Autofluorescence of atmospheric bioaerosols – spectral fingerprints and taxonomic trends of native pollen

Primary biological aerosol particles (PBAP) are important factors in atmospheric cycling, climate, and public health. Pollen is a major fraction of PBAP and is receiving increasing attention due to its high allergenic potential and the associated severe impacts on personal life quality and economy. Recently, autofluorescence-based techniques have proven to be valuable tools for real-time, in-situ quantification and classification of PBAP. First studies suggest that the autofluorescence of pollen may be sufficiently selective to be utilized for an automated and real-time monitoring of pollen in ambient air. However, the degree of selectivity autofluorescence can provide is still in question and actively debated. This study addresses the origin, properties, and selectivity of autofluorescence from native pollen (undamaged and chemically untreated) by providing fluorescence microscopy and spectroscopy measurements along with a systematic synthesis of related literature. We show that dry, native pollen reveals characteristic and reproducible autofluorescence signatures which are shaped by cell wall associated fluorophores, such as phenolic compounds and carotenoid pigments. In addition, fluorescence signals from proteins and chlorophyll a were observed occasionally. The abundance and intensity of the individual fluorescence signals show certain taxonomic trends and allow systematic differentiation from bacteria and fungal spores due to the lack of protein fluorescence from the grain surface. Principal component analysis was used to explore the discrimination potential of pollen autofluorescence and revealed a differentiation of pollen on family level. Our results help explore the levels of selectivity that autofluorescence-based techniques can provide to PBAP analysis and will support the development and application of autofluorescence-based detectors for monitoring of allergenic pollen in the atmosphere.