Surface activity of optically-trapped single subpollen particle interacting with atmospheric water

Subpollen particles (SPPs), fragments of pollen released during pollen rupture, contribute to the atmospheric bioaerosol load and are an important source of primary biological aerosol particles. These particles are small, can be suspended in the atmosphere for long periods, and can act as cloud condensation nuclei (CCN), influencing human health and climate. However, surface activities resulting from the interaction of SPPs with atmospheric water, which enable them to act as CCN, are still unclear. In this work, we applied optical-trapping Raman spectroscopy (OT-RS) to study the SPPs’ interaction with atmospheric water on a single-particle scale. By analyzing single-particle Raman spectra of SPPs, we identified different compositions, such as proteins, fatty acids, and lipids, corresponding to different fractions inside or outside of a pollen grain. We demonstrated that SPPs with higher concentrations of hygroscopic materials, such as amino acids and proteins, could act as CCN under high relative-humidity conditions. The surface modification caused by the interaction between atmospheric water and SPPs is also illustrated in this study. Additionally, we manipulated water droplets containing NaCl to coat the surface of SPPs, simulating interactions between sea-spray aerosol and SPPs. The results revealed distinct surface modifications when SPPs were exposed to saline solutions and pure water. This work illustrates the dynamic process for water uptake and cloud formation of SPPs under simulated atmospheric conditions and provides a single-particle method to explicitly characterize the surface activity between SPPs and atmospheric water.