What is the Conroy Method for Averaging PM 2.5 Hourly Values?

It might seem that the only obvious way to get a 24-hr average is to just take the average of the most recent 24 hourly values. This is certainly the most recent measured value, but it is also more about how the air was than about how it is, so we looked for another method.

Many experts in the field believe that the best 24-hr average for the AQI at hour "X" is an average of the 1-hr measurement for that hour with the previous 11 1-hr averages and the following 12 hours. In other words, the 24-hr average is centered about the current hour. We agree, but that raises a new problem concerning the average for the current hour. We don't know the values of the 12 hours following the current one! They are in the future!

Several methods have been proposed for getting around this problem. The one that was recommended at the 2004 National Air Quality Conference is called the "Conroy" method, and we propose to adopt it.

In the Conroy or "Mid-point 24-hour Average" method, the 24-hr average for hour "X" is defined as the the most recent 12-hr average combined with the most recent 4-hr average (which is weighted more heavily). The use of the weighted 4-hr average causes the computed concentration to be responsive to hourly changes.

The formula is mid-point average = (12 X (12-hr average) + 12 X (4-hr average)) / 24

Thus the Conroy method is sort of an average of what was, combined with a simple assumption (forecast) about what is coming.

FYI, the official PM 2.5 average for regulatory purposes is just the midnight-to-midnight average, which doesn't help much if you want to know what the air quality is now!