Page 80 - Fundamentals of Air Pollution

V. Air Quality Levels 53

B. Display of Air Quality Data
A very useful format in which to display air quality data for analysis is
that of Fig. 4-8, which has as its abscissa averaging time expressed in two
different time units and, as its ordinate, concentration of the pollutant at
the receptor. This type of chart is called an arrowhead chart and includes
enough information to characterize fully the variability of concentration at
the receptor.
To understand the meaning of the information given, let us concentrate
on the data for 1-hr averaging time. In the course of a year there will be
8760 such values, one for each hour. If all 8760 are arrayed in decreasing
value, there will be one maximum value and one minimum value. (For
some pollutants the minimum value is indefinite if it is below the minimum
detectable value of the analytical method or instrument employed.) In this
array, the value 2628 from the maximum will be the value for which 30%
of all values are greater and 70% are lower. Similarly, the value 876 from
the maximum will be the one for which 10% of all values are greater and
90% are lower. The 1% value will be between the 87th and 88th values
from the maximum, and the 0.1% value will lie between the 8th and 9th
values in the array.
The 50% value, which is called the median value, is not necessarily the
same as the average value, which is also called the arithmetic mean value.
The arithmetic average value is obtained by adding all 8760 values and
then dividing the total by 8760. The arithmetic average value obtained for
other averaging times, e.g., by adding all 365 24-hr values and dividing

Fig. 4-8. Sulfur dioxide concentration versus averaging time and frequency for 1980 at
U.S. National Aerometric Data Bank (NADB) Site 264280007 HOI, 8227 S. Broadway, St.
Louis, Missouri. Source: Chart courtesy of Dr. Ralph Larsen, U.S. Environmental Protection
Agency, Research Triangle Park, NC; see also Fig. 19-13.