NATIONAL AIR QUALITY AND EMISSIONS TRENDS REPORT, 2003

SPECIAL STUDIES • CHEMICAL SPECIATION OF PM 2.5 S 132003 SPECIAL STUDIES



## AbstractData from the Interagency Monitoring of PROtected Visual Environment (IMPROVE) and the Speciation Tr ends Network (STN) are used to analyze the chemical composition of PM 2.5 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 PM 2.5 mass in different regions of the United States. 

## Introduction

W

ith the promulgation of the new Particulate Matter National Ambient Air Quality Standards (PM 2.5 NAAQS), all future designatednonattainment areas and surrounding regions may need to reduce emission of fine particles and their precursors to permit those areas toattain the NAAQS. Efficient air quality management requires knowingwhich sources contribute to the problem and how much. Determining PM 2.5 source contributions is complicated due to the fact that often half or more of the PM 2.5 mass is composed of secondarily formed species, 1 hiding their point of origin. In addition, PM 2.5 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

r espective contributions to the following major urban areas: Fresno, CA, Missoula, MT, Salt Lake City, UT, T

ulsa, OK, St. Louis, MO, Birmingham, AL, Indianapolis, IN, Atlanta, GA, Cleveland, OH, Charlotte, NC, Richmond, VA, Baltimore, MD, and New York, NY. This is accomplishedby computing urban excess concentrations—by comparing annual concentrations of PM 2.5 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, severalgraphics are used to show the chemical species that make up PM 2.5 mass across the United States.


### Data Sources Ambient monitoring data from the PM 2.5 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 PM 2.5 species concentrations across the United States. The PM 2.5 STN was established by regulation 3 and is a companion network to the mass-based Federal Reference Method (FRM) network implemented in support of the PM 2.5 NAAQS. EPA established the STNnetwork to provide nationally consistent speciated PM 2.5 data for the assessment of trends at representativesites in urban areas across the country. As part of a routine monitoring program, the STN quantifies mass concentrations and PM 2.5 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 visibilityimpairment within federally designated Class I areas. 4 Ambient aerosolmass concentrations have been measured under the IMPROVE programto 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

ru ral areas that deploy the exact same aerosol monitoring protocol. As with the STN, the IMPROVE network also quantifies mass concentrations and PM 2.5 constituents.Both the STN and IMPROVE programs employ a 1-in-3-day sampling protocol. 

# Chemical Speciation of PM 2.5 

# in Urban and Rural Areas

V

enkatesh 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 27711NATIONAL AIR QUALITY AND EMISSIONS TRENDS REPORT, 2003

S 14 CHEMICAL SPECIATION OF PM 2.5 • 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

r eferences to an annual average will

r efer to these 12 months. Out of thepossible 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 moreof 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 characterizations 5 of the composition of ambient PM 2.5 , the following “relevant” chemical species that make up PM 2.5 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 networksemploy a 1-in-3-day sampling protocol, the 50% completeness criterion amounts to there being 15 or more observations per quarter. No further

r equirement 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 J anuary 2002, February 2002, March 2001

2 April 2001, May 2001, June 2001

3 J uly 2001, August 2001, September 2001
4 O ctober 2001, November 2001, December 2001 T

able 1. Quarter DefinitionsNATIONAL AIR QUALITY AND EMISSIONS TRENDS REPORT, 2003

SPECIAL STUDIES • CHEMICAL SPECIATION OF PM 2.5 S 152003 SPECIAL STUDIES

The variances in their analyticaland sampling procedures effectively result in two different operational definitions of organic and elemental carbon. 5,6 For this reason, organic (OC) and elementalcarbon (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 IMPROVEprogram are automatically blankcorrected using an appropriate blank correction factor. 6 T able 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 northe total gravimetric mass is blankcorrected in this analysis).



### Urban PM 2.5 

### 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

r ural monitoring data. These estimates are thus a first approximation of local and regional contributions of PM 2.5 mass and its chemical constituents to the urban areas investigated. Although strong regionalsimilarity 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 andanalytical methods, there are differences in the species they measure and the operational protocols they employ. To put aerosol compositiondata 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 PM 2.5 , network-wide IMPROVEmeasurements are currently lacking in this area. Ammonium concentrations are thus estimated for IMPROVE (and for comparison purposes, for STN as well) from sulfate (SO 4 ) and nitrate measurements, assuming (1) all sulfates are ammonium sulfate (NH 4 SO 4 ), and (2) all nitrates are ammonium nitrate. For now, the inter-networkmeasure 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 ruraland urban ammonium concentrations.

