Applicability of the low-cost optical particle counter OPC-N3 for microphysical measurements of fog
Abstract. Low-cost devices for particulate matter measurements are characterized by small dimensions and light weight. This advantage makes them ideal for drone measurements, where those parameters are crucial. However, they also have some issues, like the values of particulate matter from low-cost optical particle counters can be biased by high ambient humidity. In this article, we are evaluating low-cost optical particle counter Alphasense OPC-N3 for measuring the microphysical properties of fog. This study aimed to show that OPC-N3 not only registers aerosols or humidified aerosols but also registers fog droplets. The study was done on the rooftop of Institute of Geophysics, University of Warsaw, Poland, during autumn-winter 2021. To validate the results, the data from OPC-N3 were compared with data obtained from the reference instrument, which was Oxford Laser VisiSize D30. VisiSize D30 is a shadowgraph device able to register photos of individual droplets.
Taking into consideration the effective radius of droplets, it is possible to differentiate low-visibility situations between fog conditions (which are not hazardous for people) from haze events, when highly polluted air can cause health risks to people.
The compared microphysical proprieties were liquid water content (LWC), number concentration (Nc), effective radius <reff > and statistical moments of radius. The Pearson correlation coefficient for LWC was 0.91, Nc was 0.94, and for <reff > was 0.63. Overall, these results suggest a good compliance between devices. However, the OPC-N3 has to be corrected in reference to professional equipment. To conclude, our study provides the foundation for a new application of the optical particle counter Alphasense OPC-N3 within drones to measure the vertical profiles of the microphysical properties of fog.
Katarzyna Nurowska et al.
Status: final response (author comments only)
RC1: 'Comment on amt-2022-269', Anonymous Referee #2, 20 Oct 2022
- AC2: 'Reply on RC1', Katarzyna Nurowska, 28 Dec 2022
- AC4: 'Reply on RC1', Katarzyna Nurowska, 28 Dec 2022
CC1: 'Comment on amt-2022-269', Lasse Moormann, 08 Nov 2022
- AC1: 'Reply on CC1', Katarzyna Nurowska, 28 Dec 2022
RC2: 'Comment on amt-2022-269', Anonymous Referee #1, 01 Dec 2022
- AC3: 'Reply on RC2', Katarzyna Nurowska, 28 Dec 2022
Katarzyna Nurowska et al.
Katarzyna Nurowska et al.
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In the presented manuscript authors show the comparison between low-cost optical counter OPC-N3 of the Alphasense Systems with the reference instrument a shadowgraph type VisSize D30 of the Oxford Lasers. The focus of the research was to identify the usefulness of the OPC in fog detection and research.
The manuscript is important and interesting. There are however several issues that should be improved before the publication. I’ll present the list of my list of comments and suggestions below.
The instrument setup should be much more described. Was any additional housing applied for the OPC to protect it against humidity or rain? Was it lying down on the roof of the building, or on any platform above the roof? If yes how much height above the roof surface? Can you present any photos of the devices set up?
In table 2, in the text it is written that OPC sampling was 10 s, then averaged up to 1 minute, in the table it is 1 minute sampling time, please make it consistent.
Equation 3, please check if all variables are explained, what is pix^2, is i here another variable or just index?
Why there was double averaging applied? Why not straight average from 10s to 10 minutes?
In the research authors used standard deviation as the uncertainty of the OPC measurements (based on 1 min resolution). It is a basic statistical error representation, probably good enough for this study. Please elaborate on how it would change if you would calculate it from 10 s, which was as far as I understood, basic sampling time. How uncertainty would change if you also consider Poisson statistics which represents a random error in the measurements?
What was the reason to do the 1 hour averaging? Why not 0.5h or 2 hours? It should be elaborated, how was it representative? Is it 2:00:59 – 3:01:07 really an hour or a little bit more? I understand it is a minor issue, but it just looks strange.
How it differs from other periods? Can authors present the temporal evolution of droplet size distribution for all sampling periods (at least in the appendix)? The authors should explain to the readers why the analyzed period and later case study in section 4.1 was better than the rest of the time series.
Authors with good results applied the Refractive Index correction. Please elucidate if all data presented are based on RIOPC or RIwater because it is not clear to me. Can you present any figure on how the correction influenced the measurements (at least in the appendix)?
Is it possible to apply any correction function for all factors influencing the OPC measurements (internal temperature, humidity, refractive index)?
In conclusion, I find this manuscript valuable and interesting. There are still fields to improve significantly, which is why I recommend a major review after which it can be published.