Bioaerosols are colloid systems formed by tiny biological particles floating in the atmosphere, such as bacteria, fungi, viruses, and pollen. Bioaerosol particles can scatter and absorb solar radiation, communication signals, and optical signals, and can also be suspended in the air and spread long distances with the wind. Therefore, they exert an effect on many fields such as climate change, optical communication, and optical remote sensing. They have also attracted widespread attention in research areas such as functional materials, environmental protection, and disease transmission and prevention. Bioaerosol particles have diverse morphologies and rich compositions and can absorb and scatter incident light in multiple wavelengths. Additionally, their artificial cultivation techniques are relatively mature, with low cultivation costs, simple operation, and short cultivation periods. As a result, the artificially prepared bioaerosols have great development potential in the research on optical functional materials. Drying is an essential step in the artificial preparation of bioaerosols, and it can affect the broadband extinction performance of artificially prepared bioaerosols. However, the influence of different drying methods on the extinction performance of bioaerosols has not been studied. Our paper aims to investigate the influence of different drying methods on the extinction performance of artificially prepared bioaerosols and provide references for improving the extinction performance of bioaerosols through preparation.

BB0819 spores are one of the common entomopathogenic fungal spores and have been proven to have significant broadband extinction performance. Therefore, BB0819 spores are selected as the research object to investigate the effect of drying methods on the extinction performance of artificially prepared bioaerosols. In our study, two common microbial treatment methods, freeze drying and hot-air drying, are chosen to dry BB0819 spores. The preparation process consists of four main steps, including spore suspension preparation, solid culture medium cultivation, drying, and spore collection. To investigate the effect of the two drying methods on the broadband extinction performance of BB0819 bioaerosol, we build a model with detailed structural information on bio-particle morphology, and the Kramers-Kronig algorithm, discrete dipole approximation, and Monte Carlo simulation methods are adopted to calculate the extinction performance. Furthermore, Fourier transform infrared spectrum and two-dimensional correlation infrared spectrum are employed to analyze the internal composition and protein structure differences of bioaerosols after drying, explaining the changes in extinction performance. The reliability of the simulation results is confirmed through experimental validation in an aerosol chamber, where transmittance data is obtained for bioaerosols produced by different drying methods in the mid-infrared and far-infrared wavelengths.

The drying process can disrupt the internal composition of spores. This leads to changes in the absorption functional groups, protein secondary structures, and various component contents, which in turn affects the optical absorption of spores. Compared with freeze drying, hot-air drying results in a higher protein absorbance in BB0819 spores, about 7.19%. The increased protein content can enhance the optical absorption ability of spores in the 6-8 μm wavelength range. Although the protein absorbance in BB0819 spores is relatively higher after hot-air drying, the fitting results of protein secondary structure indicate that the protein absorbance of α-helix structure in spores decreases by about 5%. The α-helix is the main structure maintaining protein conformation, indicating that although the protein content is not reduced by hot-air drying, it has a significant influence on the stability of protein structure. Additionally, the rising temperature during hot-air drying leads to significant denaturation of polysaccharide substances (mainly peptide polysaccharide layer) and changes in the content of polysaccharide groups inside the material. Polysaccharides contain abundant C—O and C—C bonds, and the stretching and vibration of these chemical bonds have a strong absorption peak at around 10 μm, which may affect the optical absorption performance of spores around 10 μm. As shown in Figs. 7(c)-(f), in the 2.5-10.3 μm wavelength range, freeze-dried spores have a larger extinction cross-section and stronger extinction performance for individual spores, while in the 10.3-15.4 μm wavelength range, hot-air dried spores have a larger extinction cross-section and stronger extinction performance for individual spores. Both BB0819 bioaerosols dried by the two drying methods show significant optical attenuation ability. In the 2.5-10.3 μm wavelength range, both bioaerosols can attenuate the transmittance of incident light to below 20%, with most of them below 15%. In the 10.3-15.4 μm wavelength range, freeze-dried spores can attenuate the transmittance to between 20% and 35%, while hot-air dried spores can attenuate the transmittance below 30%.

Drying is an essential step in the artificial preparation of bioaerosols, and it affects the broadband extinction performance of artificially prepared bioaerosols. Research results show that bioaerosols after freeze drying have higher content of polysaccharides and more stable protein structures, which leads to better extinction performance in the far-infrared band. Bioaerosols obtained through hot-air drying contain more proteins for better extinction performance in the mid-infrared band. In the mid-infrared band, selecting bioaerosols obtained by freeze-dried spores can reduce the average transmittance from 11.95% to 9.14% within three minutes, while in the far-infrared band, adopting bioaerosols obtained by hot-air dried spores can reduce the average transmittance from 34.38% to 26.03% within three minutes. We clarify the effects of drying methods on the extinction properties of artificially prepared bioaerosols and provide references for improving their extinction properties through preparation.