• Sulfate : The IMPROVE program

estimates sulfate concentrations asthree times the sulfur concentration, whereas with the STN program, sulfate concentrations are used as measured. In thisanalysis, 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, m 3 Factor (μg/m 3 )
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 T

able 2. Organic Carbon (OC) Blank Correction Factors

Soil: The soil component of PM 2.5 (“crustal” material) was computed using the f ollowing formula, which is the same as that employed by the IMPROVE program 8 : PM 2.5 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

S 16 CHEMICAL SPECIATION OF PM 2.5 • SPECIAL STUDIES through 5, which show spatiallyaveraged 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 excessseen for the specific chemical constituents of PM 2.5 . One way to

r emove this effect and standardizethe 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 todetermine 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 PM 2.5 at the rural sites is generally representative of the upwind regional concentrations and is not significantly influenced by nearby emissions and that the

r egional sources (including upwind urban areas) have the same impacton 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 PM 2.5 S 172003 SPECIAL STUDIES



### Choice of Urban and Rural Sites Figure 6 summarizes the urban and

r ural 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 midportion of the United States (Cleveland, OH, Indianapolis, IN, St. Louis, MO, Tulsa, OK, and Birmingham, AL), and three urban sites in the W

est (Fresno, CA, Salt Lake City, UT, and Missoula, MT). These werechosen 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 PM 2.5 mass concentrations greater than 15 μg/m 3 , which is the level of the annual PM 2.5NAAQS. IMPROVE sites with complete data were chosen for assumed r epresentativeness of upwind background concentrations. In the case of matching the urban Atlanta and St. Louis sites to nearby rural sites, asingle available rural site with complete data was not judged to be sufficiently representative of the requisite r equirement, and therefore a multiple site approach (as explained above) was employed. T

able 3 summarizes all the STNand IMPROVE sites for their elevation and separation distances. For the analyses of urban excess, all

F resno Indy St.Louis

T

ulsa 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)

F resno, 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

T

ulsa, 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 T

able 3. STN and IMPROVE Site ParticularsNATIONAL AIR QUALITY AND EMISSIONS TRENDS REPORT, 2003

S 18 CHEMICAL SPECIATION OF PM 2.5 • SPECIAL STUDIESurban/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

r epresent 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 concentrationswere adjusted once again to the elevation corresponding to the matched urban site. Except for the St. Louis and Atlanta STN monitors and theirpairing 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 sitewas compared to the averaged concentration(s) derived from the two IMPROVE sites shown in Table 3. 

## Elevation Effects on PM 2.5 

## Concentrations As mentioned previously, all the IMPROVE data were adjusted for elevation (based on temperature andbarometric 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, thiselevation 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/PINNMissoula/MONTSLC/GRBATulsa/WIMOSt.Louis/3 SitesBirmingham/SIPSIndy/LIVOAtlanta/2 SitesCleveland/MKGOCharlotte/LIGORichmond/JARIBaltimore/DOSOBronx/BRIG -1 -0.5

0 0.5

1 1.5

2μg/m3

Unadjusted Sulfate Urban Elevation-Adjusted Sulfate Urban A

vg Unadjusted: 0.447 μ g A

vg Adjusted: 0.334 μ g Figure 7

. Effect of evaluation on rural sulfate concentrations.Fresno/PINNMissoula/MONTSLC/GRBATulsa/WIMOSt.Louis/3 SitesBirmingham/SIPIndy/LIVOAtlanta/2 SitesCleveland/MKGOCharlotte/LIGORichmond/JARIBaltimore/DOSOBronx/BRIG 0 0.5

1 1.5

2 2.5μg/m3

Unadjusted Ammonium (estimated) Urban Increment Elevation-Adjusted Ammonium (estimated) Urban A

vg Unadjusted: 0.831 μ g A

vg Adjusted: 0.787 μ g Figure 8

. Effect of evaluation on rural ammonium concentration.Fresno/PINNMissoula/MONTSLC/GRBATulsa/WIMOSt.Louis/3 SitesBirmingham/SIPSIndy/LIVOAtlanta/2 SitesCleveland/MKGOCharlotte/LIGORichmond/JARIBaltimore/DOSOBronx/BRIG 0 1 2 3 4 5 6 7μg/m3

