The Use of Low-Cost Gas Sensors for Air Quality Monitoring with Smartphone Technology: A Preliminary Study
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThe paper has very serious flaws. I suggest direct rejections. It only combines some commercial sensors with a cellphone, and then does some test experiments. I can hardly find any scientific contribution of this work toward the chemical sensor field.
Thus, I suggest rejection of this paper.
Author Response
- The paper has very serious flaws. I suggest direct rejections. It only combines some commercial sensors with a cellphone, and then does some test experiments. I can hardly find any scientific contribution of this work toward the chemical sensor field.
dear reviewer, the scientific of liturature is full of papers which "just" evaluate devices based on commercial sensors through test experiments. Just to make some example, among the papers cited in my work, we can find the Castell et al. study, Tryner et al work, Velasco et al., Demanega et al., and many others not cited in my manuscript. The scientific contribution of such papers consists in adding more information on the possible uses of electrochemical sensors. Similarly to these papers, this study evaluates a new way to use such sensors exploring the capabilities of an original device enabling the easy and quick setting up of local networks for the air quality monitoring of indoor environments composed of multiple rooms or spaces without using third services, such as, cloud services or wi-fi local networks. This feature can provide an immediate access to air quality data concerning public spaces, such as, airports, hospitals, or public offices for any user, because almost all the citizens own a smartphone. This elements make the originality of the work exposed in this manuscript and, similarly to the aforementioned studies adds more information about the possible uses of the electrochemical sensors and their limits, bringing its own scientific contribution.
- First, please clarify the novelty and significance of combining the gas sensors with the smartphone technology. As we all know, A very cheap ARM chip and a wireless unit can also function well. Why do the combination like this?
dear reviewer, as explained earlier, combining the use of gas sensors with the smartphone enables to set up very easily and quickly local monitoring networks for the air quality monitoring of indoor environments composed of multiple rooms or spaces without using third services, such as, cloud services or wi-fi local networks. This feature can provide an immediate access to air quality data of public spaces, such as, airports, hospitals, or public offices for any user, because almost all the citizens own a smartphone. This aspect is now further clarified in the introduction section of the new version of the manuscript.
- Secondly, the author finally chose electrochemical sensors. However, no papers on this type of sensor are given. Please give an introduction to the working principles and advantages of these sensors.
dear reviewer, the integrations you required are now present in the second version of the manuscript. In the subsection 2.1 the reasons leading to the selection of the electrochemical sensors are now explained. Moreover in reference [7] and [38] are provided the papers explaining the working of the electrochemical sensors and an introduction to it is now exposed.
- A detailed comparison of different experimental designs is missing.
dear reviewer, experimental designs of low-cost laboratory-made devices used as personal monitor for NO2 and CO concentrations in indoor environments with conditions comparable to this study is very hard to find. If you can suggest any, I would be happy to make a comparison. Anyway, a comparison with experiments involving electrochemical sensors for CO and NO2 are reported in the discussion section. In particular, the results related to the calibration of such sensors are compared with the work of Kang et al. from the line 386 to the line 390. From line 390 to 398, it is reported a comparison with the work of Zimmermann et al., while from line 398 to line 411, it is reported the comparison with the work of Castell et al., even though, in this study, Castell evaluated the performance of devices not designed in laboratory, but they were commercial monitors.
- Thirdly, what is the paper's scientific contribution? I believe some comparison of calibration models or research on drift compensation may be good. But the paper only describes a combination and shows the measurement results.
