the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 15 Apr 2024
Applicability of small-scale black carbon sensors to explore high resolution spatial variability of ambient black carbon
Abstract. Black Carbon (BC) is a particulate pollutant emitted as a by-product of combustion. BC has an emerging role in air quality monitoring with the current recommendations by the World Health Organization, that systematic measurements of BC should be conducted to capture the temporal and spatial variability of BC. To observe this variability, especially in urban areas, a large quantity of sensor-type measurements is required. In this study, four different types of small-scale filter-based BC sensors (AE51, MA200, MA350, Observair) were used to build a sensor network in Kumpula campus, Helsinki, Finland. Our aim was to test the applicability of the sensors to monitor ambient BC concentrations in field conditions and to study the variation of BC in high resolution. The results were compared to a reference level instrument (MAAP) for validation. During intercomparisons, the sensors had a good correlation with the reference and after a simple orthogonal regression calibration, were deemed suitable for deployment in the sensor network. During deployment, the sensor network proved to be able to capture small scale temporal and spatial differences in BC concentrations and showed potential for source-apportionment applications. The changes in temperature (T) and relative humidity (RH) were observed to induce error in the BC measurements. This error was amplified by the dualspot correction, which was worsening the measurement result under instable conditions of T and RH. This should be considered when using sensors that apply this correction automatically. The environmental compensation used by the Observair sensors reduced the error from the changing T and RH. To reduce the effect of changing T and RH, more robust environmentally controlled boxes should be developed or correction algorithms, such as environmental compensation, should be applied.
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RC1: 'Comment on ar-2024-12', Anonymous Referee #1, 24 May 2024
The manuscript presented a study of the spatial resolution of ambient black carbon collected in parallel with different types of black carbon sensors. This study is relevant and useful for atmospheric researcher. The study is very informative and well-structured. I, therefore, congratulate the authors for their efforts.
That is why I will suggest publication with minor corrections, namely:
Careful checking of the English language and the text as a whole is necessary.
There are also a few misstatements which I would advise correcting.
The author should change the title. It does not only examine the spatial variability of ambient black carbon.
In introduction: Please correct PM2.5 to PM2.5.
Page 11, Paragraph 210: A sharp ATN change |ATN| >30 was manually identified. What could be the reason for this sharp change? Please explain it.
Page 11, 3.1 section: The authors used different flow rates for the same type of sensors. This is sometime hard to follow in the article. Please create a table summarizing this. How much during each campaign, etc.
Page 12, Paragraph 255: The authors wrote that the 1st intercomparison has on average lower concentrations compared to the 2nd intercomparison. Is it not because of the different meterological condition? Please explain it.
Page 13, Figure 2: Please check the labels (date) at the xaxis. Please correct it.
Page22, Paragraph 400: The authors wrote the following: With the MA-series sensors (MA200, MA350) the change of the temperature and RH caused clearly erroneous data as seen in Fig. 12
However, we cannot see the results of MA200 sensor in the Figure 12. Please include its results in the figure.
The conclusion contains some statements that need to be clarified. For istance: what is DST? please explain it.
Citation: https://doi.org/10.5194/ar-2024-12-RC1 - AC1: 'Reply on RC1', Tapio Elomaa, 30 May 2024
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RC2: 'Comment on ar-2024-12', Anonymous Referee #2, 28 Jun 2024
The manuscript presented a comparative study on understanding the performance of portable and low-cost black carbon monitors available in the scientific community. This study is relevant and useful as it provides information on BC monitoring devices that can be used for large-scale spatial and temporal monitoring across regions. Results from this manuscript can enrich the scientific community and regulatory bodies. Hence, I recommend this manuscript be published with some major corrections.
General suggestions:
1. A language check is necessary for the entire manuscript. Some segments of the text do not follow common practices of reporting data or manuscript writing, such as consistently reporting units of mass concentration or time.
2. Section titles require renaming (based on the updated context), particularly in Section 2 (Methods) and Section 3 (Results).
3. I highly recommend adding more text discussing the results and making section 3 from "Results" to "Results and Discussions," which is a typical nature of manuscript framing in similar types of scientific journals.
