Low-cost devices for particulate matter measurements are characterised by small dimensions and a light weight. This advantage makes them ideal for UAV measurements, where those parameters are crucial. However, they also have some issues. The values of particulate matter from low-cost optical particle counters can be biased by high ambient humidity. In this article, we evaluate the low-cost Alphasense OPC-N3 optical particle counter 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 carried out on the rooftop of the Institute of Geophysics, University of Warsaw, Poland, during autumn–winter 2021. To validate the results, the data from OPC-N3 were compared with the data obtained from the reference instrument, the Oxford Lasers VisiSize D30. VisiSize D30 is a shadowgraph device able to register photos of individual droplets. Considering 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 properties were liquid water content (LWC), number concentration (N_c), effective radius r_eff and statistical moments of radius. The Pearson correlation coefficient between both devices for LWC was 0.92, N_c was 0.95 and r_eff was 0.63. Overall, these results suggest good compliance between instruments. However, the OPC-N3 has to be corrected regarding professional equipment.

We present the first estimations of single hygroscopic parameter $\kappa$ for Polish urban area. The results were obtained using an inexpensive OPC-N3 optical particulate matter counter and the ASC 1000 Acoem Aerosol Conditioning System. Our studies were carried out during the winter and spring seasons, between 2020 and 2022 in Warsaw, Poland. We study the difference of $\kappa$ between these two seasons, as the aerosol has different origins. The mean value of the hygroscopicity parameter for spring (April–June) was $0.13\pm 0.13$ and for winter (December-March) it was $0.04\pm 0.04$ (obtained using PM${}_1$). The mean value of the growth factor for spring (April–June) was $1.52\pm 0.23$ and for winter (December–March) it was $1.16\pm 0.13$ (obtained using PM${}_1$). The values for winter period suggest that during the cold season the aerosol mixture in the atmosphere is dominated by non-hygroscopic particles. However events of higher $\kappa$ > 0.5 where occurring (mostly for big aerosol particles 1–10 $\mathsf{\mu }$m in size), when the air mass was coming from North Atlantic carrying sea salt particles. Furthermore, based on $\kappa$ we propose a method to remove the dependence of PM values on relative humidity on the OPC-N3 optical particulate counter.

Based on in situ observations, remote sensing, and tethered balloon soundings, this study examines vertical profiles of microphysical and thermodynamic properties in radiative fog layers in the Strzyżów valley (Southeastern Poland). During three radiation fog events in September 2023, 74 soundings were performed, with 41 employing the OPC-N3 instrument to capture droplet spectra. All cases showed similar conditions, with liquid water path consistently above 15 g m⁻², placing most observations within the thin fog regime. Effective droplet radius decreased with height (3–4.6 µm over 100 m), while larger droplets (18.5 µm) were near the ground. Fog dissipation occurred from both top and bottom. The mature stage showed peaks in liquid water content (LWC) and droplet number concentration (N_c) at about 80% of fog depth. Larger droplets (≥ 18.5 µm) were removed within minutes, affecting fog longevity. Equivalent adiabaticity (α_eq) – the scaling of adiabatic lapse rate to match observed liquid water path (LWP) – ranged from 0 to 0.6, with one rare case of negative near-ground α_eq. Instruments above and below the fog allowed estimation of effect of the fog's impact on radiation flux. The difference of total shortwave and longwave NET (downward − upward) radiation at ground level before and after dissipation reached 150 W m⁻². A linear relationship was found between reduction in longwave radiation and LWP under optically thin conditions. Mean LWC in the fog core ranged from 0.2–0.4 g m⁻³, and N_c reached 300 cm⁻³. Near-surface effective radius was 8–10 µm, decreasing with height. Agreement between model outputs and observed fluxes supports the retrieved microphysical parameters.

This study investigates the vertical profiles of microphysical and thermodynamic properties within radiative fog layers in the Strzyżów valley (Southeastern Poland), based on in-situ, remote sensing and tethered balloon soundings data. Across three case studies of radiation fog that occurred in September 2023, 74 soundings were performed, with 41 employing the OPC-N3 instrument to capture droplet spectra. The results indicated similar weather conditions in all cases, with a liquid water path consistently above 15 g·m⁻², although no transition to dense fog was observed. The effective droplet radius decreased with height (between 3–4.6 µm for 100 m), with larger droplets (≥18.5 µm) concentrated near the ground. The fog dissipated both from the top and bottom, with the mature fog stage marked by peak liquid water content (LWC) and the droplet number concentration (N_c) near 80 % of the fog height. Theoretical calculations of droplet terminal velocity (for droplets ≥18.5 µm) indicate that larger droplets are removed from fog layers within minutes, affecting the longevity of the fog. Equivalent adiabaticity values (α_eq – the ratio by which the adiabatic lapse rate of the mixing ratio needs to be multiplied to give the same amount of liquid water path as observed in a specific cloud) ranged between 0 and 0.6. Except in one instance where negative values α_eq were observed near the ground, a phenomenon scarcely reported in existing fog studies. Having instruments measuring radiation at two different heights, it was possible to estimate the effect of fog on reducing the total shortwave and longwave (NET) radiation at ground level by 150 W·m⁻² (just before the fog disappearing and after). The measured dependence of the reduction of longwave radiation by fog depends linearly on the amount of liquid water path. As a result of the measurements, average values of liquid water content and droplet number concentrations were obtained for the observed optically thin fogs in the valley area. Mean LWC in the fog layer core was found between 0.2–0.4 g·m⁻³, with N_c up to 300 cm⁻³. The effective radius (8–10 µm) exhibited a linear height-dependent decrease, with radiation model closures yielding minimal biases, supporting the accuracy of radiation assessments within fog environments.

