How are ozone maps calculated?
The Air Quality Index (AQI) for ozone is based on the 8-hour average ozone concentration, which is computed by averaging the measured hourly ozone concentrations over an 8-hr period. In real-time, eight hours of data are not available so a method was devised to estimate the 8-hr AQI from hourly ozone data using a mid-point approach. The midpoint average is calculated as follows. At a given hour X, the 8-hr average is calculated by averaging 1-hr data from hours X-4, X-3, X-2, X-1, X, X+1, X+2 and X+3. For example, the 8-hr average for 4 PM is the average value from 12 PM through 7 PM. 8-hr calculations work fine when all of the data are available, but fail when trying to compute an 8-hr average for the current hour in real-time. A valid 8-hr average can still be computed even when just six or seven hours of 1-hr data are available, but not for a fewer number of hours. For the midpoint calculation approach, this means that the 8-hr average for the current hour only has five data values
