Ambient Air Quality Measurement with Low-Cost Optical and Electrochemical Sensors: An Evaluation of Continuous Year-Long Operation
Round 1
Reviewer 1 Report
The paper describes an attempt to estimate the usability of sold-state low-cost sensors to assess the status of air pollution.
The paper is somewhat double faceted. The case study is well described both in terms of the general approach and in the description of results. The paper shows undoubtedly a high level for what concerns the on-field activities but introduction, discussion and, especially, conclusions seem not adequate. Some points follow.
English is weak and should be checked by a professionl. Graphs are really too small, please enlarge them.
Going to the paper content, introduction is to be rewritten showing better the characteristics of the sensors, the scope of the experimental phase and the general framework. By the way, the title is all but explanatory. More in general, I do not understand why the authors do not make explicit reference to the class of low-cost solid state sensors although they cite several papers on the issue. I think that a paper like “Sensors 2020, 20, 7290; doi:10.3390/s20247290” can be used as a reference ONLY FOR THIS POINT.
Another issue is that the authors show the results of their statistical analysis, but they do not introduce properly such statistic models and instruments. I think, just for instance, that simply talking of correlation factors without any explanation, could lead to misinterpretation. The authors openly cite the recorded values and their correlations and correctly separate statistics based upon the range of the values but give little or no explanation regarding the reason for such results. I think that they should also carefully explain why correlation factors in all the analysed ranges (<10 10-100 >100) are all lower (near to zero in the first range) that the overall values. How can it happen?
Finally, I would ask the authors to enhance discussion and conclusions.
In conclusion the paper can be accepted after major changes according to the aforementioned remarks.
Author Response
English was edited by author services.
Title changed to: Ambient air quality measurement with low-cost optical and electrochemical sensors: An evaluation of continuous year-long operation.
Graphs layout of regression scatter plots and timelines was changed to single picture per page width.
The suggested parts were rewritten and partially rearranged. Description of statistic calculations added in separate paragraph of experimental section. References addressed.
Correlation matrices of PM10 sensors were found to be misplaced (overall and midrange). Newly only overall matrix is presented in full and overview table for all ranges replaces the remaining full matrices.
Reviewer 2 Report
This too short paper presents a one year comparison between low cost sensors for PM10 and for NO2. In fact, one type of low cost PM sensor and NOx sensors are used. This kind of work have been already conducted in the past for low cost sensors coming from different companies. The paper brings a complementary analysis to those already existing, but does not justified a publication in “Environments” in its present form. More work are needing before submitting a revised version of the paper, in particular for the statistical analysis of the comparison between the sensors, and for showing the interest of this work,. Or the authors could try to submit the paper to a journal having a lower impact factor. Below are some specific comments to help the authors to improve their paper.
Too many tables are provided. It is not necessary to give all the correlation matrices. Some figures can summarize the results.
Line 61: This validation is not original. Several papers already exist on this subject. The authors must conduct a more detailed bibliography research.
Line 75 : This line is too short. Can you described how you have established the validation factor ?
Legend of Figure 1a: What means: “255 per. Moving average”?
Legend of Figure 1b: What means “stc 25 val”?
Legend of Figure 3: AS for Figure 2, please provide the meaning of the abbreviations.
Line 150: What is the interest to highlight the sensor #4?
Part 3.1.1: This analysis makes sense if the errors bars of the measurements are provided for the references instruments and for the small sensors. Thus, what are the error bars? The author must verified that the error bars do not overlap before performing such analysis.
Part 3.1.2: The analysis is not really conducted. Giving the correlation matrices without serious statistical analysis is not very interesting. And some correlations values are far from 1; thus, it is too rapid to conclude that “sensors behaved similarly and produced similar results that can be considered producible and reliable”.
Author Response
Tables were revised and rearranged - correlation matrix only one remains and the others are replaced with overview table.
Introduction reviewed, statement cleared and rephrased - new position around Line 70 in revised version.
Statistics and calculations are explained in new paragraph "2.4. Statistics"
Legends, sensor coding and abbreviations explained, percentiles omitted with dataset names and their descriptions (percentiles were filled by excel as automatic description of trendlines).
Error bars are omitted as the measurement is considered indicative.
Original pragraph 3.1.2 was revised, correlation matrces were found misplaced (overall for midrange presented). Overview table is presented instead of full matrices for easier comparison.
Round 2
Reviewer 1 Report
The paper is now suitable for publication
Reviewer 2 Report
The authors have significantly improved the paper. It can be published now as it is.
