC? Please, also check for English grammar, as I believe “secondary generated” is not correct. Why do OVOC play a crucial role in tropospheric chemistry? I think the sentence could work better, if you shortended it and added causality between this statement and the ozone precursors (e.g. …as they can act as ozone precursors).
Line 18: What is the upper-middle range of VOC mixing ratios in previous studies? Which locations do you refer to?
Line 18: Here (and elsewhere in the text) you present numbers in % with one decimal place 30.0-37.8%. Considering the uncertainties of VOC and OVOC measurements and the width of this range, is it meaningful to report the one digit on the right of the decimal point?
Line 19: The overall O3 formation potential in Zibo… Do you mean the average O3 formation potential?
Line 21: Why did you combine emissions and transport in the assessment of contributions to OVOC?
Line 23: I think that the study did not include any statistical tests for significance. It might be interesting to do so if sufficient data points are available. If not, please reword and avoid phrases with “significantly”.
Line 24: Typo: overestimates
1. Introduction
Line 32: Meaningful digits?
Line 42: My suggestion for a more precise formulation … which leads to a large production of HOx…
Line 47 (and following) highlight the sources of OVOC. Please, present a complete list here and do not only show examples. Furthermore, the introduction would benefit from integrating the specific OVOC that were highlighted in the studies that you are referring to.
Line 54-55: As the results pick up on the fate of OVOC and their impact on air quality, I recommend to include a short paragraph about sinks and the ozone formation potential here.
Line 75 (and following): Please, be careful in the wording as it is important that your results can be generalized and are useful for the entire community, not only for Zibo inhabitants. I think that this is the case and that you can make it more clear here. E.g. you could present your hypothesis here.
2. Methodology
Line 99: Here, in Figure 1 and in Table S1 information about the sites is provided. Did you have also wind data from the measurement stations directly? Can you comment on the possibility that site-scale wind conditions influenced the VOC and OVOC observations?
Line 107 (and following): Please explain PAMS. I prefer to have a full list of VOC and OVOC in the SI, including information about calibration, response factors, extraction efficiency (for the OVOC) and uncertainties. Please, provide details about the sampling line. Did you remove ozone to avoid losses of the VOC or OVOC?
Line 127: Typo: Coupled
Line 154 (and following): The variation of atmospheric mixing ratios of OVOC depends on its sources and sinks. The list here includes photochemical production and emissions (both sources), regional transport (both source and sink), and deposition (sink). Why did you combine emissions and transport in this budget? Why is the photolysis of OVOC not included as a loss? I checked the two reference provided here and they did not help me understanding the approach. Tan et al. (2018a) used the same OBM, but for assessing ozone production and loss. They refer to Tan et al. (2017) who have used the model again in a different context and highlighted large uncertainties of the model calculations. Xue et al. (2014) used the OBM as well and described it in detail in their SI. However, again, I cannot find information about OVOC or the budget approach analysing production and loss terms. I find this method description crucial for understanding the results parts (3.2). With the currently provided information and description, I do not fully understand the approach and cannot judge the quality of the results.
Equations (1) and (2): In line with my previous remark, the reader needs more information and a detailed explanation/justification about the choice of production and loss terms. Please provide a suitable reference, in addition.
3. Results and discussion
Line 169 (and thereafter): You are reporting mixing ratios of VOC and OVOC in ppbV. Do not change the wording to concentrations.
Line 169 (and thereafter): What is the uncertainty of the VOC and OVOC measurement? Is it meaningful to report the first digit after the decimal point?
Line 173: What is the upper middle range? And why do you conclude that this indicates strong anthropogenic VOC emissions at Zibo city?
Line 177: Daytime maximum temperature is reported here the second time (compare line 163).
Line 179: NO2 mixing ratios
Line 183: What do you mean with “the timing of the PAMS data was matched to that of the OVOCs data”?
Line 188 (and similar starts of sentences): Please check for correct English formulation. “As for individual side” does not sound correct.
Line 188: O3 mixing ratios
Lines 189 – 212: The paragraph describes the results very qualitatively with relative wording as “slightly lower”, “comparable levels”, “higher than”. Is any of the observed differences significant? How many data points to you have? Can you do a statistical analysis with a significance test? Can you focus this paragraph on the main message and discussion point?
Lines 213—220: The mean OFP in Zibo were 410 µg m-3 with CQ (464 µg m-3), TZ (456 µg m-3), ZL (441 µg m-3), XD (422 µg m-3) and CD (279 µg m-3). So all stations wree similar except of CD, correct? Did you do a significance test? In the conclusions (lines 345-347) other numbers are stated. What is correct?
Line 220: Typo: key role.
Figure 4: The two panels show the ozone formation potential attributed to measured VOC and OVOC in absolute and relative proportions. They do not show the concentrations. However, it would be interesting to have the same stacked bar plot for the mixing ratios in addition (e.g. as a third panel).
Line 237: What do you want to say with this first sentence? Please rephrase.
Line 238: What do you mean with saying that “all five sites show good model performance”?
Lines 238: What “R values”? What “IOA values”? I cannot see those in Figure S5.
Line 243: This sentence needs to be phrased very precisely, as I think it could be misleading otherwise.
Line 245/246: What do you mean with “mainly concentrated”?
