Daily sampling of PM2.5 in Atlanta: Results of the first year of the Assessment of Spatial Aerosol Composition in Atlanta study



[1] A monitoring network was established in metropolitan Atlanta, Georgia, to measure fine particulate mass (PM2.5) and composition for approximately one year, beginning in March 1999. Annual average mass concentrations ranged from 19.3–21.2 μg/m3, maximum 24-hour concentrations were 44.3–51.5 μg/m3, and maximum 1-hour concentrations were 73.3–87.9 μg/m3. Peak mass concentrations occurred typically (but not always) at the Tucker site, which was located approximately 12 miles ENE of downtown. Mass concentrations varied significantly with season and time of day. Approximately 75% of the mass was identified using various chemical speciation analyses. Of the identifiable portion, carbonaceous material, sulfate, ammonium, and nitrate were the largest constituents.

1. Introduction

[2] In 1997, National Ambient Air Quality Standards (NAAQS) for PM2.5 were promulgated by the United States Environmental Protection Agency (USEPA) as a response to a growing number of scientific studies linking elevated fine-particle concentrations with adverse health effects [Federal Register, 1997]. These NAAQS were both short-term (less than 65 μg/m3 over a 24-hour averaging time) and long term (less than 15 μg/m3 for an annual average) in nature, and served to further enhance the effectiveness of the Clean Air Act and protect the public health within an adequate margin of safety. In May of 1999, however, the PM2.5 NAAQS were struck down by a panel of three judges from the U.S. Circuit Court of Appeals for the District of Columbia [1999], two of whom questioned the constitutionality of U.S. Environmental Protection Agency's enforcement of the standards. The circuit court's decision was subsequently reversed by the U.S. Supreme Court [2000]. As the political battle has subsided, there remains a limited, although growing, amount of PM2.5 data, and it is believed that the air quality in a significant number of areas in the country will exceed the standard.

[3] Underscoring the need for the collection of PM data was a recent study in Atlanta [Mulholland et al., 1998]. In summary, the result of that study suggested positive associations between pediatric asthma presentation rates and O3 and PM10 when modeled independently. When modeled simultaneously, however, the associations were not significant, likely due to temporal correlation of the pollutants. Spatial distributions of O3 were estimated, improving the power of the epidemiologic analysis, but insufficient data were available for spatially resolving the PM10 data. In addition, PM2.5 mass concentration data were not collected at that time and no data were available on PM speciation.

[4] Atlanta is a particularly interesting case study since it is one of the fastest growing urban areas in the country and is currently categorized by the USEPA as a “serious” nonattainment area for O3 (USEPA Green Book, available at http://www.epa.gov/oar/oaqps/greenbk/index.html). A major contributor to Atlanta's O3 problem is automobile exhaust, since the city has more daily vehicle miles traveled, VMT, per capita than any other in the country (Atlantic Regional Transportation Fact Book 2000, available at http://www.tripnet.org/GeorgiaStudyMar2000.PDF). Coincidentally, automobile exhaust also plays a significant role in PM2.5 production [Cadle et al., 1998]. The result indicates that Atlanta will experience difficulty meeting a NAAQS for PM2.5, much like the results of studies suggesting similar problems in the western United States [Chow et al., 1996, 1994; California Air Resources Board (CARB), 1991; South Coast Air Quality Management District, 1991; Solomon et al., 1989; Russell and Cass, 1984].

[5] In an effort to study the particulate problem and gain some insight regarding the sources and temporal and spatial patterns of various chemical species, the Assessment of Spatial Aerosol Composition in Atlanta (ASACA) was initiated in 1999. A system of PM2.5 monitors was deployed throughout the city beginning in March, and several detailed laboratory and field experiments were conducted. The purpose of this paper is to summarize the trends in ambient PM2.5 concentrations observed during the first year of the ASACA study, as well as to assess the representativeness of the 1999 EPA supersite for the Atlanta metropolitan area. The ASACA study is planned to continue indefinitely, developing a large database for future health, welfare and model evaluation studies.

