Diffusion behavior of particulates

[chirhone]

How do the pm2.5 (particles of 2.5 microns or less) diffuse compared to gas. I noticed when i turned on my air purifier at night and turned it off at daytime. The pm2.5 particles levels go back to high after 2 hours even when my doors and windows are closed. Do pm2.5 particles pass through microscopic holes below doors like gas? How does the diffusion of particulates differ to that of gas?

 

[ChemAir]

How are you measuring your pm2.5 particulates?

 

[chirhone]

How are you measuring your pm2.5 particulates?

A Dylos, see https://smartairfilters.com/en/blog/how-accurate-are-common-particle-counters-comparison-test/

"Overall, the three particle counters were reasonably accurate compared to the government machines. They’re all suitable for giving an approximate indoor air pollution (AQI) reading in your home."

 

[jrmichler]

Your doors and windows may be closed, but your house is always leaking air. Air leaks out, and for every cubic foot that leaks out, an equal cubic foot leaks in. Older house leak a lot, new houses in cold climates have to meet codes that require less leakage. Good search terms to start with are home air changes per hour or home air leakage.

Since air is leaking in, particulates floating in that air are coming with it. The rate of PM2.5 rise is a measure of the leakiness of your house.

 

[Cutter Ketch]

I would think a more likely explanation for variations on the few hours scale would be the dust on the surfaces of the interior of your home. Even a clean home can have milligrams of dust per square meter. My house is probably closer to grams per square meter. A small fraction of this dust is constantly being kicked up by air currents and settling back down on surfaces. The surfaces act as a semi-infinite source of airborne particles produced at a semi-constant rate (proportional to the quantity of dust, but that will only change slowly through the process of filtering the air). The rate at which the dust settles is proportional to the concentration of airborne particles. The concentration in the air rises until the rate of settling out equals the rate of generation. When you add a filter the rate of removal is still proportional to the airborne concentration, but the proportionality is now much larger. The rate of generation equals the rate of removal at a much lower airborne concentration. When you turn the filter off the concentration quickly (and 2 hours is quickly) rises to the previous equilibrium.

Over a longer time the filter will reduce the total available dust, but your coming and going not to mention air exchange with the outside will also be increasing the amount of dust. How often you dust is another critical factor. So there is a longer term equilibrium being generated by exchange of dust with the outside world which is most of what was discussed above. However I think the two hour time scale you are describing is going to be about what is already inside the home.

 

[chirhone]

Your doors and windows may be closed, but your house is always leaking air. Air leaks out, and for every cubic foot that leaks out, an equal cubic foot leaks in. Older house leak a lot, new houses in cold climates have to meet codes that require less leakage. Good search terms to start with are home air changes per hour or home air leakage.

Since air is leaking in, particulates floating in that air are coming with it. The rate of PM2.5 rise is a measure of the leakiness of your house.

Do you know how high can pm2.5 pollution get? I leased on a 32th floor. And when the windows are opened. I am getting 10 times the reading compared to when the windows are closed and air purifier running at maximum.

 

[chirhone]

I would think a more likely explanation for variations on the few hours scale would be the dust on the surfaces of the interior of your home. Even a clean home can have milligrams of dust per square meter. My house is probably closer to grams per square meter. A small fraction of this dust is constantly being kicked up by air currents and settling back down on surfaces. The surfaces act as a semi-infinite source of airborne particles produced at a semi-constant rate (proportional to the quantity of dust, but that will only change slowly through the process of filtering the air). The rate at which the dust settles is proportional to the concentration of airborne particles. The concentration in the air rises until the rate of settling out equals the rate of generation. When you add a filter the rate of removal is still proportional to the airborne concentration, but the proportionality is now much larger. The rate of generation equals the rate of removal at a much lower airborne concentration. When you turn the filter off the concentration quickly (and 2 hours is quickly) rises to the previous equilibrium.

Over a longer time the filter will reduce the total available dust, but your coming and going not to mention air exchange with the outside will also be increasing the amount of dust. How often you dust is another critical factor. So there is a longer term equilibrium being generated by exchange of dust with the outside world which is most of what was discussed above. However I think the two hour time scale you are describing is going to be about what is already inside the home.

What is your comment about diffusion behavior of Pm2.5 particles. My leased condo at 32th floor has moderate amount of pm2.5 particles. This is because the condo building is at corner of busy intersection. I tested the unit at place 3.5 kilometers with less busy street or lack of street and the pm2.5 particles are very low. My question is why didnt the particles in the busy street diffuse to that of less busy or even lack of street. Its just 3.5 kilometers away. This is why i want to understand how pm2.5 particles diffuse.

 

[Tom.G]

Just thought you would be interested in a world map of air pollution here:
http://berkeleyearth.org/air-pollution-overview/