Measurement report: Characteristics of airborne black carbon-containing particles during the 2021 summer COVID-19 lockdown in Yangzhou, China

Dai, Yuan; Wang, Junfeng; Wang, Houjun; Cui, Shijie; Zhang, Yunjiang; Li, Haiwei; Wu, Yun; Wang, Ming; Aruffo, Eleonora; Ge, Xinlei

doi:https://doi.org/10.5194/egusphere-2023-2454

Preprints

https://doi.org/10.5194/egusphere-2023-2454

Preprints

04 Dec 2023

| 04 Dec 2023

Measurement report: Characteristics of airborne black carbon-containing particles during the 2021 summer COVID-19 lockdown in Yangzhou, China

Yuan Dai, Junfeng Wang, Houjun Wang, Shijie Cui, Yunjiang Zhang, Haiwei Li, Yun Wu, Ming Wang, Eleonora Aruffo, and Xinlei Ge

Abstract. Black carbon-containing (BCc) particles are pervasive in ambient atmosphere. The unexpected outbreak of the COVID-19 pandemic in 2021 summer prompted a localized and prolonged lockdown in Yangzhou City, situated in the YRD, China, which provides a unique opportunity to gain insights into the relationship between emission sources and BCc. Satellite and ground-level measurements both demonstrated that strict emission controls effectively reduced local gaseous pollutants. Meanwhile, single particle aerosol mass spectrometer (SPA-MS) analysis showed that the number fraction of freshly emitted BCc particles decreased to 28 % during the lockdown (LD), with that from vehicle emissions experiencing the most substantial reduction. However, the uncontrolled reductions of nitrogen oxides (NO_x) and volatile organic compounds (VOCs) likely contributed to increased ozone (O₃) concentrations, increased the oxidizing capacity, which may in turn enhanced secondary PM_2.5 formation and compensated the primary PM_2.5 reduction. As a result, we did observe a slight increase of PM_2.5 concentration (21.2 μg m^-3) during the LD period compared to the period before the lockdown (BLD), and the increase of more aged BCc particles. Reactive trace gases (e.g., NO_x, SO₂, and VOCs) could form thick coatings on pre-existing particles likely via enhanced heterogeneous hydrolysis under high RH as well, resulting in significant BCc particle growth (~600 nm) during LD period. Furthermore, BCc source apportionment reveals that BCc particles were primarily of local origin (78 %) in Yangzhou during normal summertime. However, coal combustion (23 %) and vehicle emissions (21 %) were prominent non-local pollution sources, with the air mass originating from the southeast, along with biomass burning emissions (19 %) from the northeast, contributing significantly. Our research highlights that short-term, strict local emission controls may not effectively reduce PM pollution, due to the non-linear responses of PM_2.5to its precursors , further effective PM_2.5 reduction requires a comprehensive and extensive approach with a regionally coordinated and balanced control strategy through joint regulation.

Received: 21 Oct 2023 – Discussion started: 04 Dec 2023

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Yuan Dai, Junfeng Wang, Houjun Wang, Shijie Cui, Yunjiang Zhang, Haiwei Li, Yun Wu, Ming Wang, Eleonora Aruffo, and Xinlei Ge

Status: final response (author comments only)

RC1:
'Comment on egusphere-2023-2454', Anonymous Referee #2, 24 Dec 2023

The manuscript by Dai et al. reports the observation data by the single particle aerosol mass spectrometer (SPA-MS) at Yangzhou, China. The observation was conducted at around the COVID-19 lockdown (LD) period in 2021. The major finding includes enhanced abundance of aged black carbon (BC) particles during the LD period. The technique for the study sounds. The manuscript is well organized and written as a measurement report. The topic is within the scope of the journal. I suggest the publication of this manuscript after addressing the following comments.

Comments
Figure 3
Do the authors have BC and CO concentration data? Both these chemical species are emitted from incomplete combustion. Their emission ratios depend on types of sources. If the authors could provide these data, it may help supporting the conclusion that the major emission source of BC during the LD period was different.

Figure 5
The label for x-axis is missing. During the LD period, most of carbon cluster ions were smaller than C₇⁺, while C₈⁺~C₁₁⁺ ions were abundant during the before/after the LD period. Does the change in the carbon cluster ion sizes tell anything about emission sources?

