Additional Information On
AQI Changes - April 2008

• How do the AQI ranges correspond to ozone and fine particle (PM2.5) concentrations?
• Why are the AQI calculations for ozone and fine particles being changed now (April 2008)?
• Will there be times when an advisory is issued but the AQI is still in the moderate range?
• Will these AQIs match what is posted on EPA’s AirNow website?

How do the AQI ranges correspond to ozone and fine particle (PM2.5) concentrations?

The ozone AQI is calculated from 8-hour rolling average concentrations. The PM2.5 AQI is calculated using the 24-hour rolling average concentrations from the continuous monitors. The table below identifies the concentration range for each AQI level. Although this table shows concentration ranges through the “Very Unhealthy” level, we do not expect ozone or fine particle concentrations to reach these concentrations.

Why are the AQI calculations for ozone and fine particles being changed now?

Ozone
When EPA changed the ozone standard, the federal rule included a requirement to change the AQI concentration ranges. Agencies that report AQI were encouraged to time the change so that the data from the entire monitoring season are calculated using the revised ranges. In Wisconsin, the official monitoring season begins on April 15. Monitoring sites begin operating in advance of the April 15 season so April 1 was chosen as an implementation date.

Fine Particles
The federal regulation that lowered the air quality standard for fine particles did not address the update to the AQI. At that time, EPA expected that they would have a separate proposal out for public comment within a couple of months. In February 2007, EPA shared the concentration ranges under consideration; however, the proposed rule has been delayed. Agencies were encouraged to use the revised standard in their forecasting activities. During 2007, DNR used the revised standard in their decisions about issuing watches or advisories. This led to questions and confusion about why an advisory was issued when the AQI stayed in the moderate range. Because DNR has to change the AQI ranges for ozone now, we decided to revise them for fine particles at the same time. Communications about current air quality presented on the web and on our Air Quality Hotline will be more consistent with the forecasted air quality. Watches and advisories are posted on the web map and soon the voice message on the hotline will include specifics about watches and advisories too.

Will there be times when an advisory is issued but the AQI is still in the moderate range?

Yes. DNR Forecasters attempt to identify all of the counties likely to exceed the air quality standard and issue a single advisory rather than issuing several advisories during the day. Forecasters consider weather conditions, hourly pollution concentrations, and the rolling averages in their decision-making. Typically, at least one monitor will have exceeded the standard and pollution levels are rising at other monitors; however, there may be times when an advisory is called in advance of AQI values reaching the orange level. In “close calls” decisions tend to err on the side of protecting public health.

Will these AQIs match what is posted on EPA’s AirNow website?

At times, the AQIs on DNR’s web map and EPA’s AirNow website will be different. This is for two reasons: AirNow uses different formulas for calculating AQI for both ozone and fine particles and EPA has not updated the AQI concentration ranges for fine particles. DNR uses the official EPA formula for determining AQI while AirNow uses a calculation that estimates the AQI. The AirNow website provides details about the formula used [exit DNR].

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