the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 28 Nov 2024
Performance of a low-cost optical particle counter (Alphasense OPC-N3) for estimating size-resolved dust emission flux using eddy covariance
Abstract. The recent development of low-cost optical particle counters (OPCs) presents new opportunities for improving spatial coverage of particle concentration in the atmosphere as they are more affordable, compact, and consume less power than traditional OPCs. In particular, these OPCs could improve our ability to quantify dust emissions in complex environments during aeolian soil erosion. The high frequency sampling capacity (1 Hz) of some sensors may make them suitable for estimating dust emissions using the eddy-covariance method. Here, the capability of the low-cost OPC-N3 from Alfasense to estimate the size-resolved dust flux using the eddy-covariance method is evaluated. During the Jordan Wind Erosion and Dust Investigation (J-WADI) experiment, we tested one OPC-N3 against traditional reference OPCs. The OPCs were located in close proximity to a sonic anemometer, enabling the correlation of dust concentration and vertical velocity fluctuations for estimating dust fluxes. Despite the high temperature and dusty wind conditions of the campaign, the N3 monitored the dynamics and magnitude of dust concentration with reasonable precision. The turbulence characteristics of the dust concentration fluctuations measured by the N3 were similar to those from the reference OPC. After calibrating the N3 dust concentration, the N3 accurately estimated the dust emission flux, with differences of less than 30 % compared to the reference OPC. Our results confirm the potential of low-cost OPCs for dust-erosion research. Nonetheless, further evaluation of low-cost OPCs is still needed across different environments and weather conditions.
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RC1: 'Comment on amt-2024-187', Anonymous Referee #1, 24 Jan 2025
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Low-cost sensors can be an affordable and versatile tool to improve our understanding of the air quality in more diverse scenarios. Their shortcomings which mostly come from their low-cost nature and are not currently fully understood, are the main limiting factor for their wider application as a sensible alternative or supplement to the existing network. Thus, studies as this one can help in improving our understanding on the LCS and extend their use in scenarios where they can provide useful information. The study presented in the manuscript is done in a concise and scientifically correct way. I have no major concerns about the work presented and I can only suggest few things which may improve its readability and content.
- To start with, my main concern about the paper is its size. I think that the paper is way too long. While I did not find many things that were unnecessary, there is too much detail on everything. I would suggest the authors to try and reduce the size of the chapters as much as possible and add a take-away message at the end of each chapter as the lack of a discussion or a sum-up chapter is not helping in having clear conclusions of the analysis done in each chapter (which I do not suggest with the present form of the paper as it would further increase its size).
- Adding to that the abstract is poorly written in my opinion. The study has a multitude of results and while many of these point the good performance the OPC-N3 had, this is briefly demonstrated in two rather vague sentences. As the abstract is probably the first part everyone reads, I think it should include more results (even in the form of simple conclusions coming from the analysis done, mentioning what was tested), which would make the reader more interested to carry on reading the paper.
- In the Methodology chapter 2.2 while there is a detailed description of the three OPCs there is no information about their price range, even as a ratio of one against the other. Adding to that, there is no clear information on what makes the N3 cheaper and how these differences are expected to affect the quality of the data collected, as the way it is presented it appears as the N3 is the same as the others but consuming less energy and having no pump.
- The N3 is known to be greatly affected by humid conditions. While it is mentioned that what is studied in this paper is its behaviour during dust events (happening during the daytime), looking at figure 3 this can possibly be the case for the nights of the 14,17-22/9, when RH reached 80%. Was there a notable discrepancy in these periods? It would be a good opportunity to look at this and provide valuable information for one of the main problems the N3 has. I see that this is slightly discussed in the conclusions but only mentions the periods with low RH.
- Throughout the analyses done it is noted that the N3 and Promo seem to perform similarly (in good or bad cases) compared to the FIDAS. One of my guesses is that the positioning and the sampling heads are the factors that affected this outcome. Though I do not suggest the removal of the FIDAS information the question remains. If the Promo was used as the reference for the N3, what is the purpose of the FIDAS, as it makes the Promo appear to underperform as well? I suggest that you add a clear note in the results that while the FIDAS is a reference instrument its measurements are probably biased by these factors, should be considered as “background measurements” and direct comparisons should be evaluated cautiously.
Minor additions suggested
- Introduction, line 21: I would suggest adding the chemical composition of the particles on their environmental impact, as it is one of the most important factors both in their direct impact as well as for their evolution.
- Results, line 131: It is pointed that the N3 needed a restart. What was the reason for that? Stability is one of the factors affecting the reliability of low-cost sensors and any information for this is useful for future users.
- Was response time looked at? The reaction time of the instruments on fast changes is important when looking at specific conditions. This would be interesting information if available.
Citation: https://doi.org/10.5194/amt-2024-187-RC1 -
RC2: 'Comment on amt-2024-187', Amato Evan, 24 Jan 2025
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Review of “Performance of a low-cost optical particle counter (Alphasense OPC-N3) for estimating size-resolved dust emission flux using eddy Covariance” by Dupont et al.
This paper presented results from an observational study designed to evaluate the feasibility of using a so-called low-cost optical particle counter (OPC) to generate high frequency measurements of the size resolved dust concentration, with the specific application to using the eddy covariance method to estimate the vertical turbulent diffusive dust flux. Via comparison with reference instruments, the authors determined that the vertical flux estimated with the low-cost instrument was comparable to that from a (more expensive) reference instrument. I found this manuscript to be thorough in terms of evaluating/comparing the N3’s characteristics with the reference OPCs and only have a three comments that I would like to see addressed before I can recommend the manuscript for publication in AMT.
Comments:
Section 2. Can the authors report on the wavelength of light that the three OPCs operate at and how differences in this characteristic may influence the calculated size-resolved particle concentration? I’m specifically thinking about the influence of the (spectrally resolved) particle complex refractive index on the estimated particle sizes (e.g., Huang et al. 2021: Linking the different diameter types of aspherical desert dust indicates that models underestimate coarse dust emission, GRL)
Section 3.1 Can/should the apparent tendency of the N3 OPC to misclassify 1 um particles into the 1.4 um bin be accounted for by summing the particle counts for these two bins? This could also help to address the related bias shown in Fig 14b at these same sizes.
Section 3.2 I am afraid I don’t understand why the reduction in the N3 flow rate results in an overestimation of the N3 concentration relative to the Promo, given that the N3 flow rate is accounted for in the concentration calculation (line 91). Can the authors provide an explanation why accounting for the flow rate in the concentration calculation is nonetheless insufficient to account for the wind direction bias?
Citation: https://doi.org/10.5194/amt-2024-187-RC2
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