Exposure and Respiratory Tract Deposition Dose of Equivalent Black Carbon in High Altitudes

Round 1

Reviewer 1 Report

The authors analyzed and compared the exposure concentration and potential respiratory tract deposition dose of BC in three different transport microenvironments (walking, ride in microbus and cable car) in two high altitude cities La Paz and El Alto, Bolivia. The study is well conducted and the paper is well written and straightforward. It is interesting to see the study of exposure for cable cars. I believe this study is suitable for publication after addressing the following comments.

1. Line 102: Change “micro-bus” to “microbus” for consistency.
2. Section 2.3: In which season/month was this study conducted? Is the pollution level in the season/month representative of other seasons/months? How different would it be?
3. Line 169: Please double check the number. 80 hPa is not in the troposphere.
4. Line 192: Change “correction” to “adjustment”.
5. Line 193: Please briefly describe the method suggested by Alvarez et al. [36] and used in this study.
6. Lines 193-195: While equivalent black carbon (BC) mass concentrations are shown in Table 3, can you also provide a Table for the BC data before the adjustment in the supplement?
7. Table 5: Please add a column of masl for each city.
8. Line 369-370: Change “This is due to higher minute ventilation while walking” to “This is due to higher minute ventilation and longer commute time while walking”?

Author Response

Response to comments of referee #1

The authors analyzed and compared the exposure concentration and potential respiratory tract deposition dose of BC in three different transport microenvironments (walking, ride in microbus and cable car) in two high altitude cities La Paz and El Alto, Bolivia. The study is well conducted and the paper is well written and straightforward. It is interesting to see the study of exposure for cable cars. I believe this study is suitable for publication after addressing the following comments.

Reply:

We thank to referee #1 for understanding the importance and value of such studies. We also appreciate referee for his/her time, comments, and suggestions, which are intended to improve the manuscript. In the following, we would like to address the comments step-by-step.

Line by line comments

1. Line 102: Change “micro-bus” to “microbus” for consistency.

Reply:

Thank you for the notation. We replaced “micro-bus” to “microbus” as suggested.

2. Section 2.3: In which season/month was this study conducted? Is the pollution level in the season/month representative of other seasons/months? How different would it be?

Reply:

Thank you for the question. This study was conducted during May – June 2018 (indicated in Section 2.1, Line 83). The months, during which study was conducted falls into the dry period (May to August). During these months, the average rainy days are 3.5 per month, with average relative humidity of approx. 40%, and 10 mm of rainfall. We chose the dry season for our experiments due to convenience, as well as instrument safety as the measurements were performed by the volunteers exposed to elements. As per question, if our measured pollution levels are representative of other seasons/months, we can only speculate this, because to the best of the authors’ knowledge, no published data on the long-term (over a year) continuous black carbon measurements exist from Bolivia. With that being said, we note that the exposure measurements were conducted in the setting (walking, microbus), where direct emissions from the vehicles are the major contributor to the high levels of pollutants (except cable car). With this in mind, we expect exposure concentrations of black carbon while walking and/or commuting inside microbus to be somewhat similar to other season/months despite possible wet removal of pollutants (during wet season). This is based on the fact that traffic situation in both El Alto and La Paz is season/month independent. On the other hand, cable car exposure concentrations are expected to be more related to urban background concentrations of black carbon. Therefore, one may imagine that it shall be affected more by seasonal pollutant variability. However, more long-term studies, not only mobile, are needed to answer this question definitely.

3. Line 169: Please double check the number. 80 hPa is not in the troposphere.

Reply:

Thank you for the notation. Indeed, 80 hPa corresponds to altitude of 16 km. We are sorry for the mistake. We corrected the number to 6300 masl only.

1. Line 192: Change “correction” to “adjustment”.

Reply:

Thank you for the notation. We changed “correction” to “adjustment”.

1. Line 193: Please briefly describe the method suggested by Alvarez et al. [36] and used in this study.

Reply:

Thank you for the notation. In Alvarez et al. [36], it is stated that altitude of a city is an important factor when evaluating the possible effects of particulate matter on the health of the population. The pollutant concentration correction factor for high altitudes (for comparison with standard conditions) comes from the general gas law: C_St = C_L(P_St/P_L)(T_L/T_St), where St denotes the concentration (C), atmospheric pressure (P), and temperature (T) under standard, and L – under local conditions. In this study, we have used correction factor of 1.47 to adjust locally measured pollutant concentration to standard conditions (Table 2 in Alvarez et al. [36]). Based on this, we have included some explanation in the manuscript as well (Lines 198 – 204).