L308
Can these organic ion fragments be produced from POA, SOA, or both of them? Do the authors have any comments?

L492
Could condensation also contribute to the process? Or, do the authors have a strong support to demonstrate that the process was dominantly occurring by aqueous or heterogeneous reactions?

L504-505
Was Yangzhou the only one city which experienced the lockdown in the YRD region during the 3.5 months of the observation period? Could the authors be able to provide some supporting information for the statement?

Citation: https://doi.org/10.5194/egusphere-2023-2454-RC1
- AC1:
  'Reply on RC1', Xinlei Ge, 18 Mar 2024
  Responses to reviewers’ comments
  
  We thank the reviewers for their detailed, helpful, and overall supportive comments. We have revised the manuscript to account for each comment. Responses to the individual comments are provided below. Reviewer comments are in bold. Author responses are in plain text. Modifications to the manuscript are in italics. Line numbers in the response correspond to those in the revised manuscript text file.
  
  Reviewer #1:
  
  1.       (Figure 3) Do the authors have BC and CO concentration data? Both these chemical species are emitted from incomplete combustion. Their emission ratios depend on types of sources. If the authors could provide these data, it may help supporting the conclusion that the major emission source of BC during the LD period was different.
  
  We thank the reviewer for insightful comments that help improve the original manuscript. We have incorporated the concentration data of CO into Figure 3 and expanded the original discussion as follows:
  
  Lines 257-261: “Given that both BC and CO are byproducts of incomplete combustion of carbon-containing fuels (Wang et al., 2015), , and the high correlation between BC and CO (Zhou et al., 2009), it is plausible to infer that the primary emission source of BC during LD differed from that during ALD. ”
  
  2.       (Figure 5) The label for x-axis is missing. During the LD period, most of carbon cluster ions were smaller than C7+, while C8+~C11+ ions were abundant during the before/after the LD period. Does the change in the carbon cluster ion sizes tell anything about emission sources?
  
  We are grateful for the suggestion. We have supplemented the labels for the x-axis in Figure 5 and the text is updated as follows:
  
  Lines 291~295: “Previous studies have indicated that high-mass carbon ions may be linked to traffic emissions, particularly those from diesel trucks (Xie et al., 2020; Liu et al., 2019), and the observed reduction in such ions during the LD period suggests a decrease in local vehicle emissions. This trend is also consistent with the changes observed in aromatic compounds, e.g. 119[C₉H₁₁]⁺.”
  
  3.       (Line 308) Can these organic ion fragments be produced from POA, SOA, or both of them? Do the authors have any comments?
  
  Thanks for the comment. We think organic ion fragments can be produced from both POA and SOA. Primary organic aerosols originate directly from emissions sources such as combustion processes (e.g., vehicle exhaust, biomass burning) and can contain a variety of organic compounds. Secondary organic aerosols, on the other hand, are formed in the atmosphere through the oxidation of volatile organic compounds (VOCs) and subsequent condensation onto pre-existing particles. Both POA and SOA can undergo ionization and fragmentation processes, leading to the production of organic ion fragments detected by instruments like mass spectrometers. Therefore, the presence of organic ion fragments in atmospheric aerosols can be indicative of contributions from both primary and secondary sources.
  
  4.       (Line 492) Could condensation also contribute to the process? Or, do the authors have a strong support to demonstrate that the process was dominantly occurring by aqueous or heterogeneous reactions?
  
  We are grateful for the suggestion. We think condensation can contribute to the process of aerosol formation and growth, particularly in the context of secondary organic aerosol (SOA) formation. However, we also suggest that aqueous or heterogeneous reactions may be the dominant mechanisms driving the observed changes in BCc particles during the LD period.
  We do not have direct evident to support this suggestion, there are several lines of clues:
  Observations of diurnal fluctuations: Significant diurnal fluctuations are observed in the OC/Cn and SNA/Cn ratios of BCc particles during the LD period. These fluctuations suggest dynamic chemical processes rather than simple condensation, as condensation alone would not typically exhibit such pronounced diurnal variations.
  
