The Application of an Empirical Method for the Estimation of Vehicles’ Contribution to Air Pollution in an Urban Environment: A Case Study in Athens, Greece

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Dear Authors

the article presented here addresses the important topic of the impact of traffic on the amount of pollution introduced into the environment by vehicles. Conclusions based on the analysis of experimental data are presented, but the article should be supplemented for a better interpretation of the research results:

Introduction: 

1) introduction section should be expanded to include the results of other authors, showing the findings of other researchers, e.g. traffic parameters affecting emissions.

 Materials and Methods

1) Please summarise in a table the number of vehicles of a given type covering the study area in and working and non-working days.

2) The authors rightly point out that the speed of vehicles affects the emissions of NOx, CO, PM etc, which they take into account in the rv factor. The article lacks vehicle speed information. Please add statistical data (mean, median, Gaussian distribution). Please add.

3) Table 4 shows the average vehicle emissions per kilogram of fuel consumed, based on [8]. There is no information on the methodology used to determine the specific emissions (rolling road test, manufacturer's data, road test)?

4) Does the methodology presented take into account, or is it possible to take into account, the stopping time of individual vehicles at an intersection to assess pollution?

5) No information given in the article on how the speed-related fuel consumption of the vehicles included in the study was determined. Please add.

Others:

1) Page 6 line 184 Sentence:  "The primary types of emissions studied were Particulate Matter (PM), Carbon Monoxide (CO), Nitrogen Oxides (NOX), and Non-Methane Volatile Organic Compounds    (NMVOCs), excluding Methane [8]. - is repeated, see Page 6 line 161

2) references 8 and 9 are not written in accordance with the requirements of the journal

Author Response

Response to Reviewer 1

Thank you for your review of our manuscript. We have studied each comment thoroughly. The manuscript was revised following the suggestions. The sections have been rewritten, enriched with comments and corrections were made throughout the text. Figures were produced from scratch where necessary or/and corrections were made.

All the changes in the manuscript are marked up using the “Track Changes” function so as to be easily viewed by the reviewers and the editor.

Dear Authors

the article presented here addresses the important topic of the impact of traffic on the amount of pollution introduced into the environment by vehicles. Conclusions based on the analysis of experimental data are presented, but the article should be supplemented for a better interpretation of the research results:

Introduction:

1) introduction section should be expanded to include the results of other authors, showing the findings of other researchers, e.g. traffic parameters affecting emissions.

Thank you for this comment. The Introduction was rewritten to be enriched with finding from other researchers related to traffic parameters that affect emissions.

 Materials and Methods

1) Please summarise in a table the number of vehicles of a given type covering the study area in and working and non-working days.

Thank you for this comment. Table 2 was added in the section of Materials and Methods including all the necessary information for the number and type of vehicles recorded during the period of interest.

2) The authors rightly point out that the speed of vehicles affects the emissions of NOx, CO, PM etc, which they take into account in the rv factor. The article lacks vehicle speed information. Please add statistical data (mean, median, Gaussian distribution). Please add.

Thank you for this comment. The speed of each vehicle was calculated based on the equation (2) and following the methodology described in lines 262 – 267.

3) Table 4 shows the average vehicle emissions per kilogram of fuel consumed, based on [8]. There is no information on the methodology used to determine the specific emissions (rolling road test, manufacturer's data, road test)?

Thank you for this comment. The values presented in table 4 provide an estimation of fuel consumption depending on the engine capacity and fuel type of the vehicle. More detailed information on the fuel consumption of vehicles is provided in reference [8]. Clarification was added in lines 299 – 302.

4) Does the methodology presented take into account, or is it possible to take into account, the stopping time of individual vehicles at an intersection to assess pollution?

Thank you for this comment. The purpose of the present study is to calculate emissions at low vehicles speeds. Emissions were calculated during three consecutive phases of the traffic light and not during the time the vehicle was stopped at a red light. That is, emissions correspond to the starting phase of the vehicle until the next red light. Of course, this leads to an underestimation of total emissions as vehicles emit pollutant even during the stopping time at the intersection (meaning during the period the red light is on).

5) No information given in the article on how the speed-related fuel consumption of the vehicles included in the study was determined. Please add.

