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MEMORANDUM 

TO: Docket No. EPA-HQ-OAR-2012-0918 

Air Quality Designations for the 2012 PM2.5 Standards 

 

FROM: Neil H. Frank 

 Air Quality Assessment Division, OAQPS 

SUBJECT: Calculation of Urban Increments to Support the Air Quality Designations for the 

2012 PM2.5 Standards National Ambient Air Quality Standards (NAAQS) (SAN 

5706)

Introduction 

PM2.5 mass concentrations are generally higher in urban areas compared to surrounding regions. 

This “urban increment,” also known as the “urban excess,” is due to locally generated and 

largely directly-emitted PM2.5 in addition to regional contributions.1,2 Among the major 

contributors to PM2.5 mass, sulfates tend to originate from regional sources; organic carbon and 

nitrate originate from regional and local sources; and black/elemental carbon, associated soot, 

and crustal material tend to originate from local sources. Sulfate and nitrate also depend on the 

availability of ammonia, which may originate from regional or local emissions. 

The goal of the urban increment analysis3 is to estimate the local contribution to urban PM2.5 as 

measured at violating FRM/FEM/ARM monitoring sites and thereby provide additional evidence 

to consider in deciding which nearby areas contribute to a violating monitor. The urban 

increment analysis can help isolate and better explain the contributions from urban and nearmonitor emissions, separate from the regional background contributions. 

An urban increment analysis is based on the premise that so-called “rural” contributions of PM2.5

concentrations typically result from a regional distribution of contributing sources caused by 

atmospheric formation and transport of secondary aerosols.4 These are often called regional 

sources and can include a small number or large collection of large point sources, smaller 

 

1

 Rao, V. and N. Frank, A. Rush, F. Dimmick. Chemical Speciation of PM2.5 in Urban and Rural Areas. Special 

Studies, National Air Quality and Emissions Trends Report, 2003. Available at 

http://www.epa.gov/airtrends/studies.html

2

 Frank, N. H., The Chemical Composition of PM2.5 to support PM Implementation, EPA State / Local / Tribal 

Training Workshop: PM2.5 Final Rule Implementation and 2006 PM2.5 Designation Process, Chicago IL, June 20-

21, 2007, http://www.epa.gov/ttn/naaqs/pm/presents/pm2.5_chemical_composition.pdf. 

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 “Initial Area Designations for the 2012 Revised Fine Particle National Ambient Air Quality Standard” 

memorandum from Gina McCarthy, Assistant Administrator, April 16, 2013, to Regional Administrators, US EPA 

Regions I-X. 

4

 Although this memorandum refers to “urban” or “rural” areas, those are technical terms with meanings unique to 

the urban increment analysis. For purposes of this memorandum, “urban” refers to the area nearby a violation, 

while “rural” refers to the area outside the immediate area of violation. 
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stationary sources, and mobile sources – all outside the local area – as well as more distant 

contributions from other cities. Accordingly, the so-called rural concentrations of the major 

constituents of PM2.5 tend to be more spatially homogenous than the local or so-called urban 

concentrations and are less impacted by local source emissions. Due to these attributes, the urban 

increment analysis among one or more urban monitoring locations provides an indication of the 

type, size, and spatial patterns of nearby or local emission sources that are contributing to the 

concentrations experienced at the urban monitors. 

The basic approach for the urban increment analysis is to calculate the difference between the 

measured PM2.5 at an urban area monitoring site and the regional background concentration as 

measured at a nearby rural area monitoring site(s). The difference between these two 

concentrations, defined as the urban increment, provides an estimate of local contributions to 

PM2.5 mass. The urban increment is calculated as follows: 

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ൌ ܷݎܾܽ݊	݋ܥ݊ܿ݁݊ݎݐܽݐ݅݋݊௉	ெଶ.ହ	ሺ௧௢௧௔௟	௢௥	௦௣௘௖௜௘௦ሻ

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As described above, individual chemical constituents of the measured PM2.5 mass can be linked 

to specific types of emission sources. Accordingly, looking at the individual components that 

comprise the urban increment can help identify likely local or ‘nearby’ emitted pollutants that 

contribute to the design value (DV) concentration. For example, in the eastern United States, 

large stationary sources such as electric generating units are the predominant contributors to the 

sulfate component of PM2.5. High nitrate levels (i.e., from oxides of nitrogen and ammonia) often 

indicate the presence of localized mobile sources, local or regional fuel-combustion sources, a 

regional contribution from agricultural sources, or a combination of these sources. Carbonaceous 

mass – often a substantial component of urban excess – is typically associated with mobile 

sources, wood or biomass burning, and localized combustion sources. A high elemental carbon 

to organic carbon mass ratio can be a signature of diesel combustion source contributions, such 

as diesel trucks, construction engines, vehicles, ships, and trains. A high organic carbon to 

elemental carbon ratio, on the other hand, is often a signature of biomass burning. 

