Fugitive Emissions from Mobile Sources—Experimental Analysis in Buses Regulated by the Euro 5 Standard

Round 1

Reviewer 1 Report

Dear authors, your research subject and idea are interesting and innovative. However, reading the article raised a number of questions that need to be answered.
1. First of all, the problem is not very clearly formulated. The first sentence of the Abstract carries a very strong statement. And I think the message it carries is misleading. Fugitive emissions account for 50% of GHG? It needs to be explained and justified. Does this mean that if there were no Fugitive emissions, it would be 50% less GHG? Or does the authors claim that 50% of all GHGs entering the environment are via Fugitive emissions? We need a clear explanation and necessarily with scientific sources. Of course, this explanation should not be in the Abstract, but in the Introduction or Literature Review section. This is a very important point, would there be less GHG? Do they end up in the environment anyway, just in the wrong place in the process where they should? And this is where the concept of the article comes from, are the authors investigating air pollutants and GHGs entering the environment outside of the designed location (at the end of the exhaust pipe)? Would this gas in any case enter the environment if the exhaust pipe were solid (sealed)?
2. Research methodology. The sampling location (in the exhaust pipe) must be clarified. Was it the same in all buses and what principle was followed? Have you looked for places that clearly indicate that the system is leaking (clear traces of exhaust gas penetration leaving a black color trace)? As is known, internal combustion engines are equipped with various air pollution and GHG reduction measures (diesel oxidation catalyst, diesel particulate filter, selective catalytic reduction, ammonia oxidation catalyst, diesel exhaust fluid, etc.) [1]. It is very important to note where the samples were taken - before or after these systems? The key question to be answered is whether the air pollutants and GHGs would have reached the atmosphere anyway (same amount and proportions), just somewhere else in the system?
3. As is known, the major contribution to GHG from the internal combustion engine is CO2. Other GHG gases (such as N2O, CH4 make up a significantly smaller proportion [2]). It should also be emphasized that CO2 emissions in engines do not depend much on the air pollution reduction measures implemented, the main potential for reducing them is lower fuel consumption [3]. The authors should note that the main means of reducing CO2 is lower fuel consumption (this can be achieved by various organizational and technological means, in some cases replacing one type of fuel with another).
4. The authors did not investigate "normal" tailpipe emissions in their study. But wouldn't they be exactly the same as fugitive emissions (in their composition and proportion)? And if so, shouldn't it be noted in the conclusions that in principle it makes no difference to the overall air pollution and GHG levels? And is it more related to the technical state of the buses?
Sincerely, reviewer

[1] https://doi.org/10.3390/atmos12060674
[2] EMEP/EEA Air Pollutant Emission Inventory Guidebook
[3] https://doi.org/10.3390/app11188662

Author Response

Fugitive Emissions from Mobile Sources - Experimental Analysis in Buses Regulated by the Euro 5 Standard

Manuscript ID: atmosphere-2204279

Point-by-point response – letter.

Dear Editor, Ms. Nevena Popović.

First of all, we would like to thank the reviewers by the excellent and detailed revision performed, of course we understood that all the comments, suggestions and solicitations coming from the reviewers will substantially improve the submitted manuscript to reach the high-level research that any manuscript must have in order to be considered for publication in the Atmosphere Journal. Best regards.

Prof. Dr. Cid Marcos G. Andrade

Comments from reviewer #1.

1. First of all, the problem is not very clearly formulated. The first sentence of the Abstract carries a very strong statement. And I think the message it carries is misleading. Fugitive emissions account for 50% of GHG? It needs to be explained and justified. Does this mean that if there were no Fugitive emissions, it would be 50% less GHG? Or does the authors claim that 50% of all GHGs entering the environment are via Fugitive emissions? We need a clear explanation and necessarily with scientific sources. Of course, this explanation should not be in the Abstract, but in the Introduction or Literature Review section. This is a very important point, would there be less GHG? Do they end up in the environment anyway, just in the wrong place in the process where they should? And this is where the concept of the article comes from, are the authors investigating air pollutants and GHGs entering the environment outside of the designed location (at the end of the exhaust pipe)? Would this gas in any case enter the environment if the exhaust pipe were solid (sealed)?

