There is a fantastic piece in Wired discussing the understanding of aerosols and the transmission of the corona virus. It notes:
On the video call, tensions rose. At one point, Lidia Morawska, a revered atmospheric physicist who had arranged the meeting, tried to explain how far infectious particles of different sizes could potentially travel. One of the WHO experts abruptly cut her off, telling her she was wrong, Marr recalls. His rudeness shocked her. “You just don’t argue with Lidia about physics,” she says. Morawska had spent more than two decades advising a different branch of the WHO on the impacts of air pollution. When it came to flecks of soot and ash belched out by smokestacks and tailpipes, the organization readily accepted the physics she was describing—that particles of many sizes can hang aloft, travel far, and be inhaled. Now, though, the WHO’s advisers seemed to be saying those same laws didn’t apply to virus-laced respiratory particles. To them, the word airborne only applied to particles smaller than 5 microns. Trapped in their group-specific jargon, the two camps on Zoom literally couldn’t understand one another.
The real issue is that one must define an aerosol. There is a classic book by Hinds, Aerosol Technology (Wiley, 1982) worth reading. In the introduction the author notes:
The microscopic particles that float in the air are of many kinds; suspended solid particles, smoke from power generation, photochemically formed particles, salt particles formed from ocean spray, and atmospheric clouds of water droplets or ice particles....These airborne particles are all examples of aerosols.
The author continues to give examples and counter examples. But in the Wired piece one sees that the corona virus is carrier from person to person by aerosols. The questions are:
1. What is the structure of that aerosol?
2. What are its dimensions?
3. What are its aerodynamics?
4. How does it go from one person to another?
5. How is it generated in an infected person?
6. What are the dynamics of aerosol bursts?
And the list goes on. The article does not answer any of these questions. What we do know is that the virion is about 30 nm in diameter and that it may be transported in an aerosol of water about 100 nm in diameter. These small aerosols may collide and get larger or smaller, they may gather as a quasi cloud, they may do lots of things.
But we know that a particle such as this will fall according to Newtons law of gravity with a force of mg, mass times the gravitation pull. Yet against that may be a hydrostatic force due to a differing air temperature in the aerosol and drag due to the viscosity of air. This is the grossly simplistic model.
F(viscosity)=3πμVd
where we have the viscosity, velocity and diameter. In a simple single particle case the terminal velocity, the settling velocity is:
VTS=(αd2g)/(18μ)
where we first see the density of the particle. Simplified we have for small particles:
VTS=0.003d2 (cm/sec) d in microns
Now this is quite slow. One must realize the particle has a horizontal component that starts high and it too gets to a terminal state. Thus small particles can get stuck in the viscous air.
Now the fun begins.
1. The particles have lots of sizes.
2. Brownian motion occurs
3. The aerosols collide, get bigger or smaller or evaporate totally
4. There are electrostatic effects
5. and so forth!
The article concludes:
In early May, the CDC made similar changes to its Covid-19 guidance, now placing the inhalation of aerosols at the top of its list of how the disease spreads. Again though, no news conference, no press release.
The problem is that this is a really hard problem. At best we can say that "aerosols", whatever they are, spread the virus. We can also say that the CDC and WHO have not only been clueless but reckless in their approaches. We can make models, but if this pandemic has taught us anything it is that models have been not only useless but counter-productive. We are still at the stage of defining the issue. That takes measurements, data flow, monitoring, and the things we have been doing for a century.
Regrettably we have seen the worst of politics and "science" in the past year, and its intensification most recently. It is clear to me that the CDC has lost any credibility. Yet we need researchers who can tackle the issue. The Wired piece shows one such alternative. I am sure there are many more.
Also I believe, and this is a belief, that aerosols of the corona virus do linger, but the question is; for how long are they viable. RNA is very fragile, it decays and breaks down, especially single stranded. The aerosol can protect it but for how long. That is just another of the possibly hundreds of questions. We need data, not equations. They may come later, possibly.
Posts
