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

Abstract

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 behavior of respiratory fluid droplets to a droplet evaporation/sedimentation model and assess the impact on sedimentation distance, time scale, and particle phase. Above a 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 (with representative values >1 m, >2 m, and >2 m, respectively). Additionally, a respiratory droplet is likely to undergo a phase change prior to sedimenting if initially <100 μm in diameter, provided that 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.

Shielding of Viruses such as SARS-CoV-2 from Ultraviolet Radiation in Particles Generated by Sneezing or Coughing: Numerical Simulations of Survival Fractions

“…The nonsphericities appear to arise from the crystallization/precipitation of different materials in different locations. Walker et al, (2021, their figure 5c) collected dried particles of artificial saliva on a surface and examined them with scanning electron microscopy. The particles generally look smooth on the order of the apparent resolution, except for some unclear features.…”

“…These are the only studies (to the authors’ knowledge) with as much information related to morphologies of small particles from dried saliva or respiratory fluids with sizes in the range of interest (e.g., Fennelly, 2020). However, there are uncertainties in how to apply the observations of Vejerano and Marr, (2017) and Walker et al (2021) to modeling of S p of dried saliva or respiratory particles in the 0.5- to 9-µm size range.…”

“…One reason for the uncertainty is that the artificial saliva particles studied by Vejerano and Marr, (2017) and Walker et al (2021) are mostly > 20 µm. In contrast, the particles emphasized here are 0.1- to 9-µm; larger particles are significant on a per-mass basis (Jarvis, 2020), but tend to deposit relatively quickly from air.…”

“…The homogeneous-sphere-with-virions assumption is a useful case to investigate first, given the large numbers of relevant variables: mr and mi of the virion(s), particle and surface; size(s) of the virions and the particles; virion locations; wavelengths; number and angles of beams; presence of a surface or not. However, for more accurate modeling of the Sp of more complex particles, additional studies along the lines of studies by Vejerano and Marr (2017) and by Walker et al, (2021) would likely be needed. Vejerano and Marr (2017) collected droplets of artificial saliva on a superhydrophobic surface and used optical microscopy to examine the particles as they dried as the RH was lowered.…”

Airborne virus transmission via respiratory droplets: Effects of droplet evaporation and sedimentation

“…These effects include the evaporation-induced cooling of the droplet [ 28 , [44] , [45] , [46] ], airflows and ventilation effects for large droplets [ 21 , 25 , 47 ], finite evaporation-rate effects for small droplets [ 48 , 49 ], solar irradiation effect [ 50 , 51 ], and solute-induced effects, including water vapor-pressure lowering [ 52 , 53 ], local solute-concentration gradients [ [54] , [55] , [56] ], crust formation due to solute crystallization [ 54 , 57 , 58 ], liquid−liquid phase separation [ [59] , [60] , [61] ], and a possible solute-concentration dependence of the viscosity [ 62 , 63 ] and the water-diffusion coefficient [ 63 , 64 ] inside the droplet. These effects are themselves dominated by various parameters, such as the initial size of the droplet, the type and the initial volume fraction of solutes, the ambient temperature [ 47 , 50 , 65 , 66 ], the relative humidity [ 47 , 65 , [67] , [68] , [69] , [70] , [71] ], non-ideal effects due to inter-particle interactions inside the droplet [ 72 , 73 ], the internal morphology of droplets [ 59 , 74 , 75 ], and the initial height at which droplets are released into the air. Among these parameters, the relative humidity and the initial solute-volume fraction play key roles in determining the size of the droplet nuclei produced at the end of the evaporation process.…”