2. Experimental

2.1. Sampling Sites

[6] In March 1999, a four-site monitoring network was implemented to investigate PM2.5 distributions in metropolitan Atlanta. The locations and land use characteristics of the sites are shown in Figure 1. Three of the four sites (Fort McPherson, South Dekalb, and Tucker) utilized facilities formerly or currently operated by the Georgia Department of Natural Resources (DNR). The fourth site (Jefferson Street, the location of the 1999 EPA supersite) utilized facilities operated by Atmospheric Research and Analysis (ARA). PM2.5 mass data were collected using continuous instruments at Jefferson Street, Fort McPherson, and Tucker. In addition, PM2.5 data collected by the DNR using a Federal Reference Method (FRM) instrument (Thermo Anderson RAAS 100) were monitored at South Dekalb. Chemical speciation data used in these experiments were collected at Fort McPherson, South Dekalb, and Tucker. ARA collected speciated PM2.5 data at Jefferson Street [Edgerton et al., 2000], although those data are not included in these analyses. All sites except Fort McPherson were collocated with several continuous gas-phase and meteorological instruments operated by ARA and DNR. Monitoring equipment was placed on the roofs of single-story trailers, and sample inlets are located approximately 4 m above ground level. The sites are located at elevations of 300–400 m above sea level.

Figure 1.

ASACA PM2.5 monitoring network. Landuse: JST (urban/commercial), FT (residential, near major highway), SD (residential, near major highway), TU (suburban/commercial).

2.2. Sampling Equipment and Scheduling

[7] Three TEOM® Series 1400ab (Rupprecht and Patashnick, Co. Inc.) continuous mass monitors were deployed beginning in March 1999 at Jefferson Street, Fort McPherson, and Tucker. A Nafion® dryer (PD-1000, Perma Pure, Inc.,) was installed, following the procedures of Hartsell and Edgerton [1998]. In this configuration, operating temperatures were reduced from 50°C to 30°C in order to minimize volatilization of certain particulate species from the filter at elevated temperatures. Data were collected and stored every 30 min. Half-hour periods with measured concentrations less than 0.01 μg were marked as invalid (less than 5% of all data collected were invalid). Regular maintenance was performed, including exchanging the sample filter, leak testing, and flow auditing.

[8] In addition to the TEOMs, a manual, filter-based particle composition monitor (PCM) was deployed beginning in April 1999 at Fort McPherson, South Dekalb, and Tucker. As operated, the PCM (Figure 2) was a 3-channel system, based on the ARA design [Hartsell and Edgerton, 1998], to collect 24-hour (midnight to midnight EST) integrated samples for analysis of ionic, carbonaceous, and metallic species in the PM2.5 size range. The monitor was controlled by a data acquisition system (DAS) that activated sampling, sequenced the filters via solenoid valves and controlled sample flow using mass flow controllers. The DAS made it possible to remove a sample while the instrument continued to run and to maintain a 16.7 L/min flow rate through each channel. Filters were installed one day prior to sampling and were removed one day following. Forty-three field blanks (approximately 5% of samples collected) were obtained during the study period.

Figure 2.

Schematic of Particle Composition Monitor. 1 (10 μm cyclone inlet); 2 (WINS impactor); 3 (filter pack with Teflon filter for quant. of metal species); 4 (solenoid valves); 5 (annular denuder coated with sodium carbonate); 6 (annular denuder coated with citric acid); 7 (mass flow controller); 8 (parallel-plate CIF denuder).

[9] Ambient air was drawn through each channel of the monitor using large vacuum pumps and a sample inlet consisting of a 10 μm cyclone (URG, 2000-30-ENB). The sampled air then flowed through denuders (in two of the channels), for selective removal of gases, a WINS impactor, for providing a 2.5 μm size cut, and finally, the sampling media. The PCMs were mounted to the tops of monitoring shelters, approximately 2.5 m above the ground.

[10] Channels 1 and 2 were used to quantify trace metal species and ions, respectively. In channel 1, the sampled air was passed through a filter holder containing a single Teflon filter (Gelman, #R2PL047, 1μm pore size) for collection of the metal species. Ionic species, including sulfate, nitrate, and ammonium, were collected using channel 2, in which sampled air was passed through two annular denuders in series (upstream of the WINS impactor) coated with sodium carbonate and citric acid, respectively [Hartsell and Edgerton, 1998]. Following the denuders and impactor was a filter holder containing a single nylon filter (Gelman, #66509, 1 μm) used to collect ions. Channel 3 was used for quantification of elemental and organic carbon using a parallel-plate, CIF denuder [Eatough et al., 1993], which was installed upstream of a WINS impactor and filter pack assembly to remove organic gases. Dual quartz filters (Gelman, #7202) followed the impactor to collect carbon-containing species. Material that volatilized from the front filter (Q1) was captured on the backup filter (Q2). The amount volatilized was determined to be less than 5%. All filters were sealed in plastic petri dishes and stored at −10 C until sample analysis.