Thank you for this comment. Fuel consumption (in L/km) was used to calculate pollutant emissions (g/kg) based on fuel density (diesel/petrol). This conversion was necessary for the application of MLV, which links vehicle speed to the total emission calculation for each traffic stream. A relevant clarification was added also in the manuscript, in lines 262 – 268 and 299 - 302.

Others:

1) Page 6 line 184 Sentence:  "The primary types of emissions studied were Particulate Matter (PM), Carbon Monoxide (CO), Nitrogen Oxides (NOX), and Non-Methane Volatile Organic Compounds    (NMVOCs), excluding Methane [8]. - is repeated, see Page 6 line 161

Thank you for this comment. The repeated sentence was withdrawn.

2) references 8 and 9 are not written in accordance with the requirements of the journal

Thank you for this comment. References were corrected to meet the requirements of the journal.

Reviewer 2 Report

Comments and Suggestions for Authors

• Brief summary 

The study focuses on monitoring vehicle emissions at two intersections located in the Municipality of Egaleo, an urban area in Athens, Greece. By incorporating real-time data and predictive models, it aim to identify the factors influencing air pollution and propose actionable solutions to mitigate traffic-related pollution while promoting sustainable policies.

• General concept comments
  Article:

The  scope of the article is of interest the research community as reduction pollutions strategies are more and more important.

The principle of optical monitoring of traffic condition is well developed.

Weakness of the article is in the use of a too simple modelling of vehicles types and emissions factors for petrol and diesel engines.

Personally I don’t agree with the conclusions.

Review:

Modelling of vehicles types should follow the UNECE standards classification and emission factors used to estimate emissions are not consistent with Euro standards. It’s important to analyze the euro class of the various vehicles as older units (Euro 1/Euro 0)  have emissions 10 times higher than new Euro 5 units.

Average age and maintenance conditions of vehicles, that are characteristics of economic conditions of the examined area, are not taken into account. Especially contributions from ‘old’ commercial vehicles (light tracks, tracks and buses) can be significantly higher than it’s expected from the same unit with a correct maintenance (i.e. dust from diesel engines).

Finally there is no evaluation of uncertainty of the emissions parameters determined.

• Specific comments 

Row 182: classification of vehicles should be based on European classification for commercial and passengers vehicles (i.e. A-class, B-class, etc). Inconsistency are present for PM and CO factors, mainly.

Row 194: table don’t consider the age and maintenance condition of vehicles and should be based on Euro class emission factors, very different from the values showed. Table should use directly factors expressed in g/km, as given by Eu standards.

Row 228, 233, 240, 247: time frame shall be given, as the table cite ‘mass of PM’ without giving the time frame.

Row 277: I think it’s not correct to calculate the total mass of pollutants, as in order to sum different compounds (solid and gaseous) it should be necessary to use specific weight constants. For example correction weights to compare the different toxicity of various compounds could be used, otherwise I don’t understand the  reason such calculation is done.

Author Response

Response to Reviewer #2

• Brief summary 

The study focuses on monitoring vehicle emissions at two intersections located in the Municipality of Egaleo, an urban area in Athens, Greece. By incorporating real-time data and predictive models, it aim to identify the factors influencing air pollution and propose actionable solutions to mitigate traffic-related pollution while promoting sustainable policies.

• General concept comments
  Article:

The  scope of the article is of interest the research community as reduction pollutions strategies are more and more important.

The principle of optical monitoring of traffic condition is well developed.

Weakness of the article is in the use of a too simple modelling of vehicles types and emissions factors for petrol and diesel engines.

Personally I don’t agree with the conclusions.

We would like to thank Reviewer for his/her valuable comment. As many scientists all over the world, we also believe that if there are two or more competing ideas in order the same phenomenon to be explained, the simpler one has to be preferred. More complex and more precise scientific approaches require expensive equipment and recording instruments, which in that case we did not have. Thus, we decided to approach the problem in a simplified way, without losing the scientific nature of the approach and the results.

Review:

Modelling of vehicles types should follow the UNECE standards classification and emission factors used to estimate emissions are not consistent with Euro standards. It’s important to analyze the euro class of the various vehicles as older units (Euro 1/Euro 0) have emissions 10 times higher than new Euro 5 units.

Average age and maintenance conditions of vehicles, that are characteristics of economic conditions of the examined area, are not taken into account. Especially contributions from ‘old’ commercial vehicles (light tracks, tracks and buses) can be significantly higher than it’s expected from the same unit with a correct maintenance (i.e. dust from diesel engines).