This study investigates the vertical profiles of microphysical and thermodynamic properties within radiative fog layers in the Strzyżów valley (Southeastern Poland), based on in-situ, remote sensing and tethered balloon soundings data. Across three case studies of radiation fog that occurred in September 2023, 74 soundings were performed, with 41 employing the OPC-N3 instrument to capture droplet spectra. The results indicated similar weather conditions in all cases, with a liquid water path consistently above 15 g·m⁻², although no transition to dense fog was observed. The effective droplet radius decreased with height (between 3–4.6 µm for 100 m), with larger droplets (≥18.5 µm) concentrated near the ground. The fog dissipated both from the top and bottom, with the mature fog stage marked by peak liquid water content (LWC) and the droplet number concentration (N_c) near 80 % of the fog height. Theoretical calculations of droplet terminal velocity (for droplets ≥18.5 µm) indicate that larger droplets are removed from fog layers within minutes, affecting the longevity of the fog. Equivalent adiabaticity values (α_eq – the ratio by which the adiabatic lapse rate of the mixing ratio needs to be multiplied to give the same amount of liquid water path as observed in a specific cloud) ranged between 0 and 0.6. Except in one instance where negative values α_eq were observed near the ground, a phenomenon scarcely reported in existing fog studies. Having instruments measuring radiation at two different heights, it was possible to estimate the effect of fog on reducing the total shortwave and longwave (NET) radiation at ground level by 150 W·m⁻² (just before the fog disappearing and after). The measured dependence of the reduction of longwave radiation by fog depends linearly on the amount of liquid water path. As a result of the measurements, average values of liquid water content and droplet number concentrations were obtained for the observed optically thin fogs in the valley area. Mean LWC in the fog layer core was found between 0.2–0.4 g·m⁻³, with N_c up to 300 cm⁻³. The effective radius (8–10 µm) exhibited a linear height-dependent decrease, with radiation model closures yielding minimal biases, supporting the accuracy of radiation assessments within fog environments.

This study investigates the vertical profiles of microphysical and thermodynamic properties within radiative fog layers in the Strzyżów valley (Southeastern Poland), based on in-situ, remote sensing and tethered balloon soundings data. Across three case studies of radiation fog that occurred in September 2023, 74 soundings were performed, with 41 employing the OPC-N3 instrument to capture droplet spectra. The results indicated similar weather conditions in all cases, with a liquid water path consistently above 15 g·m⁻², although no transition to dense fog was observed. The effective droplet radius decreased with height (between 3–4.6 µm for 100 m), with larger droplets (≥18.5 µm) concentrated near the ground. The fog dissipated both from the top and bottom, with the mature fog stage marked by peak liquid water content (LWC) and the droplet number concentration (N_c) near 80 % of the fog height. Theoretical calculations of droplet terminal velocity (for droplets ≥18.5 µm) indicate that larger droplets are removed from fog layers within minutes, affecting the longevity of the fog. Equivalent adiabaticity values (α_eq – the ratio by which the adiabatic lapse rate of the mixing ratio needs to be multiplied to give the same amount of liquid water path as observed in a specific cloud) ranged between 0 and 0.6. Except in one instance where negative values α_eq were observed near the ground, a phenomenon scarcely reported in existing fog studies. Having instruments measuring radiation at two different heights, it was possible to estimate the effect of fog on reducing the total shortwave and longwave (NET) radiation at ground level by 150 W·m⁻² (just before the fog disappearing and after). The measured dependence of the reduction of longwave radiation by fog depends linearly on the amount of liquid water path. As a result of the measurements, average values of liquid water content and droplet number concentrations were obtained for the observed optically thin fogs in the valley area. Mean LWC in the fog layer core was found between 0.2–0.4 g·m⁻³, with N_c up to 300 cm⁻³. The effective radius (8–10 µm) exhibited a linear height-dependent decrease, with radiation model closures yielding minimal biases, supporting the accuracy of radiation assessments within fog environments.