Auto-fluorescence based detection of single aerosol particles: Applications to bioaerosols and atmospheric microplastics

Aerosol particles are an omnipresent and important component of Earth’s atmosphere. They affect human health by penetrating deep into the respiratory system and play a vital role in weather and climate by influencing cloud formation and scattering sunlight. This thesis focuses on two important aerosol classes: bioaerosols, such as pollen, fungal spores and bacteria and atmospheric microplastics (AMPs). Bioaerosols have long been recognized for their health impacts as allergens and disease vectors. More recently, their role as ice nucleating particles (INPs) has gained attention, as they can trigger the freezing of supercooled cloud droplets. As INPs not only initiate precipitation, but also change cloud albedo and lifetime, ice nucleation active bioaerosols might play an important role in Earth’s climate. AMPs are only recently being recognized as airborne pollutants. Their small size raises concerns about health risks, yet little is known about their abundance or atmospheric behavior, especially for particles smaller than 10 μm that can reach the alveolar region of the lungs. A major challenge in studying both aerosol types is their detection and classification. Their chemical and morphological complexity makes them difficult to quantify with standard techniques. Traditionally, they have been studied using separate analytical frameworks. This thesis explores a unifying detection principle: auto-fluorescence or intrinsic fluorescence on a single particle level as detected by the Wideband Integrated Bioaerosol Sensor (WIBS-5/NEO). While this light-induced fluorescence (LIF) technique is well established for detecting bioaerosols, its application to AMPs has not been demonstrated prior to this work. This thesis shows that the online detection approach of the WIBS is well-suited to detect AMPs and to classify bioaerosols in the sub-Arctic with notable selectivity. It therefore demonstrates how LIF techniques can shed light on open questions in aerosol science. The thesis comprises three peer-reviewed publications that investigate the fluorescence properties of AMPs and bioaerosols, their detectability using the WIBS and its implication for quantifying biological ice nucleation in the atmosphere. Laboratory experiments showed that the WIBS is capable of measuring fluorescence of common microplastics with high sensitivity, even detecting fluorescence of microplastics from a polyethylene terephthalate bottle of 1.2 μm diameter with 95 % efficiency. Although polymer types could not be distinguished from each other, they could be clearly separated from pollen based on fluorescence intensity. High-resolution fluorescence excitation-emission maps revealed.complex but characteristic fluorescence behavior, pointing toward potential future improvements in LIF instrumentation for AMP detection. Ayear-long field campaign in the pristine sub-Arctic (Pallas, Finland) combined WIBS measurements with traditional pollen and spore counts, environmental DNA (eDNA) analysis and INP measurements. This provided a unique high time-resolution dataset of fluorescent aerosol particles (FAPs), revealing strong seasonal cycles and clear emission drivers such as relative humidity, temperature and the absence of snow. Two distinct HFAP (highly fluorescent aerosol particle) subtypes were found to correlate positively with pollen and spore concentrations (Pearson’s r = 0.90 and 0.65, respectively, p < 0.0001). Interestingly, the WIBS proved to be more accurate in the quantification of total fungal spores than traditional microscopic techniques. Most importantly, the analysis showed a significant, strong correlation between fluorescent fungal-like particles and INP concentrations active at temperatures above–13.5 °C (r = 0.94, p <0.0001), elucidating the important role of fungal spores in high-temperature ice nucleation in the boreal environment. However, only a small fraction of spores appeared to be ice-active, and identifying specific ice-nucleating taxa via eDNA remained inconclusive due to limited existing ice nucleation activity data for boreal fungi. These findings highlight the WIBS as a powerful tool for real-time detection and quantification of bioaerosols in natural environments, offering insights into the biological contribution to atmospheric ice nucleation. This knowledge is crucial for improving INP parameterizations in climate models. At the same time, the recognition of AMPs as FAPs introduces a new complexity to interpreting fluorescence based aerosol measurements in urban or polluted environments with potential impact to applications such as allergy forecasting.