the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 04 Mar 2024
Proportional relationships between carbonaceous aerosols and trace gases in city plumes of Europe and East Asia
Abstract. The concentration of carbonaceous aerosols, black carbon (BC) and organic aerosol (OA), in the atmosphere is related to co-emitted or co-produced trace gases. In this study, we investigate the most relevant proportional relationships between both BC and OA with the following trace gases: carbon monoxide (CO), formaldehyde (HCHO), nitrogen dioxide (NO2), ozone (O3), and sulfur dioxide (SO2). One motivation for selecting these trace gases is that they can be observed using remote sensing measurements from satellite instrumentation, and could therefore be used to predict spatial changes in the amounts of BC and OA.
Airborne measurements are optimal for the analysis of both the composition of aerosols and trace gases in different environments ranging from unpolluted oceanic air masses to those in heavily polluted city plumes. The two aircraft campaigns of the EMeRGe (Effect of Megacities on the Transport and Transformation of Pollutants on the Regional to Global Scales) project have created a unique database, with flight plans dedicated to studying city plumes in two regions, Europe (2017) and East Asia (2018), along with identical instrumental payload.
Using linear regression analysis, three relevant relationships between carbonaceous aerosol and trace gases are identified:
- The BC/OA ratio observed in the Asian campaign is three times higher (≈ 0.3) than in the European campaign (≈ 0.1), whereas the Pearson correlation coefficient (R) between BC and OA is much higher in Europe (R ≈ 0.8) than in Asia (R ≈ 0.6).
- The CO/BC ratio is also observed higher in the Asian campaign (≈ 240) than in the European campaign (≈ 170), whereas the R-value between CO and BC is similar for both campaigns (R ≈ 0.7).
- The HCHO/OA ratio is similar in both campaigns (≈0.32), but the observed R-values between HCHO and OA is higher in Europe than in the Asia (R ≈ 0.7 compared to ≈ 0.3).
By focusing on heavily polluted air masses sampled downwind in the city plumes, the ratios between the observed carbonaceous aerosols and the five trace gases change, and the R-values increase with O3 for both BC and OA (R ≈ 0.5).
To assess the performance of atmospheric models with respect to the most relevant observed relationships, an air quality model ensemble is used to represent the current state of atmospheric modeling, consisting of two global and two regional simulations. The evaluation shows that these proportional relationships are not satisfactorily reproduced by the model ensemble. The relationships between BC and OA or between CO and BC are modeled with stronger correlations than the observed ones, and their higher ratios observed in Asia compared to Europe are not reproduced. Furthermore, the modeled HCHO/OA ratio is underestimated in the Asian campaign and overestimated in the European campaign.
This analysis of the proportional relationships between carbonaceous aerosols and trace gases implies that the observed relationships can be used to constrain models and improve anthropogenic emission inventories. In addition, it implies that information about the lower tropospheric concentration of carbonaceous aerosols can potentially be inferred from satellite retrievals of trace gases, particularly in the plumes from megacities.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2024-521', Anonymous Referee #1, 16 Apr 2024
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RC2: 'Comment on egusphere-2024-521', Anonymous Referee #2, 19 Apr 2024
Summary:
The paper by Deroubaix et al. investigates the proportional relationships between carbonaceous aerosols and trace gases using measurements from the EMeRGe aircraft campaigns over Europe and East Asia. The authors also evaluate the performance of regional and global models in simulating these relationships. The aircraft measurements and model evaluations provide valuable insights for the estimation of carbonaceous aerosols from the trace gases. Overall, there are potentially some interesting aspects of this paper, but at the same time there are a large number of results that are not explained or justified. The paper does not meet the publication criteria of ACP, and I cannot recommend it for publication.
Major comments:
1、 In the observational analysis, the authors primarily present their observational results without providing any physical explanations or comprehensive analysis. As a result, the paper reads more like an experimental report. The entire paper lacks thorough discussions and comparisons with relevant research. For instance, only three references are cited in Sections 3-7. The authors should compare the results with the previous studies and offer new insights to improve the scientific quality of this paper.
2、 In the model evaluation, the authors have performed only a basic comparison of model and measurements, and some logic seems misleading. The model bias in the proportional relationships should result from a combination of biases in both aerosols and trace gases. The modeled aerosol concentrations, especially in the source regions, are mainly determined by emission, transport, and particle chemistry and physical processes. The interpretations of the model bias in modeling the proportional relationships and their relation to the emission inventory are not sufficiently justified. The authors should first prove that whether the model bias in simulating proportional relationships primarily arises from the emission inventory (or model processes).
3、 The authors proposed three major scientific questions. The analysis of city plumes is critical but too brief. Additionally, the authors barely discussed the satellite-based estimation of aerosols. On the contrary, they extensively presented model results, yet they failed to integrate model evaluations into the major scientific questions.
4、 The organization of the paper is poor. The three scientific questions have been buried in Sections 3-7, making it difficult to hold the interest of readers. I recommend that the paper should be reorganized around the scientific questions.
