NATIONAL AIR QUALITY AND EMISSIONS TRENDS REPORT, 2003

SPECIAL STUDIES • CHEMICAL SPECIATION OF PM2.5 S13

2003 SPECIAL STUDIES 

Abstract

Data from the Interagency Monitoring of PROtected Visual Environment (IMPROVE) and the Speciation

Trends Network (STN) are used to

analyze the chemical composition 

of PM2.5 and to explore issues associated with interpretation of their

measurements. The data from the

largely rural IMPROVE network 

and urban STN are used to examine

spatial patterns and to develop

estimates of the local urban excess

over the regional background

concentrations. This work will give

some insights into which of the

chemical constituents are driving

urban excess of PM2.5 mass in different regions of the United States.

Introduction

With the promulgation of the new

Particulate Matter National Ambient

Air Quality Standards (PM2.5

NAAQS), all future designated

nonattainment areas and surrounding regions may need to reduce

emission of fine particles and their

precursors to permit those areas to

attain the NAAQS. Efficient air quality management requires knowing

which sources contribute to the problem and how much. Determining

PM2.5 source contributions is 

complicated due to the fact that often

half or more of the PM2.5 mass is

composed of secondarily formed

species,1 hiding their point of origin.

In addition, PM2.5 has a lifetime on

the order of several days,2 enabling

sources up to 1,500 miles away to

affect a source region.

This work examines a simple subset of the source apportionment problem by providing evidence for local

and regional source contributions and

first-order approximations of their

respective contributions to the following major urban areas: Fresno, CA,

Missoula, MT, Salt Lake City, UT,

Tulsa, OK, St. Louis, MO, Birmingham,

AL, Indianapolis, IN, Atlanta, GA,

Cleveland, OH, Charlotte, NC,

Richmond, VA, Baltimore, MD, and

New York, NY. This is accomplished

by computing urban excess concentrations—by comparing annual

concentrations of PM2.5 mass and its

most abundant chemical species at

the urban monitors with nearby rural

monitors. In the process of arriving at

the urban excess numbers, several

graphics are used to show the chemical species that make up PM2.5 mass

across the United States.

Data Sources

Ambient monitoring data from the

PM2.5 chemical Speciation Trends

Network (STN) and the Interagency

Monitoring of PROtected Visual

Environmental (IMPROVE) aerosol

monitoring network were the main

sources of data used to assess the

urban and rural PM2.5 species concentrations across the United States.

The PM2.5 STN was established 

by regulation3 and is a companion

network to the mass-based Federal

Reference Method (FRM) network

implemented in support of the PM2.5

NAAQS. EPA established the STN

network to provide nationally consistent speciated PM2.5 data for the

assessment of trends at representative

sites in urban areas across the country. As part of a routine monitoring

program, the STN quantifies mass

concentrations and PM2.5 constituents,

including numerous trace elements,

ions (sulfate, nitrate, sodium, potassium, ammonium), elemental carbon,

and organic carbon. The STN began

operation in late 1999, and there are

currently a total of 54 STN sites.

In 1987 the IMPROVE aerosol

monitoring network was established

among federal and state agencies to

provide information for determining

the types of pollutants and sources

primarily responsible for visibility

impairment within federally designated Class I areas.4 Ambient aerosol

mass concentrations have been measured under the IMPROVE program

to characterize the visibility conditions in these Class I areas since 1988.

Over the past few years, the

IMPROVE network has expanded

from its original 20 monitoring sites

to 110 sites in 2002. In addition, there

are currently over 50 supplemental

sites in regionally representative

rural areas that deploy the exact

same aerosol monitoring protocol. As

with the STN, the IMPROVE network

also quantifies mass concentrations

and PM2.5 constituents.

Both the STN and IMPROVE programs employ a 1-in-3-day sampling

protocol.

Chemical Speciation of PM2.5 in Urban and Rural Areas

Venkatesh Rao

Office of Air and Radiation

U.S. Environmental Protection Agency, Ann Arbor, MI 48105

Neil Frank, Alan Rush, Fred Dimmick

Air Quality Trends Analysis Group

U.S. Environmental Protection Agency, Research Triangle Park, NC 27711
NATIONAL AIR QUALITY AND EMISSIONS TRENDS REPORT, 2003

S14 CHEMICAL SPECIATION OF PM2.5 • SPECIAL STUDIES

Figure 1. 35 STN locations.

