Demystifying the Air Quality Index (AQI)

Here is the real time (hourly) air quality information published by the Environmental Protection Bureau in my hometown Taiyuan (local time: 02:00, 25/10/2015):

Source: http://www.tyshbj.com.cn/hbj/shishi/index.asp

I have pointed the mouse to Wucheng, the nearest monitoring station to my home. This opens a drop-down list in which the concentrations of individual pollutants are shown. These are sulphur dioxide (SO₂), nitrogen dioxide (NO₂), particulate matter between 2.5 and 10 micrometers in diameter (PM₁₀), particulate matter less than 2.5 micrometers in diameter (PM_2.5), carbon monoxide (CO), and ozone (O₃). All pollutants are given in 1-hour average concentration except for ozone which is also given in maximum 8-hour average, as well as PM₁₀ and PM_2.5 which are also given in 24-hour average. In the right side panel, the air quality index (AQI) of 48 is displayed on a green background. Beneath it, there is information about the location of the site, (air quality) index category and (air quality) index level, primary pollutant, health implications as well as recommended actions. So what is the difference between “air quality index category” and “air quality index level”? Why is the column “primary pollutant” blank? And how is AQI calculated?

To answer all these questions, I checked the Ambient Air Quality Standards (GB3095-2012) and the supporting TechnicalRegulation on the Ambient Air Quality Index (on trial) (HJ 633-2012). You might have guessed from the serial numbers – both of them were released by the Ministry of Environmental Protection (MEP) in 2012 and will be fully implemented in 2016. The current Ambient Air Quality Standards (GB 3095-1996) (revised in 2000) will henceforth be abolished.

The following table presents the current nationwide standards:

Source: http://transportpolicy.net/index.php?title=China:_Air_Quality_Standards

These two tables present the future standards, which had already been adopted by 74 major cities including Taiyuan in 2012:

Source: http://transportpolicy.net/index.php?title=China:_Air_Quality_Standards

Major amendments are:

1)       The 3^rd class of limit values is removed, which has been applied specifically to           industrial areas. The new standards are consisted of two classes of limit values only, 1^st for protected areas and 2^nd for the rest: residential, mixed and industrial areas.

2)       Limit values for PM_2.5 are introduced for the first time as well as the limit value for the maximum 8-hour average concentration of ozone.

3)       Limit values for PM₁₀, NO₂, lead (Pb) and Benzopyrene (BaP) are reduced (note the change of unit !), i.e. stricter.

There are also new regulations on the validity of statistics, which are not presented in these tables.

Along with the new ambient air quality standards, AQI is introduced to replace the former air pollution index (API). To calculate the AQI, an individual score (IAQI) is first calculated for each pollutant using the following equation:

where:

IAQI_p = individual air quality index for pollutant P

C_p = concentration of pollutant P

BP_Hi = concentration breakpoint ≥ C_p

BP_Lo = concentration breakpoint ≤ C_p

IAQI_Hi = air quality index breakpoint corresponding to BP_Hi

IAQI_Lo = air quality index breakpoint corresponding to BP_Lo

The breakpoints are set as follows:

Source: Technical Regulation on the Ambient Air Quality Index (on trial) (HJ 633-2012)

For example, given the real time PM₁₀ concentration of 48 μg/m³ at Wucheng station, its IAQI is:

Then, the total AQI is calculated using the following formula:

which simply means the highest of all IAQIs. The pollutant with the highest IAQI is referred to as the primary pollutant, when its IAQI is greater than 50. In this example, the AQI is 48 and though the primary pollutant is PM₁₀, it is not stated as such, for its IAQI is slightly below 50.

AQI is then classified as follows:

Source: https://en.wikipedia.org/wiki/Air_quality_index#cite_note-sepa.gov.cn-16

I quote this simplified table from Wikipedia, because I cannot find a full translation of the original in Chinese. Note that the “air pollution level” here should be “air quality index category” if translated word-to-word, and it really is just a description for the “air quality index level”, with category “excellent” corresponding to level I, “good” corresponding to II and so on. In the original table in HJ 633-2012, there is another column of “recommended actions” corresponding to the “health implications”.

At this point, I have to emphasize that AQI is a relative and country-specific value. Different countries have different concentration breakpoints corresponding to different air quality standards, though the function used for calculation is the same. What I have presented and explained above is the AQI used by the Chinese government, and an AQI of 48 in China does not necessarily equals to an AQI of 48 elsewhere.

For instance, let’s take a look at the AQI developed by the United States Environmental Protection Agency:

Source: http://www.airnow.gov/index.cfm?action=aqibasics.aqi

At the first sight, it appears to be identical with the one showed further above, because the color coding is the same. The descriptions for the air quality conditions are, however, quite different. While I cannot claim that with the levels III to VI, the description for level II “good” used in China is pretty misleading, because it definitely sounds more positive than the description “moderate” used in the US (and i doubt this is done unintentionally).

This table shows the US standards for PM_2.5 concentration:

Source: The National Ambient Air Quality Standards for Particle Pollution - 

Revised Air Quality Standards For Particle Pollution And Updates To The Air Quality Index (AQI)

Again, using the data presented at the beginning, a PM_2.5 concentration of 25 μg/m³ is converted to an IAQI of 35.7 in China, whereas according to the US standards, the corresponding IAQI is 78. If PM_2.5 were the primary pollutant, the air quality would be described as “excellent” and as “moderate” respectively.

In summary, AQIs are not directly transferable between countries. As a matter of fact, China’s ambient air quality standards are mostly weaker than that of the US, the EU and the WHO guidelines. Besides, AQI uses a discontinuous piecewise linear function with sudden jumps between line segments (here: AQI levels). Therefore, even though it provides an intuitive way for communicating the air quality to the public, we should still rely on absolute, observed values.