Air Quality Index (AQI)  Calculation and communication 


Student Worksheet

How is the air quality index calculated and communicated to the public? 
Procedure
1. Obtain copies of the AQI Equation (.pdf) and Breakpoints Table (.pdf).
2. Review the following examples to learn how to convert ozone ppm readings into an AQI value:
A. 8hour ozone example:
How is the air quality index (AQI) determined for an 8hour ozone concentration of 0.0874125 ppm?
First, round off the 0.0874125 ppm concentration reading to 0.087 ppm.
Second, look in the Breakpoints Table (.pdf) under the 8hour ozone column for the range of concentrations that contain this concentration (0.085  0.104 ppm).
Follow the table straight across the page to see the corresponding AQI values, in this case 101  150, which is considered "unhealthy for sensitive groups". You now have all the numbers you need to use the AQI equation:


Where IP = the index for pollutant P
CP = the rounded concentration of pollutant P
BPHi = the breakpoint that is greater than or equal to CP
BPLo = the breakpoint that is less than or equal to CP
IHi = the AQI value corresponding to BPHi
ILo = the AQI value corresponding to BPLo

In this case:
Where IP = the index for pollutant P = Ozone
CP = the rounded concentration of pollutant P = 0.087 ppm
BPHi = the breakpoint that is greater than or equal to CP = 0.104
BPLo = the breakpoint that is less than or equal to CP = 0.085
IHi = the AQI value corresponding to BPHi = 150
ILo = the AQI value corresponding to BPLo = 101
So, an 8hour ozone concentration of 0.0874125 ppm corresponds to an AQI value of 106. An AQI value of 106 is considered to be in the category of "unhealthy for sensitive groups" and is represented by the color orange. The Air Quality Index (AQI) was developed to easily communicate scientific information to the public.
B. Multiple pollutants example:
How is the air quality index (AQI) determined for multiple pollutants, an 8hour ozone value of 0.077 ppm, a PM2.5 value of 54.4ug/m3, and a CO value of 8.4 ppm?
Follow the equation three times, calculating the ozone value, the PM2.5 value and the CO value. The pollutant with the highest value, the "responsible pollutant", will determine the AQI value and category.

For Ozone (O3): 







For Particulate Matter (PM2.5): 







For Carbon Monoxide (CO): 





For this example, the AQI value is 128, with the "responsible pollutant" PM2.5. The AQI category would be "unhealthy for sensitive groups", represented by the color orange.
Assessment
3. Complete the problems on the Student Worksheet.
Extension
Use the AQI calculator to back calculate the concentration of a pollutant.