Visualization of Water Uptake by Human Respiratory Aerosol Components with In Situ Transmission Electron Microscopy

MARTIN AHN, Taylor J. Woehl, Akua Asa-Awuku, University of Maryland, College Park

     Abstract Number: 240
     Working Group: Health-Related Aerosols

Abstract
Bioaerosols are complex mixtures of biological matter (viruses, proteins, lipids) and inorganic/organic matter (salts, surfactants) that are a common respiratory virus transmission route. Liquid bioaerosol droplets with sizes between 1 – 100 μm are readily generated by human respiratory/vocal activities, such as talking, coughing, and breathing. These liquid particles are produced within the lungs, where the relative humidity (RH) is ~100%, and rapidly dry once exposed to atmospheric RH (≤ 80% RH) to form either solid or semi-solid particles. The resulting particle morphology and drying rate during this phase transition have been suggested to impact the virus viability within these bioaerosols. However, the detailed dynamics of the drying process and how it impacts the final structure of solid bioaerosols remain unknown due to a lack of observations of this nanometer scale dynamic process. In situ transmission electron microscopy (TEM) allows for the visualization of relevant nano-scale processes (e.g., phase-separation, nucleation) under controlled environmental conditions that may not be clearly resolved by conventional optical or fluorescence microscopy. Here we developed RH-controlled in situ TEM to visualize wetting and drying of bioaerosol droplets and aerosols under various environmental conditions. We utilized a MEMS-based in situ TEM sample cell, where RH control was achieved by mixing humidified and dry air streams at various volumetric flow rate ratios. Human respiratory droplets were modelled by aerosolized mixtures of bovine serum albumin (BSA) and sodium chloride (NaCl) into micron sized droplets. The dry model particles were impinged on silicon nitride sample chips and assembled into the in situ TEM sample cell and observed under controlled RH using in situ scanning TEM (STEM).