Reply from Authors: Obviously we understand the reviewer preoccupation concerning to the high percentage (50%). We must inform that it was a big typo error from our side. Actually, the percentage is just “ 5% ” instead  “ 50% ”, we sincerely apologize our error.  This percentage (5%) is partially explained along the introduction, in this point we completely agree with the reviewer, and of this form we have added to the manuscript more technical / scientific information (as requested) in order to enforce and clarified this percentage (5%).

2. Research methodology. The sampling location (in the exhaust pipe) must be clarified. Was it the same in all buses and what principle was followed? Have you looked for places that clearly indicate that the system is leaking (clear traces of exhaust gas penetration leaving a black color trace)? As is known, internal combustion engines are equipped with various air pollution and GHG reduction measures (diesel oxidation catalyst, diesel particulate filter, selective catalytic reduction, ammonia oxidation catalyst, diesel exhaust fluid, etc.) [1]. It is very important to note where the samples were taken - before or after these systems? The key question to be answered is whether the air pollutants and GHGs would have reached the atmosphere anyway (same amount and proportions), just somewhere else in the system?

Reply from Authors: We agree with the reviewer comments. The sampling location was not the same for all the buses involved in this research. The location was defined, for each vehicle, in function of visual inspection, looking for black/dark color traces (as observed in figure  2 of the manuscript) and before the after treatment exhaust system.  This information has been added to the revised version of the manuscript.

3. As is known, the major contribution to GHG from the internal combustion engine is CO2. Other GHG gases (such as N2O, CH4 make up a significantly smaller proportion [2]). It should also be emphasized that CO2 emissions in engines do not depend much on the air pollution reduction measures implemented, the main potential for reducing them is lower fuel consumption [3]. The authors should note that the main means of reducing CO2 is lower fuel consumption (this can be achieved by various organizational and technological means, in some cases replacing one type of fuel with another).

Reply from Authors: We completely agree with the reviewer comment. This information has been added to the revised version of the manuscript.

4. The authors did not investigate "normal" tailpipe emissions in their study. But wouldn't they be exactly the same as fugitive emissions (in their composition and proportion)? And if so, shouldn't it be noted in the conclusions that in principle it makes no difference to the overall air pollution and GHG levels? And is it more related to the technical state of the buses?

Reply from Authors: We completely agree with the reviewer comment. This valuable information has been added to the revised version of the manuscript, complemented by our answer to reviewer 2 (5^th comment answered below)

Reply from Authors: All the smaller points were solved/adapted in the revised version of the manuscript. Thanks very much.

Author Response File: Author Response.docx

Reviewer 2 Report

This paper reports an experimental leakage survey of selected greenhouse gases due to frictional wear and thermal fatigue in bus exhaust pipe. The research was conducted in the cities of Boa Vista, Roraima and Manaus, Amazonas. The required sample size was calculated using the free software G*Power 3.1. Fugitive gas samples were collected, using an enclosure gas collection method, from ten selected buses within a week. The gas samples were then analyzed using an analyzer (Horiba PG-300) and the results of CO, CO2 and NOx were reported. The authors found that there is a positive correlation between the fugitive emissions (i.e., CO and CO2) and the conditions of buses (i.e., mileage, age)

Comments and Suggestions

1. In the Results (line 162), if the authors do not report any HC results, there is no need to include Equation 4.
2. Please include uncertainties (e.g., standard deviation) of measurement results in the Table 5.
3. Missing measurement units for the Table 6, 7 & 8. Please apply the same units as Table 5.  
4. Error in 4.1.1. Nitrogen dioxide (CO2). It should be Carbon dioxide (CO2)
5. In the Discussion (4.2.1. CO2, 4.2.2 CO), please provide possible reasons why high mileage and engine age have a direct influence in contributing higher CO2 and CO emissions.

Author Response

Fugitive Emissions from Mobile Sources - Experimental Analysis in Buses Regulated by the Euro 5 Standard

Manuscript ID: atmosphere-2204279

Point-by-point response – letter.

Dear Editor, Ms. Nevena Popović.

