Aerial Mapping of (Bio)aerosols Using Dual UAV Platforms in an Austrian Spruce Forest.

Forests are dynamic sources and sinks of primary biological aerosol particles (PBAPs) that influence ecosystem health, atmospheric chemistry, and climate. However, the spatial and temporal distribution of these particles in and above forest canopies remains poorly understood. In this study, we deployed two complementary UAV platforms to characterize bioaerosol concentrations, composition, and environmental drivers in a spruce forest in Wienerwald, Lower Austria.

Sampling campaigns in June and July 2024 utilized a suite of sensors on the UAVs, including portable optical particle counters (POPS, 0.16 – 3.37 µm), impingers, and PM-, VOC & meteorological sensors. These aerial measurements were referenced against a ground station including a Wideband Integrated Bioaerosol Sensor (WIBS-5/NEO, 0.5 – 30 µm) and a Grimm 11-D (0.25 – 35.15 µm) aerosol spectrometer, and the same set of PM-, VOC & meteorological sensors. Two additional sampling days were conducted in June 2025 with additional UAV-based aerosol- and VOC-sampling.

We observed higher overall particle number concentrations at near-canopy altitudes (<5 m above canopy) compared to higher altitudes, with concentrations negatively correlated with total-VOC trends. In addition, we saw elevated overall particle concentrations above the canopy during morning hours, followed by a midday decrease that coincided with rising temperatures and falling relative humidity. The fluorescence data from the ground-based WIBS indicated that a substantial fraction of supermicron (>2.5 μm) particles were biological. Their fluorescence signature and elevated concentration at high relative humidities suggest a large contribution of fungal spores, which was confirmed by microscopy of samples from ground-based and, in 2025, UAV-mounted cascade impactor sampling. PBAP concentrations generally increased with high relative humidity, consistent with well-documented humidity-driven enhancements in biological particle release.

This multi-platform UAV approach provides a robust framework for resolving forest-atmosphere exchange processes, yielding critical data to improve atmospheric models and our understanding of ecosystem-climate feedback loops.