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Demystifying Air Quality Numbers

Demystifying Air Quality Numbers

There are lots of numbers used to describe air quality these days in China. This post is intended as a glossary to demystify the different numbers currently used by various sources.

Concentration
The most direct way to report air pollution would be the concentration of a given pollutant in the air. This is straight-forward science and measurement. Example: PM2.5 concentration is 950 micrograms per cubic meter (ug/m^3).

Although concentration is straightforward scientifically, there are a couple of problems with it from a public outreach perspective. First, not everyone understands what concentration is, or has context for what the numbers mean. Second, it’s difficult to compare different pollutants to each other, since each pollutant affects people differently depending on concentration. Therefore, concentrations are commonly normalized to some scale, on which multiple pollutants can be compared together and a single “score” covering multiple pollutants given to a city.

Confusingly, there are currently two such scales used to describe Chinese air quality: the US Air Quality Index (AQI) and the current Chinese Air Quality Index (AQI). Here are the technical details of each one:

US AQI
In the United States, concentrations are converted to the “Air Quality Index” using breakpoints defined in 40 CFR 58 Subpart G. (Edition: April 4, 2013)

Note: the US does not define an AQI above 500. On airnow.gov, if you use the AQI->concentration calculator and input an AQI above 500, it gives an error message.

This means that AQIs reported by the Embassy above 500 are not in accordance with US regulations. My guess is that they linearly projected using the same slope as the 400 –> 500 line.

China AQI
In February 2012, China defined a new air quality index that includes PM2.5 and ozone. It doesn’t take effect nationwide until 2016, but Beijing and many other cities have already adopted it. The China AQI breakpoints are here:

Note that the Chinese system also does not define an index above 500. This is why the data reported by the BMEMC rail at 500 for AQI. I don’t think they project as the Embassy does.

Let’s do a quick comparison of the PM2.5 cutpoints in the US and in China:

Observations
1) The US is more strict at low concentrations.
2) The systems are identical above a concentration of 150 (AQI of 200).

3) Neither system is linear, which is annoying and non-intuitive.
4) It is also very annoying that the numbers are so close (as opposed to a 1-10 index, for example). This means it is very easy to confuse AQI and concentration.

Summary
Concentration is the most accurate way of describing air pollution, but isn’t good for public awareness and comparing multiple pollutants. Both the US and China use AQI systems. Both systems go from 0-500, and are not technically defined above 500. The US and Chinese systems are identical above an index value of 200 (PM2.5 concentration of 150), but slightly different below this level.
 Because the systems aren’t identical and have different slopes, you have to be very careful when saying something like “PM2.5 is 150.” The meaning of this statement is different depending on if you mean concentration OR US AQI OR Chinese AQI.