  Increase in BC-SNA particles during nighttime: There is a noticeable increase in the proportion of BC-SNA particles during nighttime, especially when relative humidity is relatively high. This observation suggests the involvement of heterogeneous hydrolysis, a type of chemical reaction, rather than purely condensation.
  
  Comparison of particle sizes and ratios: We compared the OC/Cn and SNA/Cn ratios of BCc particles with different diameters and found pronounced diurnal variations in these ratios for BCc particles with a diameter of ~400 nm during the LD period, indicating chemical reactions as the major pathway for particle formation and growth.
  
  Role of relative humidity: The significantly higher average relative humidity during the LD period compared to the periods before and after suggests favorable conditions for aqueous or heterogeneous reactions. This supports the idea that chemical conversion of trace reactive gases and the formation of thicker coatings on BCc particles are driven by these processes during the LD period.
  
  5.       Was Yangzhou the only one city which experienced the lockdown in the YRD region during the 3.5 months of the observation period? Could the authors be able to provide some supporting information for the statement? (Line 504~505)
  
  Thanks for the comment. As far as we know, Yangzhou experienced the most severe epidemic in the Yangtze River Delta (YRD) region during the observation period. Strict city-wide lockdown measures were implemented in Yangzhou, encompassing all downtown communities, from July 28 to September 10, 2021. In contrast, other regions in the YRD, such as Shanghai and Nanjing, implemented lockdown measures in only a limited number of communities during the same period.
  
  References:
  Liu, D., Joshi, R., Wang, J., Yu, C., Allan, J.D., Coe, H., Flynn, M.J., Xie, C., Lee, J., Squires, F., Kotthaus, S., Grimmond, S., Ge, X., Sun, Y., Fu, P., 2019. Contrasting physical properties of black carbon in urban Beijing between winter and summer. Atmospheric Chemistry and Physics 19, 6749–6769. https://doi.org/10.5194/acp-19-6749-2019
  Wang, Q., Liu, S., Zhou, Y., Cao, J., Han, Y., Ni, H., Zhang, N., Huang, R., 2015. Characteristics of Black Carbon Aerosol during the Chinese Lunar Year and Weekdays in Xi’an, China. Atmosphere 6, 195–208. https://doi.org/10.3390/atmos6020195
  Xie, C., He, Y., Lei, L., Zhou, W., Liu, J., Wang, Q., Xu, W., Qiu, Y., Zhao, J., Sun, J., Li, L., Li, M., Zhou, Z., Fu, P., Wang, Z., Sun, Y., 2020. Contrasting mixing state of black carbon-containing particles in summer and winter in Beijing. Environmental Pollution 263, 114455. https://doi.org/10.1016/j.envpol.2020.114455
  Zhou, X., Gao, J., Wang, T., Wu, W., Wang, W., 2009. Measurement of black carbon aerosols near two Chinese megacities and the implications for improving emission inventories. Atmospheric Environment 43, 3918–3924. https://doi.org/10.1016/j.atmosenv.2009.04.062
  
  Citation: https://doi.org/10.5194/egusphere-2023-2454-AC1
RC2:
'Comment on egusphere-2023-2454', Anonymous Referee #1, 24 Dec 2023

Dai et al. showed the measurement result of the Black carbon-containing (BCc) particle variation and its sources in Yangzhou, China during the COVID-19 pandemic in 2021 summer. The analysis is very detailed, and the manuscript is not hard to follow. I recommend publishing it as a measurement report on ACP after dealing with the following comments.
#1. Line 29. What kinds of “local gas pollutants”?
#2. Line 28-36. These sentences are organized in a little messy. Need to be rewritten more clearly.
#3. Line 46-50. These highlights do not match the title and the major objects of this paper. The author should provide some guidance for BCc, rather than a general PM_2.5.
#4. Line 75-76, references for “Due to the complex emissions and feedback with the East Asian monsoon”
#5. Line 122. Any previous results about BCc, as BCc is the topic of your study?
#6. Line 125. You do not conduct satellite measurements. Change to “combine”
#7. Line 148. What is the flow rate and cut size for your cyclone? More detailed information is needed.
#8. Line 168. Figure 1. Remove (b) and the right corner Chinese map is not very clear. Also the author should add some cities mentioned in the text.
#9. Line 178-182. It is very unclear here. Do you mean that you used the MERRA-2 data to replace the dataset from background SO2 from TROPOMI? Also, please provide the link for your data source and change SO2 mass concentration to SO2 column concentration, as satellite only provides column concentration.
#10. Line 242. Remove “Further”
#11. Line 254. Please provide the meaning about the marked region.
#12. Line 259. The resolution of Figure 3 is too low.
#13. Line 262. Which satellite produce of PM2.5 was used? The related information should be provided in section “2.2.1 Satellite Product”
#14. Line 430. Figure 7. It is hard to see the Log-normal distribution.
#15. Line 525. The resolution of Figure 10 is too low.
#16. Line 541. Figure 11 should be removed and be used as a TOC/Abstract graphic.
#17. Line 567. The resolution of Figure 12 is too low.
#18. Line 643-651. Similar to #3, the author should highlight some discussion about BCc, rather than a general PM_2.5.

