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?
Ozone Maps
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?
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 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.
How are particle pollution maps calculated?
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.
4. Calculate the "Adjusted" 4-hour average, which is the
average of the 4 most recent "Adjusted" hourly values.
5. Estimate the Mid-24 as:
- (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.
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