[11] Regular maintenance of the PCM and its components and flow calibration were performed. Standard operating procedures (SOPs) included cleaning the PM10 inlet (monthly), washing and recoating the annular denuders (approximately bi-monthly), and cleaning the WINS impactor (bi-monthly). To reduce “particle bounce,” two drops of oil impactor oil were applied to a fresh glass fiber filter, which was installed in the impactor well each time the WINS was cleaned. The volumetric flow through each channel of the PCM was calibrated weekly using a BIOS DryCal® DC-1 primary flowmeter.

2.3. Sample Analysis

[12] Concentrations of water-soluble sulfate, nitrate and ammonium, organic and elemental carbon, and 16 trace metal species were determined from filter samples collected using the three PCMs. Ionic species were analyzed using ion chromatography (IC) [Baumann et al., 2002], carbonaceous species were analyzed using thermal optical transmittance (TOT) [Birch and Cary, 1996], and trace metal species were analyzed using inductively coupled plasma-atomic emission spectroscopy (ICP-AES).

[13] Metal species were extracted from the Teflon filters by first wetting each filter with 200 μL of ethanol. The wetted filters were then placed in 50 mL centrifuge tubes, into which was pipetted 12 mL of extracting acid (1.03 M HNO3 and 2.23 M HCl) and 28 mL of distilled, deionized water. Next the tubes were capped and heated at 70°C for 90 min in an ultrasonic bath. The tubes were allowed to cool before being centrifuged for 20 min at 2500 rpm. Finally, the extract was decanted into a 16 mm diameter borosilicate glass test tube, capped, and stored until analysis. The extraction method is described further by Cummings et al. [1984].

[14] The ICP-AES instrument (Thermo Jarrell Ash, Model 61E) was calibrated daily using multielement standards and instrument blanks. The instrument was run using an autosampler, which allowed for the analysis of up to 192 samples (including standards and blanks) per day. Results were reported as μg/mL of solution and ambient concentrations were determined by multiplying the reported value by the sample extraction volume and dividing by the total volume of air drawn through the filter. Further details of the procedure are discussed by Butler [2000].

[15] A summary of the instrument lower quantifiable limits (defined as twice the uncertainty of the field blank), filter blank values, and precision of the sample analyses is shown in Table 1. Duplicate or replicate analysis of 46 of the filters (approximately 5%) for each species was performed to determine precision, which is defined as the average coefficient of variation obtained from the measurement pairs. The filter blank value for each species is the average of all field blanks collected.

Table 1. Summary of Analytical Measurements for the Determination of Chemical Composition of PM2.5 in Atlanta During the First Year of the ASACA Study
SpeciesAnalysis MethodLower Quantifiable Limit, μg/m3Filter Blank (avg ± SD), μg/m3Analytical Precision,%
Sulfate (SO42−)IC0.0410.021 ± 0.014.2
Ammonium (NH4+)IC0.0430.02 ± 0.0045.9
Nitrate (NO3)IC0.0410.021 ± 0.015.1
Organic carbon (OC)TOT0.0760.07 ± 0.19.6
Elemental carbon (EC)TOT0.0760.01 ± 0.017.7
Silicon (Si)ICP-AES0.0160.01 ± 0.00199.2
Aluminum (AL)ICP-AES0.0910.01 ± 0.002310.2
Calcium (Ca)ICP-AES0.0350.01 ± 0.001110.0
Iron (Fe)ICP-AES0.0260.01 ± 0.0017.8
Magnesium (Mg)ICP-AES0.00320.01 ± 0.00098.0
Nickel (Ni)ICP-AES0.00180.01 ± 0.00198.1
Vanadium (V)ICP-AES0.00010.01 ± 0.0036
Cadmium (Cd)ICP-AES0.000190.01 ± 0.00539.9
Chromium (Cr)ICP-AES0.00140.01 ± 0.00086.4
Copper (Cu)ICP-AES0.00020.01 ± 0.00216.7
Manganese (Mn)ICP-AES0.00410.01 ± 0.00096.4
Potassium (K)ICP-AES0.0160.01 ± 0.001310.2
Titanium (Ti)ICP-AES0.00520.01 ± 0.00786.0
Zinc (Zn)ICP-AES0.0030.01 ± 0.00026.3
Lead (Pb)ICP-AES0.00170.01 ± 0.001616.8
Selenium (Se)ICP-AES0.0010.01 ± 0.000210.7