First of all, we would like to thank the reviewers by the excellent and detailed revision performed, of course we understood that all the comments, suggestions and solicitations coming from the reviewers will substantially improve the submitted manuscript to reach the high-level research that any manuscript must have in order to be considered for publication in the Atmosphere Journal. Best regards.

Prof. Dr. Cid Marcos G. Andrade

Comments from reviewer #2

1. In the Results (line 162), if the authors do not report any HC results, there is no need to include Equation 4.

Reply from Authors: We completely agree with the reviewer comment, Equation (4) was removed.

2. Please include uncertainties (e.g., standard deviation) of measurement results in the Table 5.

Reply from Authors:  Actually, this is a valuable information that must be included in the manuscript, uncertainties were added to the Table 5, based on the Repeatability reported by the manufacturer analyzer device used for measurements (HORIBA PG-300 Series Portable Gas Analyzer). We have included that information in the manuscript revised version.

3. Missing measurement units for the Table 6, 7 & 8. Please apply the same units as Table 5.

Reply from Authors:  We must inform that it was a typo error from our side. We completely forgot to add the measurements units on tables 6, 7 and 8. We have included that information in the manuscript revised version.

4. Error in 4.1.1. Nitrogen dioxide (CO2). It should be Carbon dioxide (CO2).

Reply from Authors:  We must inform that it was a typo error from our side. Actually, it must be “Carbon dioxide (CO2)” as mentioned by the reviewer. We have made that correction in the manuscript revised version.

5. In the Discussion (4.2.1. CO2, 4.2.2 CO), please provide possible reasons why high mileage and engine age have a direct influence in contributing higher CO2 and CO emissions.

Reply from Authors:  We agree with the reviewer solicitation. Please note that there are many possible influencing factors/parameters that can indicate the direct influence on higher CO2 and CO fugitive emissions (that we measured in this work). First, the higher the mileage the higher the chances of increase the leakage area throughout gases may exit (as fugitive emissions), this because of the natural degradation of pipeline materials. Note that this is not an isolate effect on the mileage, the same can be expected when looking at the engine age. Other important aspect here is concerning to the vehicle technical-mechanical state. There are many factors that can influencing this tendency. All the phenomenological process preceding the combustion process has strong effect, for example if cavitation is present in the fuel injector, the spray behavior will be affected, specially in terms of spray cone angle, penetration, and dispersion. Cavitation could appear in fuel injector after several time if use (directly involving mileage and usage time). It is known that transportation industries are enforced to implement rutinary and rigorously maintenance technical-mechanical prevention programs, so it should be expected that all the vehicles analyzed here undergoes that prevention programs, however we do not have access to that information. We have included this discussion in the manuscript revised version (conclusions section), including new references for support.

Reply from Authors: All the smaller points were solved/adapted in the revised version of the manuscript. Thanks very much.

Author Response File: Author Response.docx

Reviewer 3 Report

The introduction is detailed in terms of discussion of pollution and efforts to abate it - perhaps slightly too extensive. However, the introduction needs to focus more on the study itself and its motivation, rather than on pollution and GHG in general. The concept of fugitive emissions from road vehicles is mentioned, but should be better explained, early in the paper, to aid readers who may be unfamiliar with the topic.

There is perhaps too much detail on the study location (references to the rainforest and the wealth of the city where the buses operate are unnecessary and distracting). The sampling section needs to be clearer: precisely where was the gas sample collected, why was that location chosen and what is its physical significance? How exactly was the end of the copper tube connected to the exhaust? How was the gas collected in the 1-litre container? What material were the containers made from? Was a pump used? How was [ambient] air in the containers removed before the gas sample was intorduced there?

The reference to Regulation (EC) No 715/2007 may be warranted and valid, but as per my understanding, that regulation does not deal directly with fugitive emissions and the testing carried out here cannot rely on that document as a direct precedent.

The magnitude of the results for CO and CO2 are very surprising: exhaust gas from a warmed up engine running at on carbon-based fuel should contain orders of magnitude more CO2 molecules than CO molecules under almost all operating conditions. The results shown in Table 5 defy simple explanation, unless the authors are aware of an effect which could explain this phenomenon. Perhaps the empirical equations used to convert concentrations to emissions have some issue?

