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4.0 CONSTRUCTING THE WEIGHTED EMISSIONS SCORE

The emissions of PM2.5-related pollutants are an important factor considered in the

designations process. PM2.5 concentrations are formed through complex processes, with

contributions both from direct emissions and from multiple secondarily-formed

pollutants. In order to compare overall emissions among counties within a metropolitan

area, EPA developed a metric called the weighted emissions score. This chapter

discusses the methodology used to develop this metric

The first section presents the basis for the weighted emissions score metric. The second

section focuses on the calculation method for the Weighted Emissions Score (WES). The

WES is based on the urban excess (Chapter 3) and the countywide emissions. The final

section describes the sorting method used to determine the cumulative emissions scores

which were used to rank metropolitan area and nearby counties in relation to one another.

4.1 Basis and Assumptions

Nonattainment problems are caused by a combination of regional and local emissions. In

the designation process, EPA evaluated all the counties in a particular metropolitan area

(based on the 1999 and 2003 definitions), plus all counties adjacent to the metropolitan

area. For each metro area with a violating monitor, the emissions from this set of

counties were evaluated for their contribution to nearby, or “local,” PM2.5

concentrations. Because PM2.5 components such as sulfates and nitrates are formed

through atmospheric processes and can be transported many miles, sources of emissions

outside the set of counties comprised of the metropolitan area and adjacent counties were

considered to affect the regional concentration for a particular site.

For the purposes of developing a simplified emissions metric, only the pollutants SO2,

NOx, direct carbon, and direct crustal emissions were considered in the methodology.

Ammonia is recognized as a key pollutant in the formation of ammonium sulfate and

nitrate. However, for the purposes of developing this metric, it was assumed that

ammonia emissions were associated with the formation of ammonium sulfate (through

reactions with SO2) and ammonium nitrate (through reactions with NOx). Similarly,

some volatile organic compounds are recognized as precursors to secondary aerosol

formation, and others participate in the formation of ozone which is an important element

in the oxidation of sulfur and nitrogen oxides and related atmospheric chemistry

processes. However, because of the lack of speciated VOC inventories and the

uncertainty about what proportion of VOC emissions in a particular county might

participate in PM formation most directly, VOCs also were not included in the weighted

emissions score methodology. The county emissions used in this analysis were taken

from the 2001 National Emission Inventory, version 3.
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4.2 Calculating the Weighted Emissions Scores

Step 1. The counties to be analyzed in relation to each metro area were first identified.

These counties included the counties in the 1999 C/MSA, those included the 2003 metro

area definition, and any counties adjacent to those in either the 1999 or 2003 definitions.

Step 2. For each metropolitan area, the urban excess PM2.5 mass was calculated

according to the methodology described in chapter 3. According to this methodology, the

regional concentration is subtracted from the urban concentration for sulfates, nitrates,

direct carbon, and direct inorganic (or “crustal”) PM2.5. The resulting concentrations,

when added together, comprise the estimated urban excess PM2.5 for the area.

Step 3. The percentage that each PM2.5 component comprises of the total urban excess

mass is then calculated. These percentages varied from metro area to metro area, and

they served as a factor for “weighting” emissions of the pollutants associated with each

PM2.5 component.

Step 4. The next step involves calculating, for each pollutant, the percentage of CMSA

emissions attributable to each county.

Step 5. The county’s percentage of CMSA emissions for the four pollutants was then

multiplied by the corresponding PM2.5 component percentage of urban excess mass.

The calculation of the weighted emissions score is represented by the following formula:

[(County SO2 tons / CMSA SO2 tons) *

(% sulfate comprises of urban excess PM2.5)]

+ [(County NOx tons / CMSA NOx tons) *

(% nitrate comprises of urban excess PM2.5)]

+ [(County carbon tons / CMSA carbon tons) *

(% carbon comprises of urban excess PM2.5)]

+[(County crustal tons / CMSA crustal PM2.5 tons) *

(% crustal PM2.5 comprises of urban excess PM2.5)]

Step 6. This score was calculated for each county in the CMSA, and then the county

scores were sorted from highest to lowest. The sum of these CMSA county scores was

100.

 1 Myoseon Jang, N.M. Czoschke, S. Lee, and R.M. Kamens. “Heterogeneous Atmospheric Aerosol

Production by Acid Catalyzed Particle-Phase Reactions.” Science, Volume 298, 25 October 2002, pp. 874-

877.
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Step 7. The weighted emissions score was then calculated for the other counties

identified in step 1 that were outside of the 1999 metro area boundary. In the formula

above, the CMSA emissions totals for each pollutant were used as the denominator in the

equation. In this way, adjacent counties were compared with CMSA counties in a

consistent manner. These “adjacent” counties were then sorted from highest to lowest to

identify which of these counties had higher relative contributions to local PM2.5.

4.3. Comparing Weighted Emissions Scores Among Counties

The weighted emissions score was developed as a basic analytical tool used to compare

emissions across multiple counties in a metropolitan area. It should be regarded simply

as one way to assess multiple emissions all contributing to the “emissions” factor

identified in EPA guidance. In addition, the weighted emissions score was not given

more significance than the other factors in the analytical process. The assessment of

potential nonattainment area counties was based on all of the information available to the

Agency for all of the factors identified in EPA guidance. Final decisions on attainment

and nonattainment areas were based on the collective assessment of all of the nine

technical factors. EPA recognizes that there are particular uncertainties associated with

this metric. However, EPA believes that it serves as a useful tool for comparing county

emissions of multiple pollutants in a simplified way.