Accurate Representations of the Microphysical Processes Occurring During the Transport of Exhaled Aerosols and Droplets

Aerosols and droplets from expiratory events play an integral role in transmitting pathogens such as SARS-CoV-2 from an infected individual to a susceptible host. However, there remain significant uncertainties in our understanding of the aerosol droplet microphysics occurring during drying and sedimentation, and the effect on the sedimentation outcomes. Here, we apply a new treatment for the microphysical behaviour of respiratory fluid droplets to a droplet evaporation / sedimentation model and assess the impact on sedimentation distance, timescale and particle phase. Above 100 µm initial diameter, the sedimentation outcome for a respiratory droplet is insensitive to composition and ambient conditions. Below 100 µm, and particularly below 80 µm, the increased settling time allows the exact nature of the evaporation process to play a significant role in influencing the sedimentation outcome. For this size range, an incorrect treatment of the droplet composition, or imprecise use of RH or temperature can lead to large discrepancies in sedimentation distance (>1 m, >3 m and >2 m respectively). Additionally, a respiratory droplet is likely to undergo phase change prior to sedimenting if initially <100 µm diameter, provided the RH is below the measured phase change RH. Calculations of the potential exposure versus distance from the infected source show that the volume fraction of the initial respiratory droplet distribution in this size range which remains elevated above 1 m decreases from 1 at 1 m to 0.125 at 2 m.