3.0 URBAN EXCESS METHODOLOGY

Urban PM2.5 originates from local and regional (upwind) emission sources. This chapter

describes the concept of urban excess, a measure of the local contribution to urban PM2.5. These

contributions can be characterized according to the major chemical components of PM2.5

(sulfates, nitrates, carbon and crustal matter) and differentiated according to their local and

regional contributions. EPA used urban excess numbers to estimate the relative contributions of

different pollutant emissions within a given nonattainment area. The application of the urban

excess methodology to develop a “weighted emissions score” for each county assessed in the

designations process is described in detail in Chapter 4 of this Technical Support Document.

This chapter introduces the concept of urban excess, discusses uncertainties, and describes the

associated site selection process.

3.1 Concept

Both local and regional sources contribute to particle pollution. Figure 3.1 shows how much of

the PM2.5 mass can be generally attributed to local versus regional sources for 13 selected urban

areas arranged west to east. In each of these urban areas, a monitoring site was paired with a

nearby rural site. The data were derived from the national speciation monitoring networks, the

speciation trends network (STN) and the IMPROVE (Interagency Monitoring of Protected

Visual Environments) network. Local and regional contributions to PM mass can be estimated

by subtracting the rural concentration from the measured urban concentration.

Assuming that the rural concentrations represent the regional background concentration, this

difference for each species was defined as urban excess:

Urban Excess = Urban Concentration (from urban EPA network) – Regional Background (from

rural IMPROVE network) (Equation 1)

In the East, regional pollution generally contributes more than half of total PM2.5

concentrations. Rural background PM2.5 concentrations are high in the East and are somewhat

uniform over large geographic areas. These regional concentrations come from emission sources

such as power plants, urban pollution and natural sources, and can be transported hundreds of

miles.
Figure 3.1. Urban excess PM2.5 for 13 Cities

As an example, and as used in EPA’s recently released Particle Pollution Report1, the regional and

local contribution to PM2.5 mass for 13 cities are shown above using data for the year 2002. For the

selected cities shown in Figure 3.1, local contributions range from 2 to 20 μg/m3, with the West

generally showing larger local contributions than the East. In the East, local contributions are

greatest in cities with the highest annual average PM2.5 concentrations. For these illustrations, the

PM2.5 concentration is not necessarily the maximum concentration for each urban area.

Rural PM2.5 concentrations do not vary as significantly as urban concentrations. Accordingly,

urban excess concentrations typically do not change significantly when an urban site is paired

with multiple rural monitors. Rural concentrations of the major components of PM2.5 are

spatially homogenous. The regional pattern of annual average concentrations of sulfates, nitrates

and carbonaceous mass for 2002 are illustrated below.

 1

 U.S. EPA, The Particle Pollution Report, EPA 454-R-04-002, EPA Office of Air Quality Planning and Standards,

Research Triangle Park, NC, December 2004.
Figure 3.2 Rural sulfate concentrations.

Figure 3.3. Rural nitrate concentrations.
Figure 3.4. Rural total carbonaceous mass concentrations.

The spatial patterns of rural concentrations of the specific PM2.5 components result from the

geographic distribution of contributing sources, atmospheric formation of secondary aerosols,

and transport. Sulfate aerosols are formed in the atmosphere after the oxidation of sulfur

dioxide. The chemistry is sufficiently slow so that days are required for complete oxidation to

occur. During this time period, sufficient atmospheric mixing occurs such that the pollutant

generally appears well distributed regionally. As shown in Figure 3.5 (reference EPA Particle

Pollution Report, 2004) , rural and urban ambient monitors measure similar concentrations of

sulfate aerosols.

Figure 3.5 Local and regional contribution to the sulfate component of PM2.5
Carbon has the largest local contribution of the three major chemical components. Carbonaceous

mass is estimated from speciation measurements of organic and elemental carbon in combination

with a multiplier to account for the mass associated with oxygen, hydrogen, and other elements found

in carbon components found in fine particles. A multiplier value of 1.4 is used for the computation

of urban excess. Carbon particles associated with the urban excess originate from a combination of

mobile and stationary combustion sources (including power plants and other industrial facilities).

The regional contribution, which varies from 30% to 60% of the total carbon at urban locations, is

from rural emission sources such as vegetation and wildfires, as well as region-wide sources such as

cars and trucks.

Figure 3.6 Local and regional contribution to the carbonaceous component of PM2.5

Nitrates represent only about 10% to 30% of annual average PM2.5, and urban concentrations

are higher than the nearby regional levels. This is likely due to local NOx sources such as cars,

trucks, and small stationary combustion sources. The nitrates and sulfate constituent of PM2.5 are

represented as ammonium nitrates and ammonium sulfates. [The ammonium portion of particulate
matter is not separately considered. It originates from ammonia (from sources such as fertilizer and

animal feeding operations) which contribute to the formation of sulfates and nitrates]

Figure 3.7 Local and regional contribution to the carbonaceous component of PM2.5

3.2 Derivation of Urban Excess

For this exercise, the data sets used to calculate urban excess spanned the one year April 2002-

March 2003. The following steps were used to compute urban excess for this time period:

1. EPA’s speciation network (STN) was used to represent PM2.5 speciation data at urban

locations. The IMPROVE network was used to represent PM2.5 speciation data at rural

locations.

2. For the year in question, every location with monitoring data was checked for

completeness. Completeness consisted of checking whether there were a minimum of 11

valid observations of each of the major chemical species for each of the four quarters that

comprise the one year in question. Major species in the STN network include organic

carbon, elemental carbon, ammonium, crustal, sulfates, and nitrates Major species in the

rural IMPROVE network include organic carbon, elemental carbon, sulfate, crustal, and

nitrate. These criteria resulted in a total of 137 urban locations having complete data for

the year in question.

3. For each of the 137 urban sites that were complete, the nearest complete rural IMPROVE

site was identified using a geographic information system. These site pairs were then
used to compute the urban increment, component by component using Equation 1 above.

Note that this procedure often results in the same rural site being matched to various

different urban sites in close proximity to each other.

The annual average urban excess was calculated in this fashion for the major PM2.5 species:

sulfate, ammonium, nitrate, total carbon, and crustal matter. Equations used to convert measured

urban and rural speciation concentrations into estimated PM2.5 constituents and inter-network

differences and uncertainties are described elsewhere. [See the National Air Quality and

Emissions Trends Report, 2003 Special Studies Edition]. Computed urban excess concentrations

less than zero were set to zero. The complete list of matched sites and all urban excess estimates

are shown in table 1 below.
Table 1. Data Used in Calculating Urban Excess Percentages