Finally there is no evaluation of uncertainty of the emissions parameters determined.

We would like to thank the reviewer for the above comments. In the present study, the estimation of emissions is based on the assumption that vehicles operate under typical driving conditions without extreme variations in acceleration and deceleration. Additionally, the fuel consumption values used are averaged from standard driving cycles, which may not fully capture real-world variations in vehicle operation. Τhe main purpose of the present study was to apply a simple and quick approach for the calculation of emissions in cases that an estimation of roadside emissions is required. The reviewer correctly mentions the fact that older vehicles emit more pollutants and particulates compared to the newer ones. However, in the present study the age distribution of vehicles as well as the age distribution (meaning the different EURO standards) was not considered among the parameters of the approach.

• Specific comments 

Row 182: classification of vehicles should be based on European classification for commercial and passengers vehicles (i.e. A-class, B-class, etc). Inconsistency are present for PM and CO factors, mainly.

Row 194: table don’t consider the age and maintenance condition of vehicles and should be based on Euro class emission factors, very different from the values showed. Table should use directly factors expressed in g/km, as given by Eu standards.

Thank you for this comment. In the present study, fuel specific emission factors were used. It was not among authors intention to perform an in-depth analysis of vehicles characteristics but as mentioned above a simpler approach for cases an estimation of roadside emissions is required. The pollutants emitted per kilogram of fuel consumed from each vehicle type are presented in table 4. It should be mentioned that these values refer to the average emissions and the detailed methodology for their calculation is referred in) [30].

Row 228, 233, 240, 247: time frame shall be given, as the table cite ‘mass of PM’ without giving the time frame.

The duration of the emissions presented in this study is the one mentioned in table 1.

Row 277: I think it’s not correct to calculate the total mass of pollutants, as in order to sum different compounds (solid and gaseous) it should be necessary to use specific weight constants. For example correction weights to compare the different toxicity of various compounds could be used, otherwise I don’t understand the  reason such calculation is done.

Thank you for this comment. As described above, our goal was a simplistic approach to the problem. By summing the masses of pollutants (solid and gaseous), we want to show the burden of the atmosphere with these masses. Essentially, we show how much mass of each pollutant is added to the atmosphere of the examined area and what is the total mass of pollutants in the atmosphere due to traffic. In our case, we are not studying the potential toxicity of the emitted pollutants and that is why we did not mention at all the use of correction weights to compare the different toxicity of various compounds.

We would like to thank the reviewer once again for his constructive comment, as it gives us an idea for further research in the future. However, in the context of this work, we want to see in the simplest way the proportion of pollutants masses in the atmosphere due to traffic congestion in an urban hub.

Reviewer 3 Report

Comments and Suggestions for Authors

This work focuses on monitoring and analyzing air pollutant emissions, mainly from 14 passenger vehicles, at a busy urban intersection in an urban region of Athens, Greece. 

First of all, there are some issues with the citations throughout this work. Please follow the journal guidelines. It also seemed to have a tone problem, which is using the first person, such as our and we.

Table 3 shows the correction factor for emissions based on the traffic flow. What is the correction factor being different for different speed? Can the authors explained the reasons behind this?

Also, it does not seem that vehicles use benzine as fuel. Can the author check if there is an error?

Line 230 explained that PM emissions are exclusively due to benzine vehicles. What about diesel vehicles? They are known to be more polluting than gasoline vehicles.

Figure 5 shows the total emitted pollutant’s’ mass (sum of CO, PM, NOX and NMVOCs) by each vehicle during working and non-working days. It shows that benzine vehicles to more polluting than diesel vehicles, which is surprising and difficult to believe. This study is misleading if the euro emissions standard is not considered in the data collection processes.

Line 303 mentioned almost exclusively (100%), responsible for PM emissions are diesel vehicles. This conclusion clearly contradicts with Line 230. Can the authors double-check their claim and see which statement is correct? 

The limitation of this study is not mentioned in the conclusion section, which is also a major letdown. There are only 9 references being cited in this work, which is clearly subpar.

The equipment used to measure the concentrations of air pollutants from the tailpipe is not mentioned, which is also problematic. Please clearly stated the equipment used in this study. 

Author Response

Response to Reviewer #3

This work focuses on monitoring and analyzing air pollutant emissions, mainly from 14 passenger vehicles, at a busy urban intersection in an urban region of Athens, Greece. 