4. The title of the manuscript can be updated as the manuscript content does not presently match with the title.
5. Use PM2.5 everywhere
6. Line 71: Why is MAAP chosen as a reference device to compare with Aethalometers, which work slightly differently? AE33 could be a great reference device in this work. If the authors have AE33's data, adding a comparison might help understand the unit-specific offsets in measuring BC. If they don't, I highly recommend providing sufficient justification (here in Line 71) or in the methods section why MAAP is used or better suitable.
7. Line 85: Some theoretical mistakes were identified. Please check the literature and correct it. Aethelometers measure light intensity and calculate light attenuation (ATN). From ATN measurements, ATN coefficients (bATN) are derived, followed by the absorption coefficient (babs). Drinovec 2015 explained this well.
8. Line 105: There are inconsistencies around the assumptions made. If any scattering correction is made, why are they assumed to be unity? More explanations are required. Also refer to Line 152.
9. Line 118: Leakage factor changes over time. Please refer to Drinovec 2015. This assumption might change the corrections, and so might the final corrected BC concentration. Please reconsider adapting to such changes.
10. It is not clear if the MA series device's inbuilt Dual-spot corrections were used for comparison or not. Typically, MA devices have their own correction mechanism, which is not the same as Drinovec 2015. Please check and confirm with a table of corrected data or add it to Table 2.
11. Some comments on figures have been mentioned in the attached file, which mainly focuses on the visibility of the graphics and texts.
12. Section 3.1.2: A separate segment on the sensor calibration in methods sections is recommended. Also, mention how the calibrations are assessed (metrics used, such as slope, MAE). Reporting the calibration results are helpful for future studies. While presenting the data in "Result" section, please compare it with previous literature. A table might be helpful for reporting metrics from uncalibrated data, calibrated data, and literature data - with the type of calibration procedure adopted. All these elements will expand this section, which I believe is going to improve the quality of the manuscript.
13. Generally, the results and discussion section is missing references from previous literature. Please compare the reported BC levels with different devices under this work and the levels reported in previous studies in similar regions or of similar spatial characteristics.
14. Section 3.2.4: Change the section title. I recommend removing this section and adding a detailed discussion explaining the variability in previous sections. If the authors want to keep this section, will this require some detailed analysis explaining how spatial variability is captured by different devices? If there are any differences in performances? Also, some discussion was required about how spatial variability can be captured by these devices. If the true spatial variability is higher/lower than the inter-device variability studied. Finally, some recommendations /comments would be helpful for the community, such as which device performed best in what context.
15. After restructuring the result and discussion section - please update the conclusion accordingly. Avoid including conclusions that have not been discussed well in previous sections (For example, Line 436).
Some minor comments can be found in the attached file.
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AC2: 'Reply on RC2', Tapio Elomaa, 17 Sep 2024
We thank the Referee #2 for the valuable comments on the manuscript. Please find the point-to-point answers to the comments below.
- A language check is necessary for the entire manuscript. Some segments of the text do not follow common practices of reporting data or manuscript writing, such as consistently reporting units of mass concentration or time.
The manuscript has been checked in order to catch any deviancies from the guidelines.
- Section titles require renaming (based on the updated context), particularly in Section 2 (Methods) and Section 3 (Results).
Following section titles have been updated. Section 2.2 “Dualspot correction algorithms”. 2.4 “Deployment area”. 3.1 “Intercomparison periods”. 3.1.1 “Applicability of the dualspot corrections”. 3.2.2 “Weekly features in BC concentration”
- I highly recommend adding more text discussing the results and making section 3 from "Results" to "Results and Discussions," which is a typical nature of manuscript framing in similar types of scientific journals.
The title has been adjusted to the suggestion.
- The title of the manuscript can be updated as the manuscript content does not presently match with the title.
The title has been changed to:
The applicability and challenges of black carbon sensors in dense monitoring networks.
- Use PM2.5everywhere
Corrected.
- Line 71: Why is MAAP chosen as a reference device to compare with Aethalometers, which work slightly differently? AE33 could be a great reference device in this work. If the authors have AE33's data, adding a comparison might help understand the unit-specific offsets in measuring BC. If they don't, I highly recommend providing sufficient justification (here in Line 71) or in the methods section why MAAP is used or better suitable.
