Daily SO2 concentrations are based on the 24-hr average. 75%, or 18/24 hours of data are needed for a valid daily concentration. Please note that AirNow does not currently report daily AQI for SO2. If needed, see Page 16 of the EPA’s AQI Technical Assistance Document for details on calculating SO2 AQI.
AirNow does not currently report daily concentrations or AQI for NO2.
For NO2 and SO2, the hourly AQI scale is based on 1-hour averages. Raw hourly concentrations are used in a direct conversion to AQI. See AirNow.gov AQI Conversions for an easy-to-use concentration to AQI calculator.