1 MEMORANDUM

TO: Docket No. EPA-HQ-OAR-2012-0918 Air Quality Designations for the 2012 PM 2.5 Standards

FROM: Neil H. Frank Air Quality Assessment Division, OAQPS

SUBJECT: Calculation of Urban Increments to Support the Air Quality Designations for the
2012 PM 2.5 Standards National Ambient Air Quality Standards (NAAQS) (SAN 5706)

Introduction
PM 2.5 mass concentrations are generally higher in urban areas compared to surrounding regions. This “urban increment,” also known as the “urb an excess,” is due to locally generated and largely directly-emitted PM 2.5 in addition to regional contributions. 1,2 Among the major
contributors to PM 2.5 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 lo cal sources. Sulfate and ni trate also depend on the availability of ammonia, which may orig inate from regional or local emissions.

The goal of the urban increment analysis 3 is to estimate the local contribution to urban PM 2.5 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 prem ise that so-called “rural” contributions of PM 2.5 concentrations typically result from a regional distribution of contributing sources caused by atmospheric formation and tran sport 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 PM 2.5 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 PM 2.5 to support PM Implementation, EPA State / Local / Tribal

Training Workshop: PM 2.5 Final Rule Implementation and 2006 PM 2.5 Designation Process, Chicago IL, June 20- 21, 2007, http://www.epa.gov/ttn/naaqs/pm/presents/pm2.5_chemical_composition.pdf.

3 “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 memora ndum, “urban” refers to the area nearby a violation,
while “rural” refers to the area outs ide the immediate area of violation.2 stationary sources, and mobile so urces – all outside the local area – as well as more distant contributions from other cities. Accordingly, th e so-called rural concentrations of the major
constituents of PM 2.5 tend to be more spatially homogenous than the local or so-called urban concentrations and are less impacted by local sour ce emissions. Due to these attributes, the urban increment analysis among one or more urban mon itoring locations provides an indication of the type, size, and spatial patterns of nearby or loca l emission sources that are contributing to the concentrations experienced at the urban monitors.

The basic approach for the urban increment analys is is to calculate the difference between the
measured PM 2.5 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
PM 2.5 mass. The urban increment is calculated as follows:

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As described above, individual chemical constituents of the measured PM 2.5 mass can be linked to specific types of emission sources. Accord ingly, 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) concentratio n. For example, in the eastern United States, large stationary sources such as electric generating units are the predominant contributors to the sulfate component of PM 2.5 . High nitrate levels (i.e ., from oxides of nitrogen and ammonia) often indicate the presence of localized mobile sour ces, local or regional fuel-combustion sources, a regional contribution from agricult ural sources, or a combinati on of these sources. Carbonaceous mass – often a substantial component of urban ex cess – 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 calculat ion of urban increments to support initial area designations for the 2012 primary annual PM 2.5 NAAQS, the EPA developed a general approach to use 2010-2012 PM speciation measurements from the routine urba n 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.3 chemical constituents of PM 2.5 mass: sulfate, nitrate, or ganic carbon mass (OM), elemental carbon (EC), and crustal material. Th ese chemical constituents of PM 2.5 are derived from measured speciation values using the SANDWICH method, which entails a material balance to
ensure that the PM 2.5 chemical constituents sum to the total urban increment associated with average PM 2.5 and exactly represent the PM 2.5 mass measured by the PM 2.5 FRM. 6,7 The EPA
then used this information to determine th e “Urban Concentration” and the “Regional Background Concentration.”

To determine the “Urban Concentration” and mo st accurately describe the composition of the
PM 2.5 design value at violating PM 2.5 monitoring locations, the EPA used 12 consecutive calendar quarters of measur ements from CSN monitors collocated with a PM 2.5 FRM, and required that each quarter have a mi nimum of 11 observations of each PM 2.5 component. When sufficient collocated speciation data at the exact location of each violating monitor site were not available, the EPA used data fr om 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 PM 2.5 chemical constituents to account for distance from the violating location and for differences in average PM 2.5 mass.

