Surrogate Method

The health based standards for ozone and fine particles are based
on 8- and 24-hour average concentrations, respectively. During
rapidly changing air quality conditions, the use of these
averaging times creates a lag in the AQI; underestimating
pollution levels on the front end of a poor air quality event and
overestimating levels on the tail end. To better reflect
real-time conditions, the surrogate method places greater weight
on the most recent hourly data when calculating the AQI. See
EPA's AIRNow FAQ for more information, <a
href="http://www.airnow.gov/index.cfm?action=airnow.faq">EPA Air
Now</a>