Figure 2. 98 IMPROVE locations.

Data Work-Up

The time period chosen for this

analysis is the 1-year period from

March 2001 to February 2002. Any

references to an annual average will

refer to these 12 months. Out of the

possible 54 STN sites, 35 had “complete” annual data. Similarly, 98

IMPROVE sites had “complete”

annual data for this time period.

Complete data, for the purposes of

this analysis, refers to 50% or more

of the “relevant” species observations being present for the four quarters that make up the 12 months

from March 2001 to February 2002.

To be consistent with previous EPA

characterizations5 of the composition

of ambient PM2.5, the following

“relevant” chemical species that

make up PM2.5 mass are considered

in this analysis. The relevant species

for the STN are nitrate, sulfate,

organic carbon, elemental carbon,

ammonium, and the trace elements

that go into the “crustal” calculation:

aluminum, silicon, calcium, iron, and

titanium. Similarly, for IMPROVE,

the relevant species are nitrate,

sulfate, organic carbon, elemental

carbon, and the same five trace

elements that go into the “crustal”

calculation. Because both networks

employ a 1-in-3-day sampling protocol, the 50% completeness criterion

amounts to there being 15 or more

observations per quarter. No further

requirement was imposed for matching days among sites or between networks. Quarters for the 12 months

analyzed are defined in Table 1.

Figures 1 and 2 show the 35 STN

and 98 IMPROVE locations that had

complete data, as defined by the

completeness criterion defined

above, for the time period analyzed.

Quarter Months Used in Analysis

1 January 2002, February 2002, March 2001

2 April 2001, May 2001, June 2001

3 July 2001, August 2001, September 2001

4 October 2001, November 2001, December 2001

Table 1. Quarter Definitions
NATIONAL AIR QUALITY AND EMISSIONS TRENDS REPORT, 2003

SPECIAL STUDIES • CHEMICAL SPECIATION OF PM2.5 S15

2003 SPECIAL STUDIES 

The variances in their analytical

and sampling procedures effectively result in two different operational definitions of organic and

elemental carbon.5,6 For this reason, organic (OC) and elemental

carbon (EC) are not analyzed separately. Instead, total carbonaceous mass (TCM) is estimated as:

TCM = k * OC + EC for both

programs. Here k is the factor for

converting measured organic

carbon to organic carbon mass 

(to account for hydrogen, oxygen,

etc.). Historically, EPA and

IMPROVE programs have used

k=1.4 to convert from carbon to

carbon mass. Most recent findings

by Turpin et al.7 suggest that a

higher factor to convert carbon to

carbon mass may be needed in

both urban and rural areas. In this

work, both k=1.4 and k=1.8 are

used to represent TCM. In some

cases, TCM (k=1.8) is used to

show total carbonaceous mass,

whereas in other cases, comparisons are made between use of

k=1.8 and k=1.4.7

The OC measurements reported

by STN are blank-corrected data

using network-wide estimates.5 This

is consistent with the approach used

by the IMPROVE program.6 The OC

values reported by the IMPROVE

program are automatically blankcorrected using an appropriate blank

correction factor.6 Table 2 lists the OC

blank correction factors used for each

of the speciation samplers that are in

the STN network (also shown for

comparison purposes is the

IMPROVE blank correction factor). 

It should be noted that only organic

carbon concentrations for the STN

are blank-corrected (none of the

other STN chemical constituents nor

the total gravimetric mass is blankcorrected in this analysis).