The EPA derived the “Regional Background Concen tration” by averaging across multiple rural monitor sites, where available, to best represent the regional contributions to PM 2.5 mass in each calendar quarter. The EPA also used monitors in nearby, smaller ur ban areas to estimate regional background contributions as the upwind concentrati ons. Using the measured concentrations from multiple monitors to represent regional influe nce when calculating the urban increment reduces the potential bias associated with a singular rural/urba n monitor pairing. As with the violating location, the EPA required sufficient quarterly data for all PM 2.5 chemical constituents. The selected rural monitors were generally located within a 150-mile radi us of the violating PM 2.5 monitor site in order to reas onably reflect average backgroun d and the influence of upwind emissions. For this simple methodology, however, de terminations 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 locati ons 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 PM 2.5 speciation data surrounding th e DV monitoring locations. To maximize the number of background monitoring site s for this urban increment calculation, the EPA paired background locations with the viol ating monitor on a quarter-by-quarter basis. (Accordingly, although the number of included bac kground locations may vary from quarter to quarter, their average PM 2.5 component concentrations are temporally consistent with the

6 The SANDWICH technique stands for measured S ulfate, A djusted N itrate, D erived W ater, I nferred C arbonaceous

mass H ybrid Material Balance Appr oach. It identifies the PM 2.5 constituents that are represented by the measured
PM 2.5 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 56:500–511.4 analyzed data from the violating monitor.) The EPA then summarized the resulting urban increments as 3-year averag es by calendar quarter and PM 2.5 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 c oncentrations of five SANDWICHed PM 2.5 constituents (sulfate, nitrate, OM, EC, a nd 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 meas ured nitrate are provided, (i.e. NO 3 *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 A ttachment A) and can be found on the PM Designations website at http://www.epa.gov/pmdesignati ons/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. Th e EPA considered non-UCS CSN sites and
IMPROVE data as “rural.” Attachment B id entifies the rural and UCS site pairings.

4) For each calendar quarter and UCS, the EPA co mputed the average concentration of each
PM 2.5 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 aver age rural values. For the nitrate UI –

a. The UI was initially estimated as the nu merical difference between UCS and rural nitrate mass (N u and N r , respectively) which are derived from measured nitrate (NO 3 ) and expressed as ammonium nitrate by multiplying by 1.29. 9

b. The above measurement-derived differen ce is then multiplie d by the ratio of SANDWICHed UCS nitrate mass (SN u ) to the UCS measurement derived value

8 The SANDWICH estimates represent fine particle mass as measured by the PM 2.5 FRM. For sulfate and nitrates,

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

9 Because ammonium is 29% of ammonium nitrate, this multiplier converts nitrate co ncentration into ammonium
nitrate.5 (N u ). This ratio accounts for the portion of the urban increment that is retained by
the PM 2.5 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., UI nitrate = (N u – N r ) * (SN u /N u ) * 1.12

6) The EPA set to zero any negative derived qua rterly UI’s. Negative values could occur when the estimated average rural concentrati on is similar to its paired urban value.

7) For each calendar quarter, the EPA estimated th e 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 aver age 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 sp atial means of UIs from sites in the same NOAA climate region. 11

As indicated in Attachment B, the thr ee types of UIs among all eligible UCS are specified as “coloc,” “csamon” and “def ault.” 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 PM 2.5 mass at the DV site was different than the
derived PM 2.5 mass associated with the UCS (o r default value), then the EPA adjusted the estimate value from step 7 to allow the UI among the constituents to
sum to the “total” PM 2.5 UI. The EPA made this ad justment 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 n itrate that is retained by the PM 2.5

monitor. A part of this comes from local emission sour ces 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-ref erences/maps/us-climate-regions.php

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

and CSN-site PM 2.5 mass and associated increments are only attr ibuted to carbon. This default assumption is reasonable in light of the large un certainties in both the calculation of OM, and the material balance approach
inherent in SANDWICH.6 event that the initially calculated UI for OM and EC were both zero, then the EPA adjusted the largest UI am ong the other constituents. 13

In all cases, CSN measurements with less than 12 quarters of data co llocated with the DV location can be used to corroborate or adjust th e UI values derived wi th the general methods
described above.

Attachments
A: PM 2.5 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 PM 2.5 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 PM 2.5 constituents in proportion to the local emissions or in proportion to their initial calculated UI values.