Urban Increment Methodology 

To facilitate the calculation of urban increments to support initial area designations for the 2012 

primary annual PM2.5 NAAQS, the EPA developed a general approach to use 2010-2012 PM 

speciation measurements from the routine urban and rural speciation monitoring networks – 

respectively, the Chemical Speciation Network (CSN) and Interagency Monitoring of Protected 

Visual Environments (IMPROVE).5

 The urban increment methodology focuses on five major 

 

5

 At the time that states and tribes were required to make boundary recommendations to the EPA, 2010-2012 

speciation measurements were the most up-to-date information available. Urban increments derived from these 

data are representative of more recent air quality and are appropriate for use in the EPA’s analyses and technical 

support documents. 
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chemical constituents of PM2.5 mass: sulfate, nitrate, organic carbon mass (OM), elemental 

carbon (EC), and crustal material. These chemical constituents of PM2.5 are derived from 

measured speciation values using the SANDWICH method, which entails a material balance to 

ensure that the PM2.5 chemical constituents sum to the total urban increment associated with 

average PM2.5 and exactly represent the PM2.5 mass measured by the PM2.5 FRM.6,7 The EPA 

then used this information to determine the “Urban Concentration” and the “Regional 

Background Concentration.” 

To determine the “Urban Concentration” and most accurately describe the composition of the 

PM2.5 design value at violating PM2.5 monitoring locations, the EPA used 12 consecutive 

calendar quarters of measurements from CSN monitors collocated with a PM2.5 FRM, and 

required that each quarter have a minimum of 11 observations of each PM2.5 component. When 

sufficient collocated speciation data at the exact location of each violating monitor site were not 

available, the EPA used data from other nearby speciation monitors to represent the conditions at 

the violating monitor site(s). As described in the next section, the EPA then adjusted the PM2.5

chemical constituents to account for distance from the violating location and for differences in 

average PM2.5 mass. 

The EPA derived the “Regional Background Concentration” by averaging across multiple rural 

monitor sites, where available, to best represent the regional contributions to PM2.5 mass in each 

calendar quarter. The EPA also used monitors in nearby, smaller urban areas to estimate regional 

background contributions as the upwind concentrations. Using the measured concentrations from 

multiple monitors to represent regional influence when calculating the urban increment reduces 

the potential bias associated with a singular rural/urban monitor pairing. As with the violating 

location, the EPA required sufficient quarterly data for all PM2.5 chemical constituents. The 

selected rural monitors were generally located within a 150-mile radius of the violating PM2.5

monitor site in order to reasonably reflect average background and the influence of upwind 

emissions. For this simple methodology, however, determinations of a rural site’s regional 

representativeness did not take into account the influences of topography, intra-quarter 

differences in daily transport, or the likelihood that some locations may be more representative 

of upwind concentrations and corresponding emission influence. 

Some areas of the United States have limited rural monitoring locations with 12 consecutive 

quarters of sufficient PM2.5 speciation data surrounding the DV monitoring locations. To 

maximize the number of background monitoring sites for this urban increment calculation, the 

EPA paired background locations with the violating monitor on a quarter-by-quarter basis. 

(Accordingly, although the number of included background locations may vary from quarter to 

quarter, their average PM2.5 component concentrations are temporally consistent with the 

 

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 The SANDWICH technique stands for measured Sulfate, Adjusted Nitrate, Derived Water, Inferred Carbonaceous 

mass Hybrid Material Balance Approach. It identifies the PM2.5 constituents that are represented by the measured 

PM2.5 with the FRM or FEM monitor. 

7

 Frank, N. H. Retained Nitrate, Hydrated Sulfates, and Carbonaceous Mass in Federal Reference Method Fine 

Particulate Matter for Six Eastern U.S. Cities. J. Air & Waste Manage. Assoc. 2006 56:500–511. 
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analyzed data from the violating monitor.) The EPA then summarized the resulting urban 

increments as 3-year averages by calendar quarter and PM2.5 chemical constituent. 

Analytical Protocol 

The EPA used the following steps to derive the urban increment (UI) at each urban CSN location 

and DV site: 

1) The EPA generated quarterly average concentrations of five SANDWICHed PM2.5

constituents (sulfate, nitrate, OM, EC, and crustal material) for each CSN and IMPROVE 

site that produced at least 11 observations for each constituent.8 Values for estimated 

ammonium nitrate (AN) based on measured nitrate are provided, (i.e. NO3*1.29) and 

used in step 5 to produce a SANDWICHed estimate for its UI.9

 The data elements are 

described in the data dictionary (see Attachment A) and can be found on the PM 

Designations website at http://www.epa.gov/pmdesignations/2012standards/techinfo.htm. 