Urban PM2.5 Excess

Local and regional contributions

to the urban centers were estimated

by computing the differences

between the concentrations of the

annual average urban and nearby

rural monitoring data. These estimates are thus a first approximation

of local and regional contributions of

PM2.5 mass and its chemical constituents to the urban areas investigated. Although strong regional

similarity exists for each of the chemical species on a large spatial scale,

there are still local gradients that

exist in the rural concentration

domain. See, for example, Figures 3

Data Handling Protocols

Even though the STN and IMPROVE

networks use similar sampling and

analytical methods, there are differences in the species they measure and

the operational protocols they

employ. To put aerosol composition

data derived from both these networks on an as-similar-as-possible

basis, the following data handling

protocols were employed:

• Ammonium: Although directly

measured ammonium as performed by STN is important in

characterizing the composition of

PM2.5, network-wide IMPROVE

measurements are currently lacking in this area. Ammonium concentrations are thus estimated for

IMPROVE (and for comparison

purposes, for STN as well) from

sulfate (SO4) and nitrate measurements, assuming (1) all sulfates are

ammonium sulfate (NH4SO4), and

(2) all nitrates are ammonium

nitrate. For now, the inter-network

measure based on assumed ammonium sulfate and assumed ammonium nitrate compounds is more

comparable and will therefore be

used to define urban excess. These

“estimated” ammonium concentrations are the values shown on

all graphics that compare rural

and urban ammonium concentrations.

• Sulfate: The IMPROVE program

estimates sulfate concentrations as

three times the sulfur concentration, whereas with the STN

program, sulfate concentrations

are used as measured. In this

analysis, the sulfate ion measurement is used from both networks

to represent sulfates.

• Carbon: Carbon is monitored

somewhat differently by the

IMPROVE and STN programs. 

24-h Sample OC Blank Correction

Speciation Sampler Volume, m3 Factor (μg/m3)

MetOne SASS 9.6 1.40

Anderson RASS 10.4 1.28

R&P 2300 14.4 0.93

URG MASS 16.7 0.56

IMPROVE 32.8 0.4

Table 2. Organic Carbon (OC) Blank Correction Factors

Soil: The soil component of PM2.5 (“crustal” material) was computed using the 

following formula, which is the same as that employed by the IMPROVE 

program8:

PM2.5 Fine Soil = “Crustal” = 2.2[Al] + 2.49 [Si] + 1.63 [Ca] + 2.42 [Fe] + 1.94[Ti].
NATIONAL AIR QUALITY AND EMISSIONS TRENDS REPORT, 2003

S16 CHEMICAL SPECIATION OF PM2.5 • SPECIAL STUDIES

through 5, which show spatially

averaged concentrations of carbonaceous mass, sulfates, and nitrate for

the March 2001–February 2002 time

period (together with the annual

mean concentrations at each

IMPROVE monitoring location).

Thus, the location of a rural site (for

eventual pairing to an urban site to

determine urban increments) may

influence the amount of urban excess

seen for the specific chemical constituents of PM2.5. One way to

remove this effect and standardize

the choice of rural background concentrations is to use spatial interpolation to determine average concentrations for any particular urban location. Although doing this for all sites

is beyond the scope of this paper,

spatial averaging for rural concentrations was applied, albeit in a simple

manner, at two urban locations. At

the St. Louis, MO, urban site, three

nearby IMPROVE sites were used to

determine an inverse-distanceweighted annually averaged rural

concentration for each of the species.

Similarly at the Atlanta, GA, urban

site, two nearby IMPROVE sites

were used to determine an average

annual rural concentration for each

of the species. See the discussion in

the next section and Table 3 for more

information on the choice of pairing

of specific urban/rural sites. In 

general, this approach assumes 

that the PM2.5 at the rural sites 

is generally representative of the

upwind regional concentrations 

and is not significantly influenced 

by nearby emissions and that the

regional sources (including upwind

urban areas) have the same impact

on the rural monitors and the particular urban monitors. 

Figure 3. Spatial averaging of rural sulfate concentrations.

Figure 4. Spatial averaging of rural nitrate concentrations.

