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 (N_c), effective radius <r_eff > and statistical moments of radius. The Pearson correlation coefficient for LWC was 0.91, N_c was 0.94, and for <r_eff > 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.