QQ20160707-2

Are Particle Counters and Government Machines the Same?

对不起,此内容只适用于美式英文。 For the sake of viewer convenience, the content is shown below in the alternative language. You may click the link to switch the active language.

My particle counter is a beast–I’ve loved it. But I’ve always wondered how the counts of laser particle counters like mine:

11

…compare to the measurements of the huge stationary air quality monitoring stations that governments use, like this one in New Zealand:

2

If you look at the US Embassy’s Twitter feed, you’ll notice that its raw numbers are “concentration,” which it explains are micograms per cubic meter (µg/m³). The way government (BAM) machines work is that they use a source of carbon 14 that emits beta particles and then measure how many of those beta particles make it through to a detector. They then use those numbers to estimate the weight of those particles (micrograms).

In contrast, laser particle counters like mine use a laser and a photo diode sensor to estimate the number of particles in the air. I don’t see why the weight should be any more important than the number–they’re both telling you how much particulate pollution is in the air.

As an analogy, if we want to understand the crowd at a basketball game, we could count the number of people, or we could weigh those people. Of course, the more people, the heavier the total weight. And of course the two numbers won’t correlate perfectly if we have more heavy people on some days and more children on other days. But the weight and the total number should correlate highly.

The other major difference is that laser particle counters give the number of particles at that particle size and above. Government machines give the number of particles at that size and below.

To see how the two numbers compare, I put my particle counter outside my second-story window 70 times (that’s nerd dedication!) and compared my numbers to the US Embassy’s Twitter feed at the same time. Here’s what they look like:

3

They correlate at = .89, meaning the two numbers are very strongly related (remember the highest possible correlation is 1). That high correlation is especially impressive given that my house is near Gulou, and the US Embassy is out in Liangmaqiao–about 7 kilometers away.

The difference between the readings was particularly noticeable on days where a strong wind moved through Beijing. I noticed several times that my particle counts would drop before the embassy’s counts as the wind moved in from the west (where my house was) to the east (where the embassy is). (Remember, Beijing’s air gets a lot cleaner when we get winds from the west.)

If we want to get a rough conversion between the numbers, we can remove a few of the outliers and compute a regression line:

4

For example, a government concentration of 100 micrograms (four times the WHO limit!) is approximately 25,000 on the Dylos particle counter:

5

And the 24-hour WHO standard of 25 µg/m³ is about 3,000-4,000 on the Dylos.

During my home tests (before turning my filter on), the air inside my home was very often above 3,000 (even though it was still much cleaner than outside).

Conclusion: My particle counter is giving measurements that are highly related to the much larger air monitoring stations. The scale is different, but the two can be roughly converted.