Air toxics concentrations for 1990 were estimated in the Cumulative
Exposure Project with the use of computer dispersion modeling, rather
than by measurement. The best way to assess the accuracy of the
modeled pollutant concentrations is to compare them with available
real-world measurements.
To evaluate the modeled 1990 concentrations, EPA gathered more
than 700 measurements of annual average air toxics concentrations for
1990, representing 19 air toxics in up to 81 locations each. At each
monitor location, the modeled air toxics concentration was compared
to the monitored concentration by calculating a model-monitor ratio
(modeled concentration divided by monitored concentration). This
analysis found that: 1) about two-thirds of the time, the modeled
concentration is within a factor of three of the monitored concentration
(i.e., the model-monitor ratio is between 0.33 and 3.0); 2) when the
modeled concentration and monitored concentration differ by more
than a factor of three, the model is almost always too low, rather than
too high; and 3) the model underestimates monitored concentrations
about three times as frequently as it overestimates.
Several state air pollution agencies have reached similar conclusions in
their own comparisons of the modeled concentrations to air toxics
monitoring data. Some of these states do not have air toxics monitoring
data for 1990, and therefore used measurements available for more
recent years. Examples of these analyses and their findings include:
- Vermont: The State of Vermont found general agreement
between modeled concentrations and concentrations measured
in 1993 and 1994 for 10 air toxics in 5 locations. Out of the 50
comparisons, there were 12 cases of significant underestimation
(model-monitor ratio less than 0.1), and one case of significant
overestimation (ratio greater than 10).
- New York: The Northeast States for Coordinated Air Use
Management (NESCAUM) analyzed monitoring data from the
State of New York and found that modeled benzene
concentrations were all within a factor of three of measured
concentrations (model-monitor ratios between 0.33 and 3.0) at
10 monitoring sites. Six years of monitoring data (1990-95)
were used in the analysis. The 10 monitoring sites are
characterized as industrial, light industrial, urban, suburban, and
rural. No clear tendency of underestimation or overestimation
was observed.
- Mid-Atlantic States: An analysis prepared for the
Mid-Atlantic Regional Air Management Association compared
the modeled air toxics concentrations to 1996 measurements of
up to eight air toxics at three monitors located in Philadelphia,
New Brunswick, and Baltimore. Model-monitor ratios ranged
from 0.66 to 3.0; sixteen out of the 21 comparisons had ratios
of less than 2.0. Some variability in performance across different
types of HAPs was noted.
It should be noted that these results represent comparisons for a
limited number of air toxics in a limited number of geographical areas.
While these results cannot necessarily be extrapolated to other air
toxics in other areas, they provide the best indication available of how
well the model results relate to real-world data. We recommend
directly comparing ambient monitoring data, if available, when
interpreting modeled outputs for a particular area.
Contact Daniel Axelrad at
[email protected] for more information about EPA's CEP.
Also, please visit their website (below).
Visit EPA's Cumulative Exposure Project website to learn more..