1. Lines 193-195: While equivalent black carbon (BC) mass concentrations are shown in Table 3, can you also provide a Table for the BC data before the adjustment in the supplement?

Reply:

Thank you for the question/suggestion. Of course! We have included the BC data before the adjustment in the supplementary materials (Table SP-3).

7. Table 5: Please add a column of masl for each city.

Reply:

Thank you for the suggestion. We have added a column of masl for each city in table 5.

8. Line 369-370: Change “This is due to higher minute ventilation while walking” to “This is due to higher minute ventilation and longer commute time while walking”?

Reply:

Thank you for the suggestion. Referee #1 is right – RDD while walking is higher due to both higher minute ventilation as well as longer commute time. We made a change as suggested.

Author Response File: Author Response.docx

Reviewer 2 Report

Overall, this is a well executed study. Although it could have benefited from extended studies in order to generate better statistics, the general conclusions are supported by the data that is there.

A few minor comments:

On line 43, its is stated that Table SP-1 include equivalent black carbon particles. This needs clarification. Equivalent BC, eBC, is a parameter derived from light attenuation measurement but there isn't a particle type called equivalent black carbon unless you specifically say that you are referring to eBC; however, in Table SP-1 there are EC and what is called "BC" that was probably eBC. Maybe just a clarification at the very beginning that what is reported as "BC" is probably not actually BC but an estimated BC derived from various techniques. This is probably already discussed in the methodology referenced in the Madueno paper; however, please clarify in the methodology section the the BC being reported from the Aethalometers is eBC not BC.

Here in Mexico City, one of the issue with altitude is the inefficient combustion that must be even worse in La Paz and El Alto. This should be explicitly discussed, as well as why starting and stopping of the microbuses is such a problem. The acceleration is alluded to but it is worth underscoring why that produces additional pollutants.

The energy source is never described for the cable cars. Presumably the cable is driven by a wench driven by an engine, but is the engine electric or diesel?  This produces pollution if a combustion engine. What is the impact of those emissions?

I understand that this is a strictly scientific/technical presentation; however, if you are going to make recommendations that the government decrease pollution buy encouraging more people to take the cable car, you have to at least discuss how realistic this is. Most of the people who walk are not doing so for their health. They are doing so to save money since the microvan and cable car are unlikely to be free. Secondly, each cable car holds 10 people. If everyone started taking the cable car, how long would they have to wait for an open car and how much do they pay?  Some type of cost/benefit assessment would be helpful here.

Author Response

Response to comments of referee #2

Overall, this is a well executed study. Although it could have benefited from extended studies in order to generate better statistics, the general conclusions are supported by the data that is there.

Reply:

We thank to referee #2 for his/her time, comments, and suggestions, which are intended to improve the manuscript. In the following, we would like to address the comments step-by-step.

Broad Comments

1. On line 43, its is stated that Table SP-1 include equivalent black carbon particles. This needs clarification. Equivalent BC, eBC, is a parameter derived from light attenuation measurement but there isn't a particle type called equivalent black carbon unless you specifically say that you are referring to eBC; however, in Table SP-1 there are EC and what is called "BC" that was probably eBC. Maybe just a clarification at the very beginning that what is reported as "BC" is probably not actually BC but an estimated BC derived from various techniques. This is probably already discussed in the methodology referenced in the Madueno paper; however, please clarify in the methodology section the the BC being reported from the Aethalometers is eBC not BC.

Reply:

Thank you for the comment. We do agree with referee #2 that a clarification about measurement technique would benefit the manuscript. For this reason, we have added the following to the main text (section 2.3; with adopted references):

“The working principle of an aethalometer is based on measuring the optical attenuation increment, when the light is passed through particle laden filter. Equivalent black carbon mass concentration is then calculated using the known optical absorbance per unit mass of BC material. Since the MA200 is a multi-wavelength instrument, the measurements at 880 nm were interpreted as the concentration of eBC.“

We have also identified in the tables and graphs that the reported quantity is equivalent black carbon (eBC).

2. Here in Mexico City, one of the issue with altitude is the inefficient combustion that must be even worse in La Paz and El Alto. This should be explicitly discussed, as well as why starting and stopping of the microbuses is such a problem. The acceleration is alluded to but it is worth underscoring why that produces additional pollutants.