Citation: https://doi.org/10.5194/egusphere-2023-2454-RC2
- AC2: 'Reply on RC2', Xinlei Ge, 18 Mar 2024
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-2454/egusphere-2023-2454-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2023-2454-AC2
RC3:
'Comment on egusphere-2023-2454', Anonymous Referee #3, 25 Dec 2023
Review of "Measurement report: Characteristics of airborne black carbon-containing particles during the 2021 summer COVID-19 lockdown in Yangzhou, China" by Dai et al.
Manuscript number: egusphere-2023-2454
The manuscript "Characteristics of airborne black carbon-containing particles during the 2021 summer COVID-19 lockdown in Yangzhou, China" mainly investigates the changes in black carbon-containing (BCc) particles during the COVID-19 pandemic in the summer of 2021. The authors found that the number fraction of freshly emitted BCc particles decreased to 28% during the lockdown (LD). Meanwhile, the authors observed an increase of more aged BCc particles. Authors conducted ground measurements, spaceborne observations, and mass spectrometric analysis during the COVID-19 2021 summer lockdown in Yangzhou, to investigated the impact of small-scale and short-term stringent emission controls on local ambient aerosol and the mixing state of BCc particles. The authors aim to provide valuable insights for future air pollution control measures. However, COVID-19 lockdown is a special policy made by the government during the special period; meanwhile, Yangzhou is too small to explain well the impact of small-scale and short-term stringent emission controls on local ambient aerosol and the mixing state of BCc particles. Furthermore, there are many ambiguities in the manuscript. Therefore, unfortunately, I do not recommend the manuscript to be published. Please see my comments below.

Major Comments:
The authors claimed that “the number fraction of freshly emitted BCc particles decreased to 28% during the lockdown (LD), with that from vehicle emissions experiencing the most substantial reduction”, and also expressed that “the uncontrolled reductions of nitrogen oxides (NOx) and volatile organic compounds (VOCs) likely contributed to increased ozone (O3) concentrations, increased the oxidizing capacity”. My question is what are the emission sources of NOx and VOCs under the scenario of “the vehicle emissions experiencing the most substantial reduction”? Therefore, I think the authors analysis is paradoxical. Therefore, the mechanism of increase of more aged BCs particles is unreasonable.

During the observation period, precipitation occurred intermittently (Figure 2b), and the author only mentioned “the data collected during the precipitation were excluded from the analysis” in Section 3.1. This method is obviously not enough to eliminate the impact of precipitation. Additionally, the maximum daily precipitation at the observation site does not exceed 10 mm (Figure 2b), however, the author has repeatedly mentioned heavy precipitation. How is the degradation of precipitation defined?

In Section 3.5, the authors propose a method to roughly estimate the local and non-local proportions for each type of BCc particles in Yangzhou only during the ALD period. And the distribution of BCc particles during LD is unclear and there is no comparison before and after. The manuscript focuses on the COVID-19 lockdown, how do the authors provide valuable insights for future air pollution control measures? As the authors pointed out, “Since there was a heavy precipitation on July 28th (the day before lockdown) which removed most atmospheric pollutants, the air pollutants might be influenced mostly by regional transport as local emissions were significantly cut down during the LD period”. Therefore, is it necessary to analyze the distribution of BCc particles during LD period?

Minor Comments:
A detailed description of the shading in Figure 1 should be given.