The analysis and conclusions are extensive and well-written and the statistical analysis adds value – however, as noted above, I have some concerns about the numerical results themselves. 

Smaller points:

• Spelling: 'Veiche', 'Enginer'
• Units: kW are preferred over CV
• Decimal separator: inconsistent use of ',' and '.' even within the same section (e.g. Table 5)
• Equations (7-11): ‘idade’ = ‘vehicle age’?
• Figure 3: vertical axis label
• Figures 4, 5 and 6: plots a) and b) show emissions, but the axis is labelled 'Concentration'

Author Response

Fugitive Emissions from Mobile Sources - Experimental Analysis in Buses Regulated by the Euro 5 Standard

Manuscript ID: atmosphere-2204279

Point-by-point response – letter.

Dear Editor, Ms. Nevena Popović.

First of all, we would like to thank the reviewers by the excellent and detailed revision performed, of course we understood that all the comments, suggestions and solicitations coming from the reviewers will substantially improve the submitted manuscript to reach the high-level research that any manuscript must have in order to be considered for publication in the Atmosphere Journal. Best regards.

Prof. Dr. Cid Marcos G. Andrade

Comments from reviewer #3

1. The introduction is detailed in terms of discussion of pollution and efforts to abate it - perhaps slightly too extensive. However, the introduction needs to focus more on the study itself and its motivation, rather than on pollution and GHG in general. The concept of fugitive emissions from road vehicles is mentioned, but should be better explained, early in the paper, to aid readers who may be unfamiliar with the topic.

Reply from Authors: Thanks very much for this observation. In order to take around this issue, we have re-organized and complemented the section “Introduction” in the manuscript, now, there is a subsection, “fugitive emissions” in order to aid readers who are unfamiliar with both the topics, mobile sources and non-mobile sources.

2. There is perhaps too much detail on the study location (references to the rainforest and the wealth of the city where the buses operate are unnecessary and distracting).

Reply from Authors: We completely agree with this comment, that unnecessary information was removed from the revised manuscript.  

3. The sampling section needs to be clearer: precisely where was the gas sample collected, why was that location chosen and what is its physical significance? How exactly was the end of the copper tube connected to the exhaust? How was the gas collected in the 1-litre container? What material were the containers made from? Was a pump used? How was [ambient] air in the containers removed before the gas sample was introduced there?

Reply from Authors: We completely agree with this comment. Some of the questions were answered and implemented in the manuscript by recommendation of Reviewer #1 and Reviewer #2. Related to the end of the cooper tube was designed to fit the inlet of the sampling bag valve. The sampling bags made up of polyvinyl fluoride film (Tedlar). In order to not interfere in the mass flow rate throughout the leakage area, pump was not used, the sampling bags were filled by the pressure difference among the enclosured leakage area and the ambient pressure (acting in the wall of the bags). All the bags came at vacuum (with no air inside) from the laboratory were analyzes were performed. This information was added to the revised manuscript.

4. The reference to Regulation (EC) No 715/2007 may be warranted and valid, but as per my understanding, that regulation does not deal directly with fugitive emissions and the testing carried out here cannot rely on that document as a direct precedent.

Reply from Authors: We understand this comment, however, Regulation (EC) No. 715/2007 is the basis for parameterizing exhaust emissions in all regions that adopt the Euro 5 and Euro 6 standards as a reference for their emissions. Brazil, through Resolution 403/2008 follows the recommendations of Regulation (EC) No. 715/2007, therefore, the unit of measurement g/kWh adopted in its emission controls. Subsection 4.0.1., based on references (32-35), explains the need to transform the measurement units based on Resolution 403/2008, consequently, Regulation (EC) No. 715/2007.

5. The magnitude of the results for CO and CO2 are very surprising: exhaust gas from a warmed up engine running at on carbon-based fuel should contain orders of magnitude more CO2 molecules than CO molecules under almost all operating conditions. The results shown in Table 5 defy simple explanation, unless the authors are aware of an effect which could explain this phenomenon. Perhaps the empirical equations used to convert concentrations to emissions have some issue?