Lines 242-263: This discussion is difficult to follow overall as it is rather qualitative and I did not understood the methodology in the first place. In general, can you elaborate on the sensitivity of dOVOC/dt and its sensitivity towards the time-step. Did you use the measurement intervals or hourly data from the model?
Line 272: Typo:..will result in…
Line 274: Are the fractions given here the comparison of the Free Scenario with the Base case?
Line 277: How large was the fraction of HCHO of the total OVOC measured in general?
Line 280/281: I don’t understand this sentence. Do you mean the chemical reactions that include RO2, HO2 and OH, which are known and/or implemented in the MCM mechanism? This is certainly not the same.
Lines 282-306: Please, be precise in the presentation of results and avoid relative wording. It is difficult to follow here the discussion and to find out what the key message is.
Figure 7, Figure 8 and discussion in the text: These are certainly interesting results, visualizing the different production and loss pathways both for ROx and O3 production. How much do uncertainties in the model and the chemical mechanism play a role for the interpretation? What are the limitations of your approach for (1) the site comparison and for (2) the role of OVOC in the air quality analysis?
4. Conclusions
Line 339: What exactly is the upper-middle range of VOC mixing ratios in which cities?
Lines 343-347: Numbers of OFP here are different than in the rest of the manuscript.
Supplementary Information
A lot of information is provided in the supplement only, but is discussed widely in the article. Please, reconsider using some of it for clarifications of the main text.
Citation: https://doi.org/10.5194/egusphere-2024-3201-RC1 - AC2: 'Reply on RC1', kun zhang, 18 Feb 2025
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RC2: 'Comment on egusphere-2024-3201', Anonymous Referee #2, 08 Jan 2025
The article from Dai et al., “Significant influence of oxygenated volatile organic compounds on atmospheric chemistry analysis: A case study in a typical industrial city in China”, investigates the contribution of volatile organic compounds, including oxygenated VOC, to ozone formation potential using ambient data from five observation sites around Zibo, an industrial city in the North China Plain, and an observation-based model (OBM).
The topic covered by the study can help assessing correct strategies to mitigate urban air pollution, therefore I find the study of interest to the atmospheric sciences community and in scope with the journal. However, in its current form, the methodology used by the authors is not described with enough clarity and for this reason it is hard to assess the robustness of the study and the presented results. For this reason, I would suggest publication after some major revisions are considered.
The main comment I have concerns the methodology used by the authors. The experimental methods used by the authors are not sufficiently presented. The authors use measurements from five observation sites collected during five days using online gas-chromatography (GC) and offline gas-chromatography. While offline GC can require extensive timing for both sampling and analysis and can be limited by samples availability, I wonder if there is any particular reason why the online field campaigns lasted for short time periods and if the limited sampling time is representative to the field sites and conditions? Can the authors explain this choice, maybe related to previous studies conducted in the same area? The authors should also provide some information about the sampling and analytical methods used (type of analysis, GC column characteristics, desorption and separation methods, sampling inlets characteristics, use of any ozone scrubber during the sampling, samples storage) and uncertainty associated to the results. As the whole discussion is based on comparing the results among different sites there should be mention that the analytical techniques and sampling are comparable, was any test conducted to prove that?
The calculation-modelling methods miss some information as well. It is not easy to follow section 2.2 as many acronyms are present and several assumptions considered. Could the authors adjust this section in a way that it is clear which assumptions are considered and why? Could the authors provide the different equations used in their calculations and model with more detail? There is no mention on how the ozone formation potential is calculated and which data are used for that.
I suggest also to improve harmonization between the main text’s manuscript and the supplementary information. In the current form, there are many points discussed in the main text that are not immediately clear to the reader, as the main text misses to recall the corresponding information or figure actually presented in the supplement. Some of the information presented in the supplement could be included in the main text to improve clarity.
Specific comments:
line 16. What type of stations?
Line 18. Previous studies conducted where?
Line 81. 5-day field campaign: are they representative to the sites and conditions?
Section 2.1 please provide information about the methods as suggested in my general comment
Line 107. PAMS=?
I would suggest to include here a list of the measured compound, and the classification method considered by the authors to recall the chemical compounds in subsequent analysis.
Section 2.2 Please provide information about the methods and calculations as suggested in my general comment
Figure. 2 It is hard to see the contribution from acetylene in the figure.
Line 195. Is this also seen by the largest concentration of aromatics?
Line 198. Transport from where to where. Is this also the site where the largest concentration of O3 is formed?
Line 214. Please provide the method to determine OFP
Line 215. What are the contributions of other classes of VOC?
Line 216. Which OVOC contributes the largest? The authors can provide here the OVOC speciation.
Line 233. Please include a table with measured concentrations, OFP from each measured VOC and VOC group
Figure 4. There is a typo on the y axis title
Line 296. Define acronym AOC and how this value is determined in the methods
Fig. 7 &8. What is b) representing?
Line 318-319. Needed acronyms definition for O3 formation, P(O3), L(O3) and how these values are determined.
Line 339. Conducted in China or urban areas in general? What is the upper-middle range? A review of the existing studies would be helpful (if the review is the same presented in the SI, I suggest to include the review in the main text).
Line 348. Refer to SI where needed.
Line 356. Can the authors separate emissions from transport? For example using an auxiliary method, such as a source apportionment method or ancillary measurements
Citation: https://doi.org/10.5194/egusphere-2024-3201-RC2 - AC1: 'Reply on RC2', kun zhang, 18 Feb 2025
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