A novel aerosol filter sampler for measuring the vertical distribution of ice-nucleating particles via fixed-wing uncrewed aerial vehicles

Abstract. A mobile sampler for collecting aerosol particles on an uncrewed aerial vehicle (UAV) was developed and tested during three consecutive Pallas cloud experiment campaigns in the vicinity of the Sammaltunturi Global Atmosphere Watch site (67°58′ N, 24°7′ E, 565 m above sea level) in Finland. The sampler is designed to collect aerosol particles onto Nuclepore filters, which are subsequently analysed for the temperature-dependent number concentration of ice-nucleating particles (INPs) of the sampled aerosol using a freezing assay. The sampler was flown with a fixed-wing UAV in different altitudes up to 1000 m above ground level (a.g.l). The total flight times ranged from 60 min to around 100 min, depending on environmental conditions. Pressure, temperature and relative humidity were also measured to provide information about the meteorological flight conditions. The flow over the filter was maintained by a micro-diaphragm pump, providing approximately 10 standard litres per minute over a small filter (diameter of 25 mm) and around 11 standard litres per minute over a larger filter (diameter of 47 mm) at a pressure corresponding to 500 m above sea level. For a typical flight time of 1.5 h, this results in a sampled air volume of approximately 930 to 1000 standard litres per flight, giving an INP detection limit of approximately 1.1×10-3 and 1.0×10-3 INPs per standard litre, respectively. For comparison to the flight results, a similar set-up was deployed at ground level. The comparison shows a clear distinction from the water and handling blank background for both set-ups, proving the technical feasibility of the set-ups. Furthermore, for some flights, a shift between the two INP populations can be seen, indicating that ground-based INP measurements deviate from the samples collected on board the UAV.