2) Urban CSN sites (UCS) include sites that are either “URBAN AND CENTER CITY” or 

“SUBURBAN,” based on land use data. A UCS is eligible for this analysis if it provided 

12 consecutive calendar quarters with sufficient data for each chemical constituent. 

3) For each UCS and quarter, the EPA identified the eligible rural sites to be those within 

150 miles of the urban monitor or, if no rural locations were within 150 miles, then the 

EPA identified the nearest rural site. The EPA considered non-UCS CSN sites and 

IMPROVE data as “rural.” Attachment B identifies the rural and UCS site pairings. 

4) For each calendar quarter and UCS, the EPA computed the average concentration of each 

PM2.5 constituent among all eligible rural sites associated with that UCS. 

5) Except for nitrate, the EPA estimated the UI for each UCS chemical constituent as the 

numerical difference between the UCS and average rural values. For the nitrate UI – 

a. The UI was initially estimated as the numerical difference between UCS and rural 

nitrate mass (Nu and Nr, respectively) which are derived from measured nitrate 

(NO3) and expressed as ammonium nitrate by multiplying by 1.29.9

 

b. The above measurement-derived difference is then multiplied by the ratio of 

SANDWICHed UCS nitrate mass (SNu) to the UCS measurement derived value 

 

8

 The SANDWICH estimates represent fine particle mass as measured by the PM2.5 FRM. For sulfate and nitrates, 

they include the associated ammonium and particle bound water. The estimate for OM is derived by material 

balance. 

9

 Because ammonium is 29% of ammonium nitrate, this multiplier converts nitrate concentration into ammonium 

nitrate. 
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(Nu). This ratio accounts for the portion of the urban increment that is retained by 

the PM2.5 FRM.10

c. Finally, the expression is multiplied by 1.12 to produce “hydrated retained 

ammonium nitrate,” to be consistent with the SANDWICH calculations. 

 i.e., UInitrate = (Nu – Nr) * (SNu /Nu ) * 1.12 

6) The EPA set to zero any negative derived quarterly UI’s. Negative values could occur 

when the estimated average rural concentration is similar to its paired urban value. 

7) For each calendar quarter, the EPA estimated the UI at the DV site using one of three 

possible metrics: 

a. As the value at a collocated UCS, if eligible UCS data were available; 

b. Otherwise, as a distance weighted average among all eligible UCS in the same 

consolidated statistical area (CSA), if such data were available; 

c. Otherwise, as the regional default value derived from the UI among all eligible 

UCS. These regional default values are spatial means of UIs from sites in the 

same NOAA climate region.11

As indicated in Attachment B, the three types of UIs among all eligible UCS are 

specified as “coloc,” “csamon” and “default.” Attachment B also provides the 

included UCS for the csamon values. 

8) As necessary, to estimate the UI at the DV site, the EPA adjusted the initial value: 

If the 3-year quarterly average PM2.5 mass at the DV site was different than the 

derived PM2.5 mass associated with the UCS (or default value), then the EPA 

adjusted the estimate value from step 7 to allow the UI among the constituents to 

sum to the “total” PM2.5 UI. The EPA made this adjustment by scaling the OM 

and EC constituents up or down to achieve material balance with the total.12 In the 

 

10 The SANDWICH procedure is designed to account for atmospheric particle nitrate that is retained by the PM2.5

monitor. A part of this comes from local emission sources and the remainder from more distant sources. With this 

ratio, the retained fractions of each part are identical. 

11 http://www.ncdc.noaa.gov/monitoring-references/maps/us-climate-regions.php 

12 The adjustments, which are only made for OM and EC, essentially assume that the difference between DV-site 

and CSN-site PM2.5 mass and associated increments are only attributed to carbon. This default assumption is 

reasonable in light of the large uncertainties in both the calculation of OM, and the material balance approach 

inherent in SANDWICH. 
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event that the initially calculated UI for OM and EC were both zero, then the EPA 

adjusted the largest UI among the other constituents.13

In all cases, CSN measurements with less than 12 quarters of data collocated with the DV 

location can be used to corroborate or adjust the UI values derived with the general methods 

described above. 

Attachments 

 A: PM2.5 chemical constituent data dictionary 

 B: Urban rural pairings and types of UI for each DV site 

 

 

13 In the event that examination of monitor siting and potential influence of local emissions suggest that noncarbonaceous sources may be responsible for the local urban gradient in PM2.5 concentrations in the vicinity of the 

DV monitor, then an alternate estimate of the DV composition and its urban increment should be considered. Such 

alternative approaches may include assigning the excess mass to PM2.5 constituents in proportion to the local 

emissions or in proportion to their initial calculated UI values.