Figure 5. Spatial averaging of rural TCM (k=1,8) concentrations.
NATIONAL AIR QUALITY AND EMISSIONS TRENDS REPORT, 2003

SPECIAL STUDIES • CHEMICAL SPECIATION OF PM2.5 S17

2003 SPECIAL STUDIES 

Choice of Urban and Rural Sites

Figure 6 summarizes the urban and

rural locations chosen for this analysis. There are five urban sites (Bronx,

NY, Baltimore, MD, Richmond, VA,

Charlotte, NC, and Atlanta, GA) in

the Northeast and Mid-Atlantic

States, five urban sites stretching

from north to south in the mid

portion of the United States (Cleveland, OH, Indianapolis, IN, St. Louis,

MO, Tulsa, OK, and Birmingham,

AL), and three urban sites in the

West (Fresno, CA, Salt Lake City, UT,

and Missoula, MT). These were

chosen due to their data being complete for the year in question as well

as their ease in matching up with

nearby IMPROVE rural (discussed

further below) sites for the urban

excess study. Except for Tulsa, they

were also selected to represent states

with reported PM2.5 mass concentrations greater than 15 μg/m3, which

is the level of the annual PM2.5

NAAQS. IMPROVE sites with complete data were chosen for assumed

representativeness of upwind background concentrations. In the case of

matching the urban Atlanta and St.

Louis sites to nearby rural sites, a

single available rural site with complete data was not judged to be sufficiently representative of the requisite

requirement, and therefore a multiple site approach (as explained

above) was employed.

Table 3 summarizes all the STN

and IMPROVE sites for their elevation and separation distances. For

the analyses of urban excess, all

Fresno Indy

St.Louis

Tulsa

Missoula

SLC

Charlotte

Baltimore

Atlanta

Cleveland

Richmond

Birmingham

Bronx

16 rural IMPROVE sites

13 urban STN sites

Figure 6. Thirteen urban/rural site paintings.

Urban Location/Site Elevation (m) Rural Location/Site Elevation (m) Distance Apart (km)

Fresno, CA 96 Pinnacles National Monument, CA 317 28

Missoula, MT 975 Monture, MT 1,293 72

Salt Lake City, UT 1,306 Great Basin National Park, NV 2,068 277

Tulsa, OK 198 Wichita Mountains, OK 487 298

St. Louis, MO 0 Cadiz, KY 188 296

Hercules-Glades, MO 423 322

Bondville, IL 211 220

Birmingham, AL 174 Sipsy Wilderness, AL 279 100

Indianapolis, IN 235 Livonia, IN 298 142

Atlanta, GA 308 Okefenokee National Wildlife Refuge, GA 49 324

Shining Rock Wilderness, NC 1,621 236

Cleveland, OH 206 M.K. Goddard, PA 383 129

Charlotte, NC 232 Linville Gorge, NC 986 132

Richmond, VA 59 James River Face, VA 300 179

Baltimore, MD 5 Dolly Sods/Otter Creek Wilderness, WV 1,158 256

Bronx, NY 0 Brigantine National Wildlife Refuge, NJ 9 165

Table 3. STN and IMPROVE Site Particulars
NATIONAL AIR QUALITY AND EMISSIONS TRENDS REPORT, 2003

S18 CHEMICAL SPECIATION OF PM2.5 • SPECIAL STUDIES

urban/rural pairings were elevationadjusted to account for the effect of

24-h average sample volume density

on aerosol concentration. Both

IMPROVE- and STN-reported data

represent local conditions. This elevation adjustment was done in two

steps: (1) all the concentrations from

the IMPROVE sites were adjusted to

sea-level conditions, and (2) all these

sea-level-adjusted concentrations

were adjusted once again to the elevation corresponding to the matched

urban site. Except for the St. Louis

and Atlanta STN monitors and their

pairing with rural IMPROVE monitors, all other STN sites were

matched one-on-one with the rural

monitors listed in Table 3. In the case

of St. Louis, the three IMPROVE

monitors shown in Table 3 as

matched sites were inverse-distance

weighted, and the urban Atlanta site

was compared to the averaged concentration(s) derived from the two

IMPROVE sites shown in Table 3.

Elevation Effects on

PM2.5 Concentrations

As mentioned previously, all the

IMPROVE data were adjusted for

elevation (based on temperature and

barometric pressure correction factors) twice: once to adjust to sea level

and then again, as necessary, to

adjust to the elevation of the

matched urban site. Basically, this

elevation adjustment is a small technical correction to make the “urban

excess” calculation more meaningful.

