This post is intended to be a brief explanation of the Chinese air pollution index, API. This post will not include comparisons to other international systems; I will write about that sometime in the next couple of days.
The API is a single number indicating the air quality on a given day. The API goes from 0 to 500. The higher the number, the worse the air quality is on that day. China assigns an overall air quality to different ranges of APIs, as shown in this image:
(Note: the English translations here may be under revision.)
A "Blue Sky Day" is defined as a day for which the API is 100 or below; in other words, a day in which the air quality is either "excellent" or "good" according to Chinese designations.
The API is determined from only three pollutants: SO2, NO2, and PM10. The concentration of each pollutant is measured at various stations throughout the city over a 24-hour period (noon to noon). The average daily concentration of each pollutant is then converted to a normalized index using the following table:
Linear interpolation is performed between each set of points to determine an API for each pollutant from that pollutant's concentration.
Once normalized, there are technically three APIs, one for each pollutant. (Note: Individual pollutant APIs for each measuring station are reported daily by the Beijing EPB on the Olympic air quality website.) Beijing's single API reported daily by MEP is the simply the highest of the three averages. In Beijing, the highest of the three is almost always PM10.
To convert from API to PM10 concentration (assuming PM10 is the highest on that day), use the following formulas:
For API 0-51: PM10 concentration = API/1000
For API 51-200: PM10 concentration = (API - 25)/500
For API 201-300: PM10 concentration = (API + 300)/1429
For API 301-400: PM10 concentration = (API + 225)/1250
For API 401-500: PM10 concentration = (API + 100)/1000
Again, for these formulas to be applicable the reported API must be the API for PM10.