3. Results and Discussion

3.1. Material Balance of Chemical Species

[16] Table 2 summarizes maximum and average concentrations of PM2.5 and its chemical constituents at the three sites. The most abundant species were sulfate, organic carbon, ammonium, and nitrate. Trace metals contributed less than 1% to the total fine mass on average, but were occasionally important. The monitoring sites were similar regarding the maximum and average values observed for most species.

Table 2. Summary of Continuous and Integrated PM2.5 Measurements in Atlanta During the First Year of the ASACA Study
Chemical SpeciesFort McPhersonSouth DekalbTuckerJefferson Street
Average, μg/m3Maximum, μg/m3Average, μg/m3Maximum, μg/m3Average, μg/m3Maximum, μg/m3Average, μg/m3Maximum, μg/m3
1-hour mass19.390.021.287.921.097.3
24-hour mass19.545.119.9140*21.251.521.648.6
Sulfate (SO42−)
Ammonium (NH4+)
Nitrate (NO3)
Organic carbon (OC)6.717.86.820.96.416.4
Elemental carbon (EC)
Silicon (Si)0.0440.830.0764.050.0280.81
Aluminum (Al)
Calcium (Ca)
Iron (Fe)0.0370.930.0430.790.0332.6
Magnesium (Mg)0.0140.160.0120.20.0140.16
Nickel (Ni)0.00270.160.00460.320.00360.39
Vanadium (V)0.00050.00920.00090.0670.00050.0098
Cadmium (Cd)0.00030.00560.0010.190.000240.0052
Chromium (Cr)0.00150.220.00310.220.00350.52
Copper (Cu)0.00190.0620.00280.250.00210.18
Manganese (Mn)0.00770.140.00630.130.00670.14
Potassium (K)
Titanium (Ti)0.00970.160.00930.340.00780.16
Zinc (Zn)0.0150.0880.0150.290.0130.24
Lead (Pb)0.00930.0630.0172.30.00820.059
Selenium (Se)0.0730.970.072.70.0591.02

[17] The sum of the individual chemical concentrations for PM2.5 should be less than or equal to the 24-hour averaged continuous mass measurements. This sum includes chemicals quantified from the nylon (SO42−, NO3, NH4+), Teflon (trace metals), and quartz (OC, EC) filters. To account for metal oxides present in crustal material, Al2O3, SiO2, CaO, and Fe2O3 were determined as [1.89*Al], [2.14*Si], [1.4*Ca], and [1.43*Fe] using the method of Solomon et al. [1989]. Oxygen and hydrogen associated with OC (which is not quantified using the TOT method) were estimated by applying a factor of 1.4 to the measurement to obtain “organic material” (1.4*OC = OM). This multiplicative factor is an estimate, generally ranging from 1.2–2.0. Application of a larger factor yields a more “closed” material balance.

[18] Figures 3a, 3b, and 3c show scatterplots of PM2.5 sum of species versus 24-hour mass for each of the three sites. The plots contain a solid line indicating the one-to-one relationship and a dashed line representing the linear regression of the data. The slopes of the regression lines are less than unity, implying that the sum of species calculations slightly underestimate the total mass measurements, and account for approximately 86% of the variance at Fort McPherson and Tucker. As mentioned previously, PM2.5 mass was measured using a FRM sampler at South Dekalb by the Georgia DNR. The FRM instrument is significantly different from the continuous TEOM®, although Allen et al. [1997] found reasonable agreement. Still, several potential outliers are indicated in Figure 3c that affect the regression. The reconstructed mass measurements accounted for approximately 71% of the variance in the FRM measurements at South Dekalb. Several data points at each site are above the one-to-one line, indicating reconstructed mass values greater than the respective FRM and TEOM measurements. This situation may result from positive or negative artifact bias due to gas adsorption (PCM) and volatility (TEOM, FRM), respectively.

Figure 3a.

Scatterplot of PM2.5 sum of species versus 24-hour mass at Fort McPherson. Solid line indicates one-to-one relationship; dashed line indicates linear regression of data.

Figure 3b.