Other than at the Dolly Sods/

Baltimore rural/urban pairing of

sites, however, the urban/rural

elevation differences were small, and

these adjustments are very minor as

can be seen in Figures 7 through 11,

which show the effects of elevation

adjustments for all the chemical

species of interest at the 13

urban/rural paired combinations. 

Fresno/PINN

Missoula/MONT

SLC/GRBA

Tulsa/WIMO

St.Louis/3 Sites

Birmingham/SIPS

Indy/LIVO

Atlanta/2 Sites

Cleveland/MKGO

Charlotte/LIGO

Richmond/JARI

Baltimore/DOSO

Bronx/BRIG

-1

-0.5

0

0.5

1

1.5

2

μg/m3

Unadjusted Sulfate Urban Elevation-Adjusted Sulfate Urban

Avg Unadjusted: 0.447μg

Avg Adjusted: 0.334 μg

Figure 7. Effect of evaluation on rural sulfate concentrations.

Fresno/PINN

Missoula/MONT

SLC/GRBA

Tulsa/WIMO

St.Louis/3 Sites

Birmingham/SIP

Indy/LIVO

Atlanta/2 Sites

Cleveland/MKGO

Charlotte/LIGO

Richmond/JARI

Baltimore/DOSO

Bronx/BRIG

0

0.5

1

1.5

2

2.5

μg/m3

Unadjusted Ammonium (estimated) Urban Increment

Elevation-Adjusted Ammonium (estimated) Urban

Avg Unadjusted: 0.831μg

Avg Adjusted: 0.787μg

Figure 8. Effect of evaluation on rural ammonium concentration.

Fresno/PINN

Missoula/MONT

SLC/GRBA

Tulsa/WIMO

St.Louis/3 Sites

Birmingham/SIPS

Indy/LIVO

Atlanta/2 Sites

Cleveland/MKGO

Charlotte/LIGO

Richmond/JARI

Baltimore/DOSO

Bronx/BRIG

0

1

2

3

4

5

6

7

μg/m3

Unadjusted Nitrate Urban Increment

Elevation-Adjusted Nitrate Urban Increment

Avg Unadjusted: 1.40 μg

Avg Adjusted: 1.38 μg

Figure 9. Effect of evaluation on rural nitrate concentration.
NATIONAL AIR QUALITY AND EMISSIONS TRENDS REPORT, 2003

SPECIAL STUDIES • CHEMICAL SPECIATION OF PM2.5 S19

2003 SPECIAL STUDIES 

Fresno/PINN

Missoula/MONT

SLC/GRBA

Tulsa/WIMO

St.Louis/3 Sites

Birmingham/SIPS

Indy/LIVO

Atlanta/2 Sites

Cleveland/MKGO

Charlotte/LIGO

Richmond/JARI

Baltimore/DOSO

Bronx/BRIG

0

5

10

15

20

25

30

μg/m3

Urban Rural

Figure 12. Urban excess for total PM2.5 gravimetric mass.

Urban Increments of

PM2.5 Mass and the

Chemical Species

Urban sites were paired with

matched rural sites as listed in Table

3, and the annual average concentrations were calculated for both the

urban sites and the companion rural

site(s). All rural values reflect elevation-adjusted values. These averaged

rural concentrations were subtracted

from the appropriate urban concentrations to arrive at the urban increments of mass and increments of the

individual chemical species. 

Shown first in Figure 12 is the

comparison of urban concentrations

to estimated regional background for

total measured gravimetric mass.

The difference is the “urban increment.” The height of each bar

represents the annually averaged

urban gravimetric mass. Overlaying

the nearby rural gravimetric mass on

top of the urban mass levels shows

how much of the total mass can be

attributed to rural vs. urban sources.

It can be seen that Fresno, Cleveland,

and Birmingham are the urban sites

in this analysis with the largest

urban PM2.5 mass during the time

period investigated. The largest

urban increment in PM2.5 mass is

seen to be at the Fresno, CA, site,

with an average excess of about

18 μg/m3. The smallest urban increment for mass is seen to be at the St.

