Daily PM2.5 AQI
The daily PM2.5 AQI is calculated by taking the 24-hour concentration average from midnight to midnight (Local Standard Time) and converting to AQI using the PM2.5 cutpoints. 75%, or 18/24 hours of data are needed for a valid daily AQI calculation.
Example 1: Valid
Example 2: Insufficient Data for Calculation
Hourly PM2.5 AQI
NowCast AQI method
For real-time and other hourly readings, the NowCast method is used to calculate the AQI for each hour. In 2021, the NowCast also replaced the midpoint averaging method (described below) for hourly AQI from older hours. For information on how to calculate the NowCast for PM2.5, click here.
Midpoint AQI method (obsolete as of 2021)
Previously, if viewing/calculating hourly PM2.5 AQI for times >=6 hours old, the hourly AQI calculations would be performed using the midpoint 24-hour average concentration converted to AQI. For a given hour, this included the previous 12 hours, the given hour, and the following 11 hours. 18 of 24 hourly concentrations (75%) were needed for a midpoint 24-hour average.
24-hour average midpoint concentrations were truncated to the nearest .1 ug/m3
Example 1: No Missing Data (click image for larger view)
Example 2: Some Missing Data (click image for larger view)
