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What is air mass concentration and what is its importance?

Update: 2021-07-28 10:02 Source: LUFTMY Reading: ↻ News
 

Simply put, "air mass concentration" is a way of representing particles in the air as masses in a fixed volume of air.For example, the World Health Organization's PM2.5 air quality guideline is 10μg/m3.That number is a mass concentration: 10 micrograms of PM2.5 dust particles per cubic meter of air.

 

The widespread use of air mass concentrations means that PM2.5 sensors need to provide readings in this format, even if it is not the format used to make the original measurement.This is where things get tricky.

What is air mass concentration and what is its importance?

How do you measure particles?

The vast majority of dust particle sensors available today, whether hundreds of dollars or 10,000 dollars, are optical particle sensors.These devices work by counting particles in the air.As air is drawn into the sensor, particles in the airflow pass through the laser beam, causing the light from the laser to be diffracted.The light will fall on a sensor placed perpendicular to the laser beam.Based on changes in the intensity of light received by the sensor, individual particles can be counted and their size determined.These individual particles are grouped according to their size (called bins).Particles between 0.3 and 0.5 microns may be placed in one container, while particles between 0.5 and 1.0 microns will be placed in another.

 

In the world of clean rooms, filter manufacturing, and certain industries, these particle counts are the final readings that end users are looking for.There are many international and ISO standards for hospital and cleanroom air quality that use particle counts rather than mass concentrations.For example, an operating room where brain surgery is performed is very demanding and might use particle counting.In this case, even a single particle in the air can have serious consequences.A microgram of PM2.5 can contain thousands of particles, so a mass concentration reading of 0μg/m3 may not tell the whole story.

 

So how do optical particle counters such as lasers produce accurate mass concentration readings?

 

At first glance, it seems relatively simple: Take the number of particles in the air (the original reading), assign them masses based on their size, and then add them all up to get the mass concentration.Simple, right?

 

The challenge here is that not all particles are created equal.Imagine two vats, one filled with 1,000 golf balls and the other with 1,000 ping-pong balls.For particle counters, they look very similar.There are 1,000 white balls, and they're all about the same size.The output of the particle counter will say "1,000 particles, all of the same size."But in trying to calculate the weight of those 1,000 particles, should a particle counter use the mass of a golf ball or a ping-pong ball?Whichever one is chosen, one of the buckets will end up being miscalculated.

 

Assumptions can be made about the particle's density and refractive index, both of which affect the mass concentration, which is what most particle counters do.Most consumer devices simply have a default correction factor built into the device that will always be used whether your particle is a golf ball or a ping-pong ball.Professional-grade devices often allow users to enter these details manually, but knowing the density of the particles you are measuring is not easy and usually requires sending particle samples to a laboratory for analysis.

 

In real life, particle density and optical properties can be heavily influenced by geography read more about this in our feature article.Beijing's particulates are likely to come mainly from coal burning, while London's particulates are likely to come mainly from vehicle emissions.These particles are very different, and using the wrong factors when calculating the mass concentration can result in very different results.

 

Also, in one location, the source of pollution may vary with the season (or even the time of day!)And change, resulting in incorrect results.Simply using the default factor for all devices means that the results are wrong more often than they are correct.

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