Louis site, which shows an average

urban excess of about 5 μg/m3 total

PM2.5 mass. Although this result

suggests that there are more local

sources influencing urban PM2.5

mass at the Fresno, CA, location than

at the St. Louis, MO, location, the

selected rural sites in the eastern

United States may be more reflective

of background concentrations. The

Fresno site may be influenced by

other PM2.5 sources throughout the

Fresno/PINN

Missoula/MONT

SLC/GRBA

Tulsa/WIMO

St.Louis/3 Sites

Birmingham/SIPS

Indy/LIVO

Atlanta/2 Sites

Cleveland/MKGO

Charlotte/LIGO

Richmond/JARI

Baltimore/DOSO

Bronx/BRIG

0

5

10

15

μg/m3

Unadjusted TCM (k=1.8) Urban Increment

Elevation-Adjusted TCM (k=1.8) Urban Increment

Figure 10. Effect of elevation on rural TCM (k=1.8) concentrations.

Fresno/PINN

Missoula/MONT

SLC/GRBA

Tulsa/WIMO

St.Louis/3 Sites

Birmingham/SIPS

Indy/LIVO

Atlanta/2 Sites

Cleveland/MKGO

Charlotte/LIGO

Richmond/JARI

Baltimore/DOSO

Bronx/BRIG

-0.5

0

0.5

1

1.5

μg/m3

Unadjusted Crustal Urban Increment

Elevation-Adjusted Crustal Urban Increment

Figure 11. Effect of elevation on rural crustal concentrations.
NATIONAL AIR QUALITY AND EMISSIONS TRENDS REPORT, 2003

S20 CHEMICAL SPECIATION OF PM2.5 • SPECIAL STUDIES

Sulfate Ammonium Nitrate TCM (k=1.8) Crustal

0

2

4

6

8

10

μg/m3

Urban Rural

Figure 15. Urban excess at St. Louis, MO.

San Joaquin Valley. In general, the

total excess mass ranges from 4

to16 μg/m3, with the West generally

showing more mass urban excess

than the East. On average, the urban

excess in PM2.5 mass for the investigated 13 site combinations is seen to

be about 8 μg/m3.

Figures 13 through 16 show a

comparison of urban concentrations

with estimated regional background

for four example sites (urban sites:

Fresno, CA, St. Louis, MO, New

York, NY, and Charlotte, NC—see

Table 3 for the matched rural sites for

these urban locations) out of the total

13 urban/rural pairings investigated.

The height of each bar represents the

average urban concentration by

species. Overlaying the nearby rural

concentrations by chemical component on the urban chemical component concentrations, the example

stacked bar charts (Figures 13-16)

show that the estimated regional

background represents varying proportions of the total urban concentrations by component and location.

Specifically, TCM and nitrates dominate Fresno particulate aerosol,

whereas carbon and sulfates are the

highest among the example eastern

sites. In terms of urban excess, all

four of these examples show TCM

and nitrate concentrations to be the

major components. Urban increments of TCM are seen to range from

13 μg/m3 at the Fresno, CA, location

to about 3 to 4 μg/m3 at the other

three locations. Similarly, nitrate

urban excess is seen to be 6.5 μg/m3

at the Fresno, CA, location and is in

the 0.5 to 1.3 μg/m3 range at the

other sites studied. As stated earlier,

the Fresno values are probably reflective of contributions from the San

Joaquin Valley.

Another interesting way to look 

at urban excess at the 13 selected

urban/rural pairs is by examining

Sulfate Ammonium Nitrate TCM (k=1.8) Crustal

0

5

10

15

20

μg/m3

Urban Rural

Figure 13. Urban excess at Fresno, CA.

Sulfate Ammonium Nitrate TCM Crustal

0

2

4

6

8

10

μg/m3

Urban Rural

Figure 14. Urban excess at Charlotte, NC.

