Simulation and field campaign evaluation of an optical particle counter on a fixed-wing UAV

Abstract

Unmanned aerial vehicles (UAVs) have great potential to be utilised as an airborne platform for measurement of atmospheric particulates and droplets. In particular, the spatio-temporal resolution of UAV measurements could be of use for the characterisation of aerosol, cloud, and radiation (ACR) interactions, which contribute to the largest uncertainty in the radiative forcing of climate change throughout the industrial era (Zelinka et al., 2014). UAV-instrument combinations must be extensively validated to ensure the data are repeatable and accurate. This paper presents an evaluation of a particular UAV-instrument combination: the FMI-Talon fixed-wing UAV and the UCASS open-path optical particle counter. The performance of the UCASS was previously evaluated on a multi-rotor airframe by Girdwood et al. (2020). However, fixed-wing measurements present certain advantages – namely endurance, platform stability, and maximum altitude. Airflow simulations were utilised to define limiting parameters on UAV sampling – that is, an angle of attack limit of 10∘ and a minimum airspeed of 20 m s−1 – which were then applied retroactively to field campaign data as rejection criteria. The field campaign involved an inter-comparison with reference instrumentation mounted on a research station, which the UAV flew past. Cloud droplets were considered the ideal validation particle; since the underlying Mie assumptions used to compute droplet radius were more valid, future work will focus on the instrument response to aerosol particles. The effective diameter measured by the UAV largely agreed within 2 µm. The droplet number concentration agreed within 15 % on all but five profiles. It was concluded that UCASS would benefit from a mechanical redesign to avoid calibration drifts, and UAV attitude variations during measurement should be kept to a minimum.