Unadjusted Nitrate Urban Increment Elevation-Adjusted Nitrate Urban Increment A

vg Unadjusted: 1.40 μ g A

vg Adjusted: 1.38 μ g Figure 9

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

SPECIAL STUDIES • CHEMICAL SPECIATION OF PM 2.5 S 192003 SPECIAL STUDIESFresno/PINNMissoula/MONTSLC/GRBATulsa/WIMOSt.Louis/3 SitesBirmingham/SIPSIndy/LIVOAtlanta/2 SitesCleveland/MKGOCharlotte/LIGORichmond/JARIBaltimore/DOSOBronx/BRIG 0 5 10 15 20 25 30μg/m3 Urban Rural Figure 12

. Urban excess for total PM 2.5 gr a vimetric mass. 

## Urban Increments of PM 2.5 

## Mass and the Chemical Species Urban sites were paired with matched rural sites as listed in Table3, and the annual average concentrations were calculated for both the urban sites and the companion ruralsite(s). All rural values reflect elevation-adjusted values. These averaged

ru ral concentrations were subtractedfrom 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

r epresents 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 PM 2.5 mass during the time period investigated. The largest urban increment in PM 2.5 mass is seen to be at the Fresno, CA, site, with an average excess of about 18 μg/m 3 . 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/m 3 total PM 2.5 mass. Although this result suggests that there are more local sources influencing urban PM 2.5 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 PM 2.5 sources throughout theFresno/PINNMissoula/MONTSLC/GRBATulsa/WIMOSt.Louis/3 SitesBirmingham/SIPSIndy/LIVOAtlanta/2 SitesCleveland/MKGOCharlotte/LIGORichmond/JARIBaltimore/DOSOBronx/BRIG 0 5 10 15μg/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/PINNMissoula/MONTSLC/GRBATulsa/WIMOSt.Louis/3 SitesBirmingham/SIPSIndy/LIVOAtlanta/2 SitesCleveland/MKGOCharlotte/LIGORichmond/JARIBaltimore/DOSOBronx/BRIG -0.5 0 0.5

1 1.5μg/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

S 20 CHEMICAL SPECIATION OF PM 2.5 • SPECIAL STUDIES Sulfate Ammonium Nitrate TCM (k=1.8) Crustal 0 2 4 6 8 10μg/m3 Rural Urban Figure 15

. Urban excess at St. Louis, MO. San Joaquin Valley. In general, the total excess mass ranges from 4 to16 μg/m 3 , with the West generally showing more mass urban excess than the East. On average, the urban excess in PM 2.5 mass for the investigated 13 site combinations is seen to be about 8 μg/m 3

. 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 Y

ork, NY, and Charlotte, NC—see T

able 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 ruralconcentrations by chemical component on the urban chemical component concentrations, the example stacked bar charts (Figures 13-16) show that the estimated regionalbackground 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 themajor components. Urban increments of TCM are seen to range from 13 μg/m 3 at the Fresno, CA, location to about 3 to 4 μg/m 3 at the other three locations. Similarly, nitrate urban excess is seen to be 6.5 μg/m 3 at the Fresno, CA, location and is in the 0.5 to 1.3 μg/m 3 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 20μg/m3 Rural Urban Figure 13

. Urban excess at Fresno, CA.

Sulfate Ammonium Nitrate TCM Crustal 0 2 4 6 8 10μg/m3 Rural Urban Figure 14

. Urban excess at Charlotte, NC.

Sulfate Ammonium Nitrate TCM (k=1.8) Crustal 0 2 4 6 8 10μg/m3 Rural Urban Figure 16

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

SPECIAL STUDIES • CHEMICAL SPECIATION OF PM 2.5 S 212003 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 PM 2.5 mass urban excess for these 13 urban/rural site combination pairs. The urban mass is derived from the STN speciation samplers. The urbansites 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 PM 2.5 mass, and, at the western sites, nitrates also play a role in determining the total PM 2.5 mass increments for the time period investigated. The average excess urban mass seen in the eastern sites is 5 to 8 μg/m 3 with carbon contributing between 3 and 5 μg/m 3 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/m 3 , with carbon contributing about 5.0 to 6.5 μg/m 3 to the total mass increment. Birmingham probably has local (urban) emissions sources that are contributing to the PM 2.5 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 theselected 13 urban/rural combinations. Table 4 presents these same findings through summary statistics.Fresno/PINMissoula/MOSLC/GRBTulsa/WIMSt.Louis/3Birmingham/SIIndy/LIVAtlanta/2Cleveland/MKGCharlotte/LIGRichmond/JABaltimore/DOSBronx/BRI 0 5 10 15 20μg/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