Specific comments (in chronological order)：
1、 Lines 87-89: The paper focuses on the observed proportional relationships between aerosols and trace gases, as well as the model performance in simulating these relationships. The introduction (Lines 53-87) mainly emphasizes the significance of aircraft-based and satellite-based measurements for aerosols and trace gases. The authors should concentrate more on relevant observations and model simulations about the proportional relationship between aerosols and trace gases.
2、 Line 94: "cities plumes" -> "city plumes"; also check elsewhere
3、 Lines 100-104: The structure of the paper seems like a mechanical repetition and requires reorganization (see Major comments #4).
4、 Figure 2： How can we ensure consistency in time and space between aircraft observations and models with different resolutions? Additionally, the scatter points in the figure overlap with each other. All figures (including Figures 3-5) should be replotted to make them more clear.
5、 Lines 168-169 and Lines 172- 173: The author has not provided physical explanations for the higher R-value in Europe or for the much higher BC/OA ratio in Asia. Although the author mentioned wildfires in Indochina in Line 182, the explanation is not convincing and requires more comparisons with previous studies.
6、 Lines 179-181: Why do all the models simulate higher R-values than observations ? Please explain.
7、 Section 3.2, Section 4.2, and Section 5.2: The analysis of urban plumes is crucial but too brief. Also, suggest merging these sections.
8、 Figure A1: The city plumes of Asia and Europe come from different environments, so can they be plotted together in Figure A1? Can the models well reproduce the city plumes? Additionally, how can we ensure consistency in time and space between observations and models? Please explain.
9、 Lines 189-191: In the city plumes, why does the observed R-value decrease in observations but hardly decrease in the models? Please explain.
10、 Lines 209-213 and Table 1: Why are the R-values of the observed BC with the trace gases (CO, HCHO, NO2, and SO2) higher in Europe than those in Asia? Why is the R-value of the observed BC with O3 lower in Europe than that in Asia? Why are the R-values of the observed BC with the three trace gases (NO2, O3, and SO2) lower than the R-values of BC with the two trace gases (CO and HCHO) for the two campaigns? It requires more comparisons with previous studies and discussions.
11、 Line 216: The CO/BC ratio is 168.01 ppb per µg/m3 of BC in Europe and 243.75 ppb per µg/m3 of BC in Asia?
12、 Lines 216-219: Could you provide any comments on the differences in observed CO/BC and HCHO/BC ratios between Europe and Asia？
13、 Table 1：Could you provide any comments on the zero R-value of BC with O3 in the model (e.g., WRFchem–FNL and WRFchem–ERA5)? Why are the modeled R-values between BC and O3 by CAMS–forecast negative in Europe but positive in Asia? Please explain.
14、 Lines 222-223: Why is the observed CO/BC ratio in Europe much lower than that in Asia? Additionally, why is the CO/BC ratio in Europe modeled by CAMchem-CESM2 twice as high as that in Asia? Please explain.
15、 Lines 224-225 and Lines 232-233: Why do models typically overestimate the HCHO/BC ratio in Europe? Please explain.
16、 Lines 236-237: Please be careful in discussion. It is not proving that the model bias primarily comes from emission inventory.
17、 Lines 246-249 and Table 2: Why do the R-values of BC with the four trace gases (CO, HCHO, NO2, and SO2) decrease in city plumes? Additionally, why does the R-value of BC with O3 increase in city plumes? Is this related to the aging of carbonaceous aerosols during transport? Please explain.
18、 Lines 262-267 and Table 3: Why are the R-values of the observed OA with the trace gases (CO, HCHO, NO2, and SO2) higher in Europe than those in Asia? Additionally, why is the R-value of the observed OA with O3 lower in Europe than that in Asia? It requires more comparisons with previous studies and discussions. I recommend merging the discussion with Specific Comment #10.
19、 Lines 269-271: Why is the observed CO/OA ratio in Europe much lower than in Asia? Additionally, why is the observed HCHO/OA ratio similar in Europe and Asia? It requires more comparisons with previous studies and discussions. I recommend merging the discussion with Specific Comment #12.
20、 Lines 284-286: Could you provide any comments on the strong linear relationship of CO/OA in regional simulations compared to observations?
21、 Lines 295-297 and Table 4: Why do the R-values of OA with the four trace gases (CO, HCHO, NO2, and SO2) decrease in city plumes? Why does the R-value of OA with O3 increase in city plumes? I recommend merging the discussion with Specific Comment #17.
22、 Section 6：The discussions are critical, but the majority of this section lacks support from current analysis (see Major Comment #2).
23、 Lines 377-389: The analysis of city plumes is too brief. Additionally, there is little analysis of satellite-based estimation of aerosols, but extensive results of model evaluations (see Major Comment #3). I recommend integrating the model evaluations into the scientific questions.
Citation: https://doi.org/10.5194/egusphere-2024-521-RC2
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