First of all, there are some issues with the citations throughout this work. Please follow the journal guidelines.

Thank you for this comment. Regarding references corrections were made where necessary to meet journal guidelines.

It also seemed to have a tone problem, which is using the first person, such as our and we.

Thank you for this comment. The whole text was corrected in accordance to the reviewer’s comment.

Table 3 shows the correction factor for emissions based on the traffic flow. What is the correction factor being different for different speed? Can the authors explained the reasons behind this?

Thank you for this comment. The correction factor is necessary because more pollutants are emitted during low-speed driving conditions. Therefore, higher values of the rv factor are applied for lower speed (Vi) values.

Also, it does not seem that vehicles use benzine as fuel. Can the author check if there is an error?

Line 230 explained that PM emissions are exclusively due to benzine vehicles. What about diesel vehicles? They are known to be more polluting than gasoline vehicles.

Figure 5 shows the total emitted pollutant’s’ mass (sum of CO, PM, NOX and NMVOCs) by each vehicle during working and non-working days. It shows that benzine vehicles to more polluting than diesel vehicles, which is surprising and difficult to believe. This study is misleading if the euro emissions standard is not considered in the data collection processes.

Line 303 mentioned almost exclusively (100%), responsible for PM emissions are diesel vehicles. This conclusion clearly contradicts with Line 230. Can the authors double-check their claim and see which statement is correct? 

We would like to thank the reviewer for the above comments. Corrections were made to tables 7 – 9 as well as in figures 3 – 5. More significant digits were added to the values so as the contribution of vehicles using benzine as fuel to be clear. The contribution of diesel fuelled vehicles to PM emissions was found to be more important.

The limitation of this study is not mentioned in the conclusion section, which is also a major letdown.

Thank you for this comment. Lines 449 – 456 were added at the end of the conclusions to mention the limitations of the present study.

There are only 9 references being cited in this work, which is clearly subpar.

Thank you for this comment. More references were added throughout the manuscript and mainly in the introduction to enrich the text with updated findings from other researchers.

The equipment used to measure the concentrations of air pollutants from the tailpipe is not mentioned, which is also problematic. Please clearly stated the equipment used in this study. 

Thank you for this comment. There was no specific equipment used in this study for measuring the exhaust emissions. Results are based on the calculation of emissions through the equations included in table 3 as well as equations (1) & (2). However, in studying the problem, a simple video camera was used. With the camera, traffic was recorded on video, every day and period that this campaign was carried out. Then, each video was studied separately in detail and the transit times, the type of cars (small or medium), the brand of cars, the type of fuel they use, etc., were measured. Finally, the methodology described was followed and the equations listed in the text of the specific paper were used to extract the results.

As many scientists all over the world, we also believe that if there are two or more competing ideas in order the same phenomenon to be explained, the simpler one has to be preferred. More complex and more precise scientific approaches require expensive equipment and recording instruments, which in that case we did not have. Thus, we decided to approach the problem in a simplified way, without losing the scientific nature of the approach and the results.

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

row 302: pls correct 'PM emissions are almost exclusively due to benzine vehicles'

figure 5 is misleading as different pollutants don't have the same health effect and I think it's not correct to give the total sum without a correct weight (i.e. PM is more dangerous than NOx) 

Author Response

Comment 1: row 302: pls correct 'PM emissions are almost exclusively due to benzine vehicles'

Response 1: We would like to thank the reviewer for his valuable remark. The specific sentence has been removed from the text, which has been appropriately formatted.

Comment 2: Figure 5 is misleading as different pollutants don't have the same health effect and I think it's not correct to give the total sum without a correct weight (i.e. PM is more dangerous than NOx)

Response 2: We would like to thank the reviewer for his valuable remark. This particular Figure 5 has been removed from the text, as well as the corresponding paragraph in the text that referred to Figure 5.

Reviewer 3 Report

Comments and Suggestions for Authors

After the authors took into consideration the comments raised in the previous review report, the quality of this manuscript has significantly improved. The number of references cited in this work increased from 9 to 31, which is important to ensure the scientific background is solid in this work.

Author Response

Comment: After the authors took into consideration the comments raised in the previous review report, the quality of this manuscript has significantly improved. The number of references cited in this work increased from 9 to 31, which is important to ensure the scientific background is solid in this work.

Response: We would like to thank the reviewer for his valuable remark.