r egional sources of SO 2 . This small estimated urban excess in the East (0.0-0.5 μg/m 3 ) is attributed at least in part to sulfur emissions associated with fuel combustion from stationary and mobile

sources. • N itrates 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 NO x /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 plannednext year by the IMPROVE program. This was initiated, in part, because there is concern that the IMPROVE protocol may produce

r elatively lower concentrations of nitrates, so some of the reported difference may be measurement

r elated.

• Carbonaceous mass is shown to

have a substantial urban excess (2.9 to 13.2 μg/m 3 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 “crustalmaterial.” 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

S 22 CHEMICAL SPECIATION OF PM 2.5 • SPECIAL STUDIES Therefore, this difference also

r eflects 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 PM 2.5 to urban areas were investigated at 13 urban locations in the United States. This was accomplished by matching urban sites to nearby ruralsites 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:

T

otal Carbon Mass (TCM) (k=1.8):

Crustal:

0.0 0.4 0.8 F resno SLC Missoula

T ulsa St. Louis

Birmingham
Atlanta Charlotte Richmond Bronx

Baltimore Cleveland Indy Figure 18

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

W

est (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

T

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

T

otal 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 T

able 4. Minimum, Maximum, and Average Urban Excess in μg/m 3 f or 13 STN/IMPROVE Combinations Concentrations are μg/m 3

.NATIONAL AIR QUALITY AND EMISSIONS TRENDS REPORT, 2003

SPECIAL STUDIES • CHEMICAL SPECIATION OF PM 2.5 S 232003 SPECIAL STUDIES

there is uncertainty in the measuredmass and in other measurement protocols, it is clear that carbonaceousmass is prevalent everywhere (average of 5.1 μg/m 3 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/m 3 , whereas in the eastern sites, TCM urban excess is in the range of 2 to 5.4 μg/m 3 . TCM, based on k=1.8, varies from a range of 5.3 to 13.2 μg/m 3 in the West and to a range of 2.9 to 6.7 μg/m 3 in the East. Similarly, nitrates are prevalent in the urban excess estimates for the North and We st (2 to 6 μg/m 3 ). Consistent with the theory that most sulfates are transported from regional sources of SO 2 , 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

r egional and local sources that contribute to PM 2.5 mass. More work is needed in the areas of estimating

r egional background associated with specific urban areas using spatialanalysis, identifying specific emission sources with the estimatedurban excesses using source apportionment techniques, more refineddata 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. In Proceedings of the International Symposium, Dubrovnik, Yugoslavia
, 7-14 September 1977 ; Pergamon Press: Oxford, 1978.
3. U.S. Environmental ProtectionAgency, Office of Air Quality Planning and Standards, Emissions, Modeling, and Analysis Division, Monitoring and Quality Assurance Group. Particulate Matter (PM2.5) Speciation Guidance, Final Draft, Edition 1 ; October 17, 1999.4. Malm, W.C.; Sisler, J. F.; Huffman, D.; Edred, R. A.; Cahill, T. A.Spatial and seasonal trends in particle concentration and optical extinction in the United States. J. Geophys. Res

. 1994 , 99, 1347-1370.
5. U.S. National Air Quality and Emissions Trends Report, 1999 ; EPA454/R-01-004; Research Triangle Park, NC, March 2001. 6. Dimmick, F. Recent analysis of PM 2.5 Speciation Data with Emphasis on Carbonaceous Mass. EPA Memorandum, October 3, 2002. 7. Turpin, B.; Lim, H-J. Species contributions to PM 2.5 mass concentrations: revisiting common assumptions for estimating organic mass. Aerosol Sci. Technology

. 2001 , 35, 602- 610. 8. IMPROVE Web Site. http:// vista.cira.colostate.edu/improve/

. Accessed October 2002.NATIONAL AIR QUALITY AND EMISSIONS TRENDS REPORT, 2003

S 24 CHEMICAL SPECIATION OF PM 2.5 • SPECIAL STUDIES