Scatterplot of PM2.5 sum of species versus 24-hour mass at South Dekalb, with potential outliers identified. Solid line indicates one-to-one relationship; dashed line indicates linear regression of data.

Figure 3c.

Scatterplot of PM2.5 sum of species versus 24-hour mass at Tucker. Solid line indicates one-to-one relationship; dashed line indicates linear regression of data.

[19] The average PM2.5 chemical composition at each site for the entire study period is shown in Figure 4. There was little spatial variation in chemical species on average, but moderate variation was observed on given days, resulting from local meteorological or source effects. Average meteorological data collected by the DNR at the South Dekalb and Tucker sites during the study period are shown in Table 3. Figure 5 shows the major species on one of the highest PM2.5 days (8/4/99), which experienced a 24-hour concentration of 49.6 μg/m3, averaged over all three sites. The high concentrations occurring on that day indicate a very large contribution of secondary processes, since sulfate, nitrate, and ammonium constituted approximately 45% of the PM2.5 mass on average. In addition, if 50–60% (or more) of the organic carbon resulted from secondary processes, as was calculated for August by Butler [2000] using the methods of Turpin and Huntzicker [1991] and Castro et al. [1999], then as much as 60% of the total mass may be attributed to secondary processes.

Figure 4.

Material balance on the chemical composition of annual average PM2.5 collected in metropolitan Atlanta, Georgia, from April 1999 to February 2000. Values indicate annual arithmetic mean concentration in μg/m3.

Figure 5.

Material balance on the chemical composition of PM2.5 collected in metropolitan Atlanta, Georgia, on August 4, 1999. Values indicate 24-hour concentration in μg/m3.

Table 3. Average Meteorological Data Collected by the DNR at the South Dekalb and Tucker Sites, March 1999 Through February 2000
ParameterAnnual AverageSpring 1999Summer 1999Autumn 1999Winter 2000
  • a

    Average of South Dekalb and Tucker measurements.

  • b

    Measured at Tucker.

Temperature, degrees C17.215.725.116.98.7
Relative humidity, %
Precipitation, inchesa0.0600.0450.0760.0700.047
Wind speed, m/s1.
Wind direction, degrees Na205.8208.5205.7194.9217.8
Solar radiation, W/m2b174.7193.0210.8143.9124.5

[20] The seasonal contributions of all species to total PM2.5 mass are shown in Figures 6a, 6b, and 6c. The annual trends in chemical composition at each site are virtually the same, in that the major constituents, except nitrate, were most abundant during warmer, more photochemically active months and lowest during the colder months. This behavior supports the notion that secondary aerosol formation was a significant source of PM2.5. Nitrate peaked during winter, presumably owing to its gas/particle equilibrium temperature dependence, the presence of shallower mixing heights during cold months, or lower amounts of sulfate. Likewise, the metal species peaked during the winter months as a result of the shallower mixing heights. A second peak for elemental carbon (EC) also occurred in the winter months (August 1999 had high EC values). Figure 7 shows a seasonal plot of average carbon monoxide concentration measured by the DNR at two metro Atlanta locations (Roswell Road, 17 miles north of downtown; Dekalb Technical College, 10 miles east), which reinforces the presence of shallower wintertime mixing heights, since CO emissions are relatively invariant with season.

Figure 6a.

Seasonal mass balance of PM2.5 species at Fort McPherson (April 1999 to February 2000).

Figure 6b.

Seasonal mass balance of PM2.5 species at South Dekalb (April 1999 to February 2000). PM2.5 mass data collected by the Georgia DNR using an FRM instrument.

Figure 6c.

Seasonal mass balance of PM2.5 species at Tucker (April 1999 to February 2000).

Figure 7.

Seasonal trend in carbon monoxide concentration in Atlanta during the first year of the ASACA study. Values indicate averages from Roswell Road (north of downtown) and Dekalb Tech (east of downtown) monitoring locations.

[21] The difference between the total mass measurements and the sum of species calculations (i.e., unidentifiable material, UM) was also investigated for seasonal trends. Figure 8 displays the variation at each site and indicates little seasonal dependence, although Tucker showed strong peaks in October 1999 (7.7 μg/m3) and February 2000 (9.8 μg/m3), and Fort McPherson was less than zero in December. Negative UM indicates that the sum of species calculation exceeds the total mass measurement, which may result from volatilization artifacts that bias the mass measurements low. In addition, UM is directly affected by the factor applied to OC to account for oxygen and hydrogen [Turpin and Huntzicker, 1991], and may suggest that slight adjustments to the factor may be required during an annual cycle under certain conditions.