Sulfate Ammonium Nitrate TCM (k=1.8) Crustal

0

2

4

6

8

10

μg/m3

Urban Rural

Figure 16. Urban excess at New York City, NY.
NATIONAL AIR QUALITY AND EMISSIONS TRENDS REPORT, 2003

SPECIAL STUDIES • CHEMICAL SPECIATION OF PM2.5 S21

2003 SPECIAL STUDIES 

the urban increment of gravimetric

mass as it compares to the urban

increments of each of the chemical

species that drive that mass. This is

shown in Figure 17. The top line in

Figure 17 depicts the total PM2.5

mass urban excess for these 13

urban/rural site combination pairs.

The urban mass is derived from the

STN speciation samplers. The urban

sites are arranged to reflect a west-toeast trend as you go from left to right

on the graph. At all locations, total

carbonaceous mass is seen to be the

major contributor to PM2.5 mass,

and, at the western sites, nitrates also

play a role in determining the total

PM2.5 mass increments for the time

period investigated. The average

excess urban mass seen in the eastern

sites is 5 to 8 μg/m3 with carbon

contributing between 3 and 5 μg/m3

to the mass increment. The exception

to this average is the Birmingham,

AL, urban site. This site is paired

with the Sipsy Wilderness rural site

(~100 km away) to estimate urban

excess. Birmingham shows a mass

increment of about 12 μg/m3, with

carbon contributing about 5.0 to 6.5

μg/m3 to the total mass increment.

Birmingham probably has local

(urban) emissions sources that are

contributing to the PM2.5 mass. To

understand why the mass is so much

higher in the urban Birmingham area

compared with the other eastern sites

studied, more work is needed to

investigate how these sources differ

from emissions sources in the other

eastern locations.

National Map of Urban

Excess

The estimated urban excess concentrations are displayed in the national

map shown in Figure 18 for the

selected 13 urban/rural combinations. Table 4 presents these same

findings through summary statistics.

Fresno/PIN

Missoula/MO

SLC/GRB

Tulsa/WIM

St.Louis/3

Birmingham/SI

Indy/LIV

Atlanta/2

Cleveland/MKG

Charlotte/LIG

Richmond/JA

Baltimore/DOS

Bronx/BRI

0

5

10

15

20

μg/m3

PM2.5 Mass

Nitrate

Sulfate TCM (k=1.8)

Crustal

Ammonium

TCM (k=1.4)

Figure 17. Comparison of mass urban increment to chemical species.

Those urban excess numbers that

were less than zero were set equal to

zero in Table 4 (the “minimum”

values for sulfate and crustal concentrations in the “East” and “Overall”

columns). However, the actual numbers, both positive and negative,

were used to compute average concentrations (of urban excess concentrations). 

The significant points and important caveats are as follows:

• The estimate for urban excess

sulfate is invariably very small in

the eastern United States, which is

consistent with the notion that

most sulfates are transported from

regional sources of SO2. This small

estimated urban excess in the East

(0.0-0.5 μg/m3) is attributed at

least in part to sulfur emissions

associated with fuel combustion

from stationary and mobile

sources.

• Nitrates are seen to be in excess 

in the more northern and western

locations, showing a larger local

contribution than sulfates or any

other species except carbon. This

is assumed to reflect local nitrogen

sources (e.g., mobile), nitric acid

from NOx/VOC reactions, and

preferential winter-time nitrate

formation compared to sulfates.

However, more work is needed to

assess the comparability of nitrate

measurements and monitoring

methods between networks. To

that end, a major study is planned

next year by the IMPROVE program. This was initiated, in part,

because there is concern that the

IMPROVE protocol may produce

relatively lower concentrations of

nitrates, so some of the reported

difference may be measurement

related.

• Carbonaceous mass is shown to

have a substantial urban excess

(2.9 to 13.2 μg/m3 when k=1.8). 

It is clearly the largest among 

all reported chemical components

in this “urban excess” analysis. 

It appears to be attributed to local

emissions, with mobile sources as

a possible major contributor.

• Some locations also show a sizeable urban excess of “crustal

material.” The estimation procedure used in the IMPROVE protocol includes the measurement of

iron and other trace elements.
NATIONAL AIR QUALITY AND EMISSIONS TRENDS REPORT, 2003

S22 CHEMICAL SPECIATION OF PM2.5 • SPECIAL STUDIES

Therefore, this difference also

reflects oxidized particulate metals, some of which may be attributed to road dust or industrial

sources in urban areas.