Reply:

Thank you for the comment. We have included the following to manuscript text (section 3.1; with adopted references) to facilitate the discussion related to high pollutant concentrations observed in both El Alto and La Paz:

“The increased levels of traffic related pollutants in La Paz and El Alto may be a result of three separate causalities, which all together contribute to elevated roadside pollution. Firstly, both La Paz and El Alto are high altitude cities (3600 and 4100 masl, respectively). Previous studies suggest that at high altitudes, internal combustion engines operate at reduced efficiency, which relates to higher emission of pollutants (air/fuel ratio of the mixtures is far richer than stoichiometric) [43-44]. Secondly, intensive traffic and frequent passenger requests to hop on/off along the route results in vehicle speedups, slowdowns, stops, and starts. Such behavior of increased vehicle acceleration/deceleration results in rich air/fuel mixture, and is known to produce significantly higher emissions compared to steady driving conditions [45-46, and references therein]. Lastly, approx. 60% of Bolivia’s total vehicle fleet is more than 10 years old, whereas, 80% of the public transport fleet is comprised of 17 to 44 year-old vehicles [47]. Outdated engine technology was shown to produce extremely higher emissions [17, 40]. All previously mentioned causalities work in favor of increased pollution levels in the cities.”

3. The energy source is never described for the cable cars. Presumably the cable is driven by a wench driven by an engine, but is the engine electric or diesel? This produces pollution if a combustion engine. What is the impact of those emissions?

Reply:

Thank you for the comment. According to Global Infrastructure Hub (https://inclusiveinfra.gihub.org/case-studies/mi-teleferico-cable-car-bolivia/), the energy source of the cable car in La Paz and El Alto is 100% electric, with most of its power requirements being supplied by hydroelectric power plants. With that being said, from our experience during the field work, we believe that some kind of heavy-duty diesel generator might also be present in order to supplement Mi Teleferico electric motors in case of power interruption. Despite this, the exposure concentration of equivalent black carbon close to end station of cable car is relatively low (0.5 – 2.0 µg/m³; see Fig. R1), indicating that the emission (if such exist) from combustion engines is well controlled. It suggests that emission of cable car has a minimal impact on urban equivalent black carbon exposure concentrations.

Fig. R1. Median exposure concentration of equivalent black carbon while walking in El Alto. Red circle indicates the end station of cable car.

4. I understand that this is a strictly scientific/technical presentation; however, if you are going to make recommendations that the government decrease pollution buy encouraging more people to take the cable car, you have to at least discuss how realistic this is. Most of the people who walk are not doing so for their health. They are doing so to save money since the microvan and cable car are unlikely to be free. Secondly, each cable car holds 10 people. If everyone started taking the cable car, how long would they have to wait for an open car and how much do they pay? Some type of cost/benefit assessment would be helpful here.

Reply:

Thank you for the comment. The insights of referee #2 are on point – a single cabin of cable car indeed holds finite number of people, which results in a severely long waiting queues at some of the stations (e.g. yellow line, El Alto). Passengers have to wait a significant amount of time on street, being exposed to the direct emissions from passing vehicles. One could even argue that in such case, lower exposure concentrations of pollutants inside cable car is out weighted by elevated exposure to pollutants while waiting for the commute. On the other hand, although not as long, waiting queues were also present in case of commute by a microbus. In this study, however, we excluded waiting times from data analysis. This is because we did not always consistently end up waiting in long queues (saving time and resources), as well as huge variability in waiting times.