Line 177-181, NO2 (NRTI/L3 NO2) obtained from the TROPOspheric Monitoring Instrument (TROPOMI) with a spatial resolution of 3.5×7 km²; Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2 SO2SMASS) with a spatial resolution of 69×55 km². My question is, how do the authors think about the impacts of resolution difference between two datasets on the results?

Line 228, “dramatically”, the authors should objectively describe and carefully consider the modifiers.

Line 244-246, “surface O3 concentration showed an increase of 12.6 μg m-³ (19%) during the LD period compare to the BLD period, which may attribute to the reduction of fresh NO emissions that alleviates O3 titration”. In fact, the O3 concentration showed sustained higher values during the ALD period compare with those in BLD and LD periods (Figure 3). What is the reason?

Line 246-248, “However, the average concentrations of PM2.5 (19.9 vs. 21.2 μg m-³) and SO2 (9.4 vs. 9.5 μg m-³) were very close between BLD and LD stages (Figure 3).” According to this logic, TVOC was also very close between BLD and LD stages (Figure 3). Why was it not mentioned?

Line 254, There is a discrepancy between the segmentation of the chart and the textual expression. Additionally, are all observation elements conducted at a rooftop laboratory 20 m above ground? If yes, is it appropriate to use “surface”?

Line 270-273, “Such results highlight the short-term, limited-scale, and human-induced reduction in air pollutants as a result of the lockdown measures in Yangzhou, and demonstrate the effectiveness of regional stringent emission control in reducing local atmospheric pollutant concentrations”. From the analysis of the results, it is obvious that the result cannot be obtained. Furthermore, this conclusion is completely opposite to that in Section “Abstract” (Line 46-50).

Line275-280, Calculations were only conducted for the regions with PM2.5 > 10 µg m^-3, NO2 > 0.2 Dobson units (DU), and SO2 > 0.2 DU in the BLD period. What is the reason for defining these thresholds? Please provide an explanation.

Line 297, Figure 5, please add a horizontal axis identifier.

Line 354, As shown in Figure 1, it can be seen that significant precipitation occurs during the LD period. The authors’ expression here is “the day before lockdown”, please unify it.

Line 462, 477, RH is a key element which is responsible for the formation of BCOC and BC-SNA particles. Is there a simple positive or negative correlation between them?

Line 474-477, “As shown in Figure 9, BCc particles with ~400 nm Dva exhibited significant diurnal fluctuations in the OC/Cn and SNA/Cn ratios, during the LD period. Moreover, there was a noticeable increase in the proportion of BC-SNA particles during nighttime when RH was relatively high”. Compared with the LD period, BCc particles exhibited more significant diurnal fluctuations in the OC/Cn and SNA/Cn ratios during the ALD period. What is the reason?

Line 490-491, Please confirm the relationship between “BCc particles” and “RH” again to prevent conflicting results.
Citation: https://doi.org/10.5194/egusphere-2023-2454-RC3
- AC3: 'Reply on RC3', Xinlei Ge, 18 Mar 2024
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-2454/egusphere-2023-2454-AC3-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2023-2454-AC3

Yuan Dai, Junfeng Wang, Houjun Wang, Shijie Cui, Yunjiang Zhang, Haiwei Li, Yun Wu, Ming Wang, Eleonora Aruffo, and Xinlei Ge

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https://doi.org/10.5194/egusphere-2023-2454-supplement

Data sets

Data of BCc particles during the 2021 summer COVID-19 lockdown in YZ Yuan Dai https://doi.org/10.6084/m9.figshare.24427795

Yuan Dai, Junfeng Wang, Houjun Wang, Shijie Cui, Yunjiang Zhang, Haiwei Li, Yun Wu, Ming Wang, Eleonora Aruffo, and Xinlei Ge

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Short summary

Short-term strict emission control can improve air quality but the effectiveness requires assessment. During 2021 summer COVID-19 lockdown in Yangzhou, we showed that the PM_2.5 level did not decrease with decrease of gaseous pollutants as aged black carbon-containing particles increased substantially, due to enhanced atmospheric oxidizing capacity and high relative humidity. The results highlights the importance of a regionally balanced control strategy for future air quality management.


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