dear reviewer, I think that one novelty of this study consists in considering and testing the use of an original handheld device for air quality monitoring in real situations. This study explore the capabilities of such idea and its limits. This work also highlights the limits of some electrochemical gas sensors, such as those used for NO2 and CO, in indoor environments. In other papers already published, this aspects are not fully exposed, see for example, the Tryner et al. study. Other papers "just" expose a performance assessment of commercial available low-cost monitors, not even designed or developed in laboratory, but they are equally useful to understand the potentialities of this particular technology, and therefore, bring their scientific contribution by adding more information about the use of electrochemical sensors. Some example of such papers are the one of Castell et al., or the work of Demanega et al. Many other papers can be found in literature reporting evaluations or use case study of electrochemical gas sensors and devices based on their functioning (see also Velasco et al. cited in this work). This study explores and evaluates an original device evaluated in real scenarios, thus it adds more information about the possible uses of such technologies and the related limits. As concerns the drift of sensors, its consideration makes sense only when we consider long periods of use of the sensors. For experiments lasting less than one month, as it is the case of this study, the drift can be considered as negligible, so it does not make any sense considering it. For this reason, this aspect was not analyzed.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsThe need of llow-cost sensitive and selective gas sensors for air quality monitoring is recognized worldwide.
This paper presents an integration of Alphasense sensors in a system connected to a mobile phone. This seems very attractive. The question is in what extent we can believe in the results.
The system completely depends on the quality of Aphasense devices. Are they enough selective to measure such a low concentration of NO2? What a magical membrane filter is used to prevent the influence of other molecules (cross-sensitivity)? And what are these molecules? Did the authors check what signals the get from other gases?
The authors report NO2 concentration of 64 and 38ppm while the noise is 2-10ppb. They should splice the measured curves.
To sum up, I recommend to publish this paper after highlighting the abovementioned issues.
Author Response
- The system completely depends on the quality of Aphasense devices. Are they enough selective to measure such a low concentration of NO2?
Dear reviewer, as stated by the manufacturer, the NO2-B43F sensors can be used to measure NO2 levels under 100 ppb if supported by an appropriate electronic circuitry. This information is detailed in the sensor datasheet available on the manufacturer website at the page: https://ametekcdn.azureedge.net/mediafiles/project/oneweb/oneweb/alphasense/products/datasheets/alphasense_no2-b43f_datasheet_en_4.pdf . It is now clear that the electronic board supporting the sensor working is fundamental for measuring such low gas concentrations. This is one of the technical challenges addressed by this study. Same considerations can be made for the CO sensor. To improve the clarity of the paper, I will point out this information in the new version of the manuscript.
- What a magical membrane filter is used to prevent the influence of other molecules (cross-sensitivity)?
as you can find in the article of Hossain, co-authored by two members of Alphasense ltd. (namely Saffell and Baron) and cited in my work, the NO2 sensors are equipped with a membrane made of MnO2/PTFE microparticle composite filter powder. This information is also reported in the Alphasense website at the page: https://www.alphasense.com/products/view-by-target-gas/ozone-sensors-o3. As concerns the CO sensor, the manufacturer shows on the sensor datasheet that it has a membrane to filter out the H2S gas, but it is not specified its structure. This information is present at the page https://ametekcdn.azureedge.net/mediafiles/project/oneweb/oneweb/alphasense/products/datasheets/alphasense_co-b4_datasheet_en_2.pdf . The new version of the manuscript is now updated with this information.
- And what are these molecules?
The interfering gases of NO2-B43F sensor and CO-B4 sensors can be viewed on the sensor datasheets at the links earlier exposed. Anyway the main interfering gas for the NO2-B4 is the ozone and chlorine, while we have the Hydrogen and the hydrogen sulfide for the CO-B4. This information are now integrated in the new version of the manuscript.
- Did the authors check what signals the get from other gases?
Dear reviewer, the information on sensor cross-sensitivity are reported on their datasheets available on the manufacturer website; moreover, this work is focused on exploring the capabilities of an handheld monitor used for evaluating the personal exposure to some air pollutants, therefore, fine characterization of commercial sensors is off the topic of this paper in my opinion.
- The authors report NO2 concentration of 64 and 38ppm while the noise is 2-10ppb. They should splice the measured curves.
Dear reviewer, I’m so sorry but I cannot really figure out what do you mean. Please, could you be more specific? Thank you.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Comments and Suggestions for AuthorsThe paper has been improved and is ready for publication.