Reply from Authors: Thanks very much for the logical observation. We made a mistake with the CO2 measurements units. It came from the gas analyzer in percentage and along the data postprocessing we assumed as ppm. There is a factor to be used ([percentage data]% x 10^4 = ppm). We have corrected the units from CO2 and corrected the tables 5 and 6 as well as the equation 8 (now equation 6 in the revised version of the manuscript), and figure 4.  The figure (in-Portuguese) below shows the report from the laboratory were analyzes were performed including the units (In this case % for CO2). We apologize for the mistake.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Dear Editor and Authors,

Thank you for the corrections. I think the quality of the paper has improved significantly. Although English is not my native language, but I think it still needs to be improved anyway (I think it will be done in the final editing stage). I wish the authors success in their further work.

Sincerely, reviewer

Author Response

First of all, we would like to thank the reviewers by the excellent and detailed revision performed, of course we understood that all the comments, suggestions and solicitations coming from the reviewers will substantially improve the submitted manuscript to reach the high-level research that any manuscript must have in order to be considered for publication in the Atmosphere Journal.

Best regards.

Prof. Dr. Cid Marcos G. Andrade

Reviewer 3 Report

This revised version has been much improved and is now almost ready for publication. Nevertheless, I would request that the authors consider the following issues:

Twice the manuscript uses "Please note" - according to my understanding of how scientific papers should be prepared in English, this is too conversational in tone and might be replaced with "It should be noted" or "It is worth noting".

The following fragment requires revision: "The  long the cooper tube the lower the chance of kinetics evolution in function of the heat transfer process (sample gas - cooper tube wall)" - the element Cu is called copper in English. The first part should use the comparative structure, i.e. it should read "The longer the tube, the lower..."

Figure 3 seems to have a mistake in terms of values, or perhaps units, for the CO2 points - fugitive CO2 values were up to ~1.7 g/kWh (Table 5), but the log-scale graph shows those values as being ~1000 times too high, i.e. up to ~1.7 kg/kWh. This must be carefully reviewed before publication and I would recommend double-checking all presented values for potential unit conversion errors of this type. 

Author Response

Fugitive Emissions from Mobile Sources - Experimental Analysis in Buses Regulated by the Euro 5 Standard

Manuscript ID: atmosphere-2204279

Point-by-point response – letter.

First of all, we would like to thank the reviewers by the excellent and detailed revision performed, of course we understood that all the comments, suggestions and solicitations coming from the reviewers will substantially improve the submitted manuscript to reach the high-level research that any manuscript must have in order to be considered for publication in the Atmosphere Journal.

Best regards.

Prof. Dr. Cid Marcos G. Andrade

Comments from reviewer #3

1. Twice the manuscript uses "Please note" - according to my understanding of how scientific papers should be prepared in English, this is too conversational in tone and might be replaced with "It should be noted" or "It is worth noting".

Reply from Authors: Thanks very much for this observation. We have corrected all similar terms as suggested by a respected reviewer.

2. The following fragment requires revision: "The long the cooper tube the lower the chance of kinetics evolution in function of the heat transfer process (sample gas - cooper tube wall)" - the element Cu is called copper in English. The first part should use the comparative structure, i.e. it should read "The longer the tube, the lower..."  

Reply from Authors: We completely agree with this comment, The verbal structure has been modified.  

3. Figure 3 seems to have a mistake in terms of values, or perhaps units, for the CO2 points - fugitive CO2 values were up to ~1.7 g/kWh (Table 5), but the log-scale graph shows those values as being ~1000 times too high, i.e. up to ~1.7 kg/kWh. This must be carefully reviewed before publication and I would recommend double-checking all presented values for potential unit conversion errors of this type.

Reply from Authors: We completely agree with this comment. We found a multiplicative factor of x10 impacting the values of CO2 concentrations. We corrected Equations 3 according to reference 34 in addition to Tables (5-6) and Figures 3 and 4. New checks were performed in all conversions of measurement units.

Reply from Authors: All the smaller points were solved/adapted in the revised version of the manuscript.

Thanks very much.