Frequently Asked Questions

What does "Not currently forecasting" mean?

Air Quality forecasts are provided by state and local agencies. Many areas produce air quality forecasts only during ozone "season." The typical ozone season is May to September, and many areas stop daily air quality forecasting September 30 and begin again on May 1. Forecasts are still available for areas where other pollutants are forecast or where the ozone "season" is longer.

Why is a map not available?

The first ozone map of the day is usually available by 9:00 a.m. EST during ozone "season." The typical ozone season is May to September, and many areas stop providing data for maps after September 30 and begin again on May 1. Sometimes maps are not available because of data collection and transmission problems. When these problems occur they are corrected as soon as possible.

Particle Pollution Maps

Particle Pollution maps are available year-round for many cities. However, some state and local air pollution agencies are not yet participating in this program. Sometimes maps are not available because of data collection and transmission problems. When these problems occur they are corrected as soon as possible.

How are AQI 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 available; the previous 4 hours and the current hour. This means that the 8-hr concentration needs to be estimated.

To estimate the 8-hr average based on the current 1-hr concentration, linear regression was performed on historical data to develop an equation for the relationship between the 1-hr and 8-hour concentrations. The relationship is explained by the following equation:

where:

value = 8-hr surrogate ozone concentration
a = multiplier used for 1-hr ozone value "x"
b = offset

Every monitoring site in the AIRNow system has a 1-hr to 8-hr surrogate relationship. On average, the 8-hr average concentrations tend to be about 85% of the 1-hr concentrations.

The AQI for particle pollution was developed for assessing air quality conditions over a 24-hour period. To assess air quality conditions at a given time using the AQI, one would ideally use the average particle pollution measurement over a 24-hour window centered about the hour being measured (i.e., mid point of the 24 hour range or Mid-24) to compute the AQI. The issue, however, in protecting public health via the AQI for particles is that twelve hours of future data are not available. Therefore a surrogate or estimation method was developed which uses a combination of hourly particle concentrations from previous hours to estimate the Mid-24 average.

This surrogate method combines both the 4-hour average and the 12-hour average in the following manner:

1.  Calculate the average of the previous 12 hours.

2.  Calculate the ratio of the most recent hour to the average of the previous 12 hours.

3.  Calculate an "Adjusted" hourly value:

• The adjusted hourly value is equal to the actual hourly value, if the actual hourly value is < 30 ug/m3.
  
  
• The adjusted hourly value is equal to the actual hourly value, if the actual hourly value is > 30 ug/m3 and the ratio of most recent hourly value to the average of most recent 12 hourly values is < 0.9 or > 1.7.
  
  
• Otherwise, the adjusted hourly value is equal to 0.75 times the actual hourly value.

• (12*(12-hour average) + 12*(4-hour adjusted average))/24.

In summary the surrogate method is the average of the 12-hour average and an adjusted 4-hour average for particles.

These estimated concentrations are used until 18 out of 24 values or more are available for calculating a "real" mid-point 24-hour average for each hour of the day.

How to order EPA Publications

You can order these publications directly via EPA's National Service Center for Environmental Publications (NSCEP) (http://www.epa.gov/ncepihom/)web site. Your publication requests can also be mailed, called or faxed directly to:

U.S. Environmental Protection Agency
National Center for Environmental Publications (NSCEP)
P.O. Box 42419
Cincinnati, OH 42419
1-800-490-9198/(513) 489-8695 (fax)

Please use the EPA Document Number, which is usually bolded or highlighted, when ordering from NSCEP.