Bioaerosol Characterization with Vibrational Spectroscopy: Overcoming Fluorescence with Photothermal Infrared (PTIR) Spectroscopy.

Aerosols containing biological material (i.e., bioaerosols) impact public health by transporting toxins, allergens, and diseases and impact the climate by nucleating ice crystals and cloud droplets. Single particle characterization of primary biological aerosol particles (PBAPs) is essential, as individual particle physicochemical properties determine their impacts. Vibrational spectroscopies, such as infrared (IR) or Raman spectroscopy, provide detailed information about the biological components within atmospheric aerosols but these techniques have traditionally been limited due to the diffraction limit of IR radiation (particles >10 μm) and fluorescence of bioaerosol components overwhelming the Raman signal. Herein, we use photothermal infrared spectroscopy (PTIR) to overcome these limitations and characterize individual PBAPs down to 0.18 μm. Both optical-PTIR (O-PTIR) and atomic force microscopy-PTIR (AFM-PTIR) were used to characterize bioaerosol particles generated from a cyanobacterial harmful algal bloom (cHAB) dominated by Planktothrix agardhii. PTIR spectra contained modes consistent with traditional Fourier transform infrared (FTIR) spectra for biological species, including amide I (1630-1700 cm^-1) and amide II (1530-1560 cm^-1). The fractions of particles containing biological materials were greater in supermicron particles (1.8-3.2 μm) than in submicron particles (0.18-0.32 and 0.56-1.0 μm) for aerosolized cHAB water. These results demonstrate the potential of both O-PTIR and AFM-PTIR for studying a range of bioaerosols with vibrational spectroscopy.