Aerosol/Droplet Transformation

Evaporation and hygroscopic growth

 

High level of certainty:

1. Aerosols and droplets are generated under physiological conditions of high relative humidity (often assumed to be >99% RH) and temperature (37°C) and, thus, are dominated by their water content. The majority (~99% by number) are of respirable size.

2. Breathing generally produces the fewest particles by number and mass, followed by speaking, then coughing, and then sneezing. Emission rates for these activities are further detailed in the Expiration mechanism.

3. Once generated by an expiratory event into an environment at, typically, lower RH and temperature, the droplets and aerosols lose moisture moving towards a state of equilibrium water content. This decrease in size occurs over a timescale less than 1 s for the smallest particles to greater than 10 s for the larger droplets, depending on RH and temperature.

4. The evaporation of the droplets and aerosols is coupled with the forward momentum of the jet and with sedimentation. The interplay of these governs the transmission distance of the aerosols and droplets and the fraction that remain airborne. A 100-µm droplet sediments 1 m in 3 s, a 1-μm particle takes 8 hours to sediment. Once generated by an expiratory event into an environment at, typically, lower RH and temperature, the droplets and aerosols lose moisture moving towards a state of equilibrium water content. This decrease in size occurs over a timescale less than 1 s for the smallest particles to greater than 10 s for the larger droplets, depending on RH and temperature.

Low level of certainty:

1. The hygroscopic response (moisture content with variation in RH) and evaporation kinetics of droplets and aerosols created from expiratory events, e.g. saliva, deep lung fluid, etc. Most models assume the aerosols/droplets behave as pure water or salt solution. While surrogate formulations for respiratory fluids have been reported, it is recognised that these may not reflect the true complexity of real exhaled particles.

2. The impact of phase behaviour (e.g. crystallization) and the slow release of water from viscous, partially dried particles on aerodynamic size, sedimentation rate and if the virus remains infectious.

3. The size distributions (number and mass) and compositions of aerosols and droplets expired by individuals with COVID 19 and differences to a healthy individual.

4. The interactions between virons and of other airborne pollutants (e.g. organics, oxidants) and effect on biological properties.

5. The impact of the particles’ evolving surface composition, bulk structure and physical properties on the deposition behaviour of respiratory droplets and aerosols on surfaces, i.e. do they bounce or stick. The evolution of cohesivity and their subsequent resuspension behaviour.

References:

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