Conclusions

In this work, the local and regional

source contributions of PM2.5 to

urban areas were investigated at 

13 urban locations in the United

States. This was accomplished by

matching urban sites to nearby rural

sites and then comparing the appropriate concentrations of chemical

constituents and mass. Although

2.9 8.1 13.2

0.4 3.5 6.5

0.0 0.9 1.9

0.0 0.4 0.9

Sulfate:

Ammonium:

Nitrate:

Total Carbon Mass

(TCM) (k=1.8):

Crustal:

0.0 0.4 0.8

Fresno

SLC

Missoula

Tulsa

St. Louis

Birmingham

Atlanta

Charlotte

Richmond

Bronx

Baltimore

Cleveland

Indy

Figure 18. National map depicting urban excess by component for 13 example areas.

West (3 sites) East (10 sites) Overall (13 sites)

Chemical Species Min Max Average Min Max Average Min Max Average

Sulfate 0.4 0.9 0.6 0 0.8 0.3 0 0.9 0.3

Estimated Ammonium 0.4 2.3 1.4 0.3 1.1 0.6 0.3 2.3 0.8

Nitrate 1.0 6.5 3.7 0.4 1.5 0.8 0.4 6.5 1.5

Total Carbonaceous

Mass (k=1.4) 4.2 10.5 6.6 2.4 5.4 3.3 2.4 10.5 4.1

Total Carbonaceous

Mass (k=1.8) 5.3 13.2 8.3 2.9 6.7 4.2 2.9 13.2 5.1

“Crustal” -0.1 0.5 0.2 0 0.8 0.2 0 0.8 0.2

Table 4. Minimum, Maximum, and Average Urban Excess in μg/m3 for 13 STN/IMPROVE Combinations

Concentrations are μg/m3.
NATIONAL AIR QUALITY AND EMISSIONS TRENDS REPORT, 2003

SPECIAL STUDIES • CHEMICAL SPECIATION OF PM2.5 S23

2003 SPECIAL STUDIES 

there is uncertainty in the measured

mass and in other measurement protocols, it is clear that carbonaceous

mass is prevalent everywhere (average of 5.1 μg/m3 with k=1.8) and is

the major component of urban excess

at all the sites studied. In the western

sites, the TCM (based on k=1.4)

urban excess varies from 4.5 to 10.5

μg/m3, whereas in the eastern sites,

TCM urban excess is in the range of

2 to 5.4 μg/m3. TCM, based on k=1.8,

varies from a range of 5.3 to 13.2

μg/m3 in the West and to a range of

2.9 to 6.7 μg/m3 in the East. Similarly,

nitrates are prevalent in the urban

excess estimates for the North and

West (2 to 6 μg/m3). Consistent with

the theory that most sulfates are

transported from regional sources of

SO2, the urban excess of this chemical component is invariably very

small in the eastern United States.

These results may be viewed as a

first step in differentiating between

regional and local sources that contribute to PM2.5 mass. More work is

needed in the areas of estimating

regional background associated with

specific urban areas using spatial

analysis, identifying specific emission sources with the estimated

urban excesses using source apportionment techniques, more refined

data analysis that includes meteorological variables, and examination of

the data on finer time resolution to

get to the next and more refined level

of urban excess concentrations. These

will be the subjects of future papers

in this area.

Disclaimer

The views and opinions expressed in

this paper are solely those of the

authors and do not necessarily reflect

those of the U.S. Environmental

Protection Agency.

References

1. Schichtel, B.A.; Husar, R.B.

Aerosol types over the continental

U.S.: spatial and seasonal patterns.

Presented at the A&WMA Conference, Kansas City, MO, 1992; Paper

92-60.07. 

2. Husar, R.B.; Lodge Jr., J.P.;

Moore, J.D. Sulfur in the atmosphere.

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Oxford, 1978.

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7. Turpin, B.; Lim, H-J. Species

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Aerosol Sci. Technology. 2001, 35, 602-

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8. IMPROVE Web Site. http://

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Accessed October 2002.
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