Unfortunately, no AE33 data was available during this campaign. MAAP was utilized as the reference instrument as it was the highest-grade instrument available during this period.
- Line 85: Some theoretical mistakes were identified. Please check the literature and correct it. Aethelometers measure light intensity and calculate light attenuation (ATN). From ATN measurements, ATN coefficients (bATN) are derived, followed by the absorption coefficient (babs). Drinovec 2015 explained this well.
Error fixed. “The measured variable by the instrument is the attenuation coefficient batn(λ) [m-1] calculated from the measured attenuation and the operational parameters of the instrument as described in Eq. 2.”
- Line 105: There are inconsistencies around the assumptions made. If any scattering correction is made, why are they assumed to be unity? More explanations are required. Also refer to Line 152.
Scattering correction is the 𝑠(𝜆)𝜎sp(𝜆) part of the general correction scheme (Eq. 3). This refers to the scattering from the aerosols and is ignored in this study which was meant by saying “assumed unity”. The multiple scattering correction (Cref) refers to the enhanced attenuation due to the filter fibers. For this the manufacturer recommendations are used. This statement is rephrased to:
“The aerosol scattering correction requires measurement of the scattering coefficient, which in many cases is not possible due to the lack of instrumentation. Due to this the aerosol scattering correction is often voided as in this study”
Text has been adjusted that in reference to these to corrections, aerosol scattering correction is used for the 𝑠(𝜆)𝜎sp(𝜆) part and multiple scattering correction for the Cref.
- Line 118: Leakage factor changes over time. Please refer to Drinovec 2015. This assumption might change the corrections, and so might the final corrected BC concentration. Please reconsider adapting to such changes.
The line referring to the leakage factor has been removed from the text and equations. This was an error in the manuscript. The leakage factor was tested during data processing of the manuscript but later removed.
- It is not clear if the MA series device's inbuilt Dual-spot corrections were used for comparison or not. Typically, MA devices have their own correction mechanism, which is not the same as Drinovec 2015. Please check and confirm with a table of corrected data or add it to Table 2.
The MA series inbuilt corrections are used. Meaning the dualspot corrected data that the instrument gives is used as is.
Confusion to this is related that the authors were not aware of the exact correction algorithm used by the MA-series instruments. It was assumed that Dualspot refers to the correction outlined in Drinovec et al. (2015). This is now considered and refered in the modified version of the manuscript.
- Some comments on figures have been mentioned in the attached file, which mainly focuses on the visibility of the graphics and texts.
These comments have been taken into account and the plots updated.
- Section 3.1.2: A separate segment on the sensor calibration in methods sections is recommended. Also, mention how the calibrations are assessed (metrics used, such as slope, MAE). Reporting the calibration results are helpful for future studies. While presenting the data in "Result" section, please compare it with previous literature. A table might be helpful for reporting metrics from uncalibrated data, calibrated data, and literature data - with the type of calibration procedure adopted. All these elements will expand this section, which I believe is going to improve the quality of the manuscript.
Calibration methodology is added to section 2.5. The results of the calibration are showed in section 3.1.2
- Generally, the results and discussion section is missing references from previous literature. Please compare the reported BC levels with different devices under this work and the levels reported in previous studies in similar regions or of similar spatial characteristics.
Some references to previous studies in Helsinki have been added to section 3.2.1.
- Section 3.2.4: Change the section title. I recommend removing this section and adding a detailed discussion explaining the variability in previous sections. If the authors want to keep this section, will this require some detailed analysis explaining how spatial variability is captured by different devices? If there are any differences in performances? Also, some discussion was required about how spatial variability can be captured by these devices. If the true spatial variability is higher/lower than the inter-device variability studied. Finally, some recommendations /comments would be helpful for the community, such as which device performed best in what context.
This section is removed and merged with section 3.2.1.
- After restructuring the result and discussion section - please update the conclusion accordingly. Avoid including conclusions that have not been discussed well in previous sections (For example, Line 436).
The conclusions have been updated to correspond to the results and discussion provided in the manuscript.
The comments provided in the attached file have also been answered with point-to-point answers and can be found in the attached file.
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AC2: 'Reply on RC2', Tapio Elomaa, 17 Sep 2024
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