Secondly, referee #2 asks, how realistic is for more people to use cable car, motivating that people commute by walking not because of their health, but because other means of transportation cost money. Although such discussion is beyond the scope of this article, the referee #2 is partly right. The cost of cable car varies between 3 to 5 Bolivianos (approx. 40 – 70 Euro cents). The trip by microbus, on the other hand, is between 1 – 2 Bolivianos (approx. 15 – 30 Euro cents). Moreover, we did observe that passengers using microbus would more frequently take some cargo, which would not necessarily be allowed inside a cable car – making transportation by microbus not only cheaper, but also more convenient. Also, a cable car only connects two points with no possibility to jump out in between, while microbus would stop on request. In other words, it is hard to imagine that cable car would be able to 100% replace regular means of transportation, nevertheless, during the course of this study, results clearly showed that both exposure to equivalent black carbon, as well as respiratory tract deposition dose of such pollutant is lowest inside cable car. This we were able to demonstrate from 2 month mobile measurements in three different modes of transportation (walking, microbus, cable car). As referee #2 already noticed, this study is rather a scientific/technical presentation of exposure concentrations of pollutants in high altitudes rather an investigation of strategies towards improving transport system. For this very reason, we restrained from a definite recommendation of cable car as a panacea to solve transport related shortcomings. Much more studies, including transdisciplinary approaches (e.g. health, social, environmental disciplines) will be needed to reveal true benefits and drawbacks of such mode of transportation. This we state in manuscript summary and conclusion section (Lines 374 – 377, and 383-384). We absolutely agree with referee #2 that cost/benefit assessment using cable car as main mode of transportation would benefit both government and population tremendously. However, such study would deviate from the original goal of this article, which is to explore exposure and respiratory tract deposition dose of black carbon in high altitudes. Therefore, it would be constructive to present such discussion in a separate study. Some of the required technologies for the integration of different transport modes that are adaptive to the social and economic dynamics of urban areas are discussed by Bürger [24]. Moreover, a recent study on travel time savings from La Paz-El Alto cable car usage indicated that travel by cable car reduces travel time on average by 9 minutes, representing an average net benefit to the commuter of US$0.58 [R1]. However, things have changed from the time this study was carried-out. The longest and fastest line was inaugurated in August 29, 2017. Given the fact that this line also connects La Paz and El Alto, it is likely that time saved by cable-car users is even higher. In the same study, authors concluded that future studies should focus on determining cable car implementation costs, capacity, and reliability. Moreover, cost-benefit analysis is suggested to assess the socioeconomic costs (e.g. high income vs. poorest population) and benefits of investing in such mode of transportation. Martinez et al. [R2] studied the effects of areal cable cars on mode of transport, time use and employment. It was found that the economic benefits of the cable car outweigh costs by a ratio of 1.05 to 2.16. With that being said, studies that focus on reduction of exposure to pollutants in cable car are still lacking. It seems that cable car as a mode of transportation is only starting to gain much needed scientific attention.

References:

R1. Garsous, G.; Suárez-Alemán, A.; Serebrisky, T. Cable Cars In Urban Transport: Travel Time Savings From La Paz-El Alto (Bolivia). Transport Policy 2019, 75, 171-182.

R2. Martinez, S.; Sanchez, R; Yáñez-Pagans, P. Getting a Lift: The Impact of Aerial Cable Cars in La Paz, Bolivia. IDB 2018, http://dx.doi.org/10.18235/0001205.

Author Response File: Author Response.docx

Reviewer 3 Report

In this manuscript, the authors report unique measurements of personal exposure to black carbon (BC) aerosol particles during commutes in two cities at high altitude, La Pez and El Alto, Bolivia. The experimental design is well thought out, including walking and riding microbus/cable car stages that are representative for residents of these cities. The selection and implementation of instrumentation is sound, centred around inter-compared micro-aethalometers. The analysis and discussion of the measurements is compelling, particularly in the context of the increased minute ventilation of those living at high altitude. Furthermore, the conclusion that commuting by cable car is very effective at minimizing respiratory deposited dose provides insights for policy makers in cities at high elevation. The manuscript may be published in Atmosphere as is, but I would recommend a final revision of the text, with a focus on English language editing.

Author Response

Response to comments of referee #3

In this manuscript, the authors report unique measurements of personal exposure to black carbon (BC) aerosol particles during commutes in two cities at high altitude, La Pez and El Alto, Bolivia. The experimental design is well thought out, including walking and riding microbus/cable car stages that are representative for residents of these cities. The selection and implementation of instrumentation is sound, centred around inter-compared micro-aethalometers. The analysis and discussion of the measurements is compelling, particularly in the context of the increased minute ventilation of those living at high altitude. Furthermore, the conclusion that commuting by cable car is very effective at minimizing respiratory deposited dose provides insights for policy makers in cities at high elevation. The manuscript may be published in Atmosphere as is, but I would recommend a final revision of the text, with a focus on English language editing.

Reply:

We thank to referee #3 for understanding the importance and value of such studies. As suggested, we have revised manuscript text with a focus on English language editing. All edits are highlighted in a new version of manuscript.

Author Response File: Author Response.docx