POLLUTION LOCATOR|EPA Comparisons of NATA Data with 1996 Monitoring Data

To evaluate the accuracy of the exposure modeling used by the National-Scale Air Toxics Assessment, EPA has compared predicted air quality estimates with currently available, but geographically limited ambient air monitoring data. A representative subset of seven HAPs (benzene, perchloroethylene, formaldehyde, acetaldehyde, cadmium, chromium and lead) was selected for EPA's evaluation. These pollutants were selected because they represent impacts by different combinations of mobile, area and point sources; include reactive and non-reactive compounds; and include those with both primary emissions and secondary formation in the atmosphere. They also include those HAPs with the largest number of monitoring sites.

In general, EPA has found that the modeled estimates for most of the pollutants examined are typically lower than the measured ambient annual average concentrations when evaluated at the exact location of the monitors. When the maximum modeled estimate for distances up to 10-20 km from the monitoring location are compared to the measured concentrations, the modeled estimates are closer to monitored concentrations. This result can be attributed, in part, to spatial uncertainty of the underlying emission and meteorological data, and the tendency of current air toxics monitoring networks to focus on the highest air pollution impact areas.

The following table displays the summary results of EPA's monitor to model data analyses:

Agreement of Model and Monitors by Pollutant, on a point-to-point basis.
Pollutant Number of Sites Median of Ratios Within Factor of 2 Within 30% Underestimated
Benzene 87 0.92 89% 59% 59%
Perchloroethylene 44 0.52 55% 32% 86%
Formaldehyde 32 0.65 53% 28% 88%
Acetaldehyde 32 0.60 59% 22% 91%
Lead 242 0.17 18% 10% 91%
Cadmium 20 0.18 15% 5% 85%
Chromium 36 0.15 28% 19% 83%

NATA model estimates are systematically lower than the monitor averages. Possible explanations for the systematic underestimation in NATA include:
1. The emission rates are systematically underestimated and/or many sources are missing from the emissions inventory.
2. Many of the monitors were likely sited to find peak concentrations. Often, the ambient concentration falls off quickly around the peak area. Even under the scenario of a "perfect" model and "perfect" monitors, if the monitor is situated right at the peak and the emissions or meteorological inputs are even slightly inaccurate, the model will tend to underestimate results. This is especially likely for pollutants dominated by point sources with elevated releases, because any errors in release height, exit velocity, and/or emissions location will likely cause the model to find a peak concentration area different from the true peak.

Note that for some of the pollutants in EPA's evaluation, there are very few monitoring sites across the country. The table below shows the number of monitors and the number of states covered for each pollutant.

Geographic Coverage of Monitoring Data, by Pollutant.
Pollutant Number of Sites Number of States
Benzene 87 16
Perchloroethylene 44 8
Formaldehyde 32 10
Acetaldehyde 32 10
Lead 242 28
Cadmium 20 7
Chromium 36 6

There are only 20 monitors for cadmium, 13 of which are in Illinois and New York. Acetaldehyde and formaldehyde have exactly the same set of 32 monitors, 28 of which are in the northeastern and Great Lakes states. Of the 36 chromium monitors, 32 are in California, New York, and Illinois. Further, all 10 chromium monitors in New York are in Staten Island. More monitor data is available for both benzene and lead. In general, the southeastern, northwestern, Great Plains, and Rocky Mountain states are very sparsely monitored. Conversely, the northeastern and mid-Atlantic states, Great Lakes states, California, Texas, and Louisiana are fairly well monitored.

For further information, see EPA's complete report on its Comparison of NATA Modeling System Results to Monitored Concentrations