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
For real-time readings and readings less than 6 hours old, the NowCast is used to calculate the AQI for each hour. For information on how to calculate the NowCast for PM2.5, click here.
If viewing/calculating hourly PM2.5 AQI for times >=6 hours old, the hourly AQI calculations should be done by using the midpoint 24-hour average concentration converted to AQI. For a given hour, this includes the previous 12 hours, the given hour, and the following 11 hours. 18 of 24 hourly concentrations (75%) are needed for a midpoint 24-hour average.
24-hour average mdpoint concentrations are 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)