Figure 8.

Seasonal variation in unidentifiable material (UM). * Indicates incomplete data for PM2.5 mass.

3.2. Diurnal Variations of PM2.5 Mass

[22] A significant benefit of continuous instruments is the ability to resolve the concentration fluctuations associated with short-term variability in emissions and meteorology. In Figures 9a, 9b, 9c, and 9d, diurnal variations in PM2.5 are presented for each of the sites, aggregated seasonally. The Jefferson Street and Tucker sites behaved similarly, in general, during the spring (Figure 5.7); however, JST showed a more marked difference between its highest (19.9 μg/m3, 7–8:00 AM EST) and lowest (11.1 μg/m3, 5–6:00 PM) mass concentrations. The levels at Tucker, which is located further from major freeways and downtown, peaked later in the morning (21.6 μg/m3, 9–10:00 AM) and decreased less dramatically in the afternoon (15.8 μg/m3, 2–3:00 PM). The vector-averaged wind from the southwest likely influenced the values at Tucker as well. The morning peaks were likely associated with rush hour traffic patterns, but there was no corresponding afternoon/evening peak. Both sites experienced an increasing overnight trend due to the nighttime temperature inversion.

Figure 9a.

Diurnal variations in PM2.5 mass at two metropolitan Atlanta monitoring stations during the first year of the ASACA study (Spring 1999, March–May).

Figure 9b.

Diurnal variations in PM2.5 mass at three metropolitan Atlanta monitoring stations during the first year of the ASACA study (Summer 1999, June–August).

Figure 9c.

Diurnal variations in PM2.5 mass at three metropolitan Atlanta monitoring stations during the first year of the ASACA study (Autumn 1999, September–November).

Figure 9d.

Diurnal variations in PM2.5 mass at three metropolitan Atlanta monitoring stations during the first year of the ASACA study (winter 1999–2000, December–February).

[23] Unlike spring, the patterns observed at all three sites were somewhat different from each other during the summer months, as shown in Figure 10, with the only similarities being morning (28.6–34.4 μg/m3) and overnight (29.5–36.1 μg/m3) peaks.

[24] The hour-averaged concentrations at all sites remained elevated in autumn, as shown in Figure 11, with more consistent trends observed among the three sites. Each site experienced concentration peaks in the morning, a trough in the afternoon, and a second peak overnight. The patterns observed at Fort McPherson and Tucker were quite similar, with Tucker higher at every hour by 1–4 μg/m3. Jefferson Street experienced a more marked difference between its peak and low values than the other sites. In addition, Jefferson Street was also different from the others in that its nighttime peak (31.8 μg/m3, 8–9:00 PM) exceeded its morning peak (27.3 μg/m3, 7–8:00 AM).

[25] The patterns observed during the winter for the three sites were again quite similar, as shown in Figure 12. The morning peak observed at each site was roughly twice as high as the afternoon low.

[26] The PM2.5 mass data provide a strong indication of homogeneity throughout the ASACA domain. Local differences due to highway proximity and wind direction were observed, but overall, values obtained at Jefferson Street (the supersite) were representative.

4. Summary and Conclusions

[27] Results from the first year of an ongoing study to elucidate PM2.5 distributions in Atlanta for health, welfare and air quality model evaluation found high concentration levels at the four sites. Annual PM2.5 mass concentrations for the period March 1999 through February 2000 were 19.3 μg/m3 at Fort McPherson, 19.9 μg/m3 at South Dekalb, 21.0 μg/m3 at Jefferson Street, and 21.2 μg/m3 at Tucker. Each site exceeded the annual NAAQS of 15 μg/m3, although two more years of data are required for comparison. Only the South Dekalb site violated the daily NAAQS of 65 μg/m3.

[28] Composition was primarily sulfate, related ammonium and OC, with significantly smaller amounts of EC and metal species. Most species peaked during the summer months; however, nitrate, metals and EC showed some enhancement in the winter due to lower inversion heights.

[29] Diurnally, there were discernible early morning and late night peaks that corresponded to rush-hour traffic patterns and inversion heights, respectively. Light winds, with vector-averaged direction from the southwest, contributed to the occurrence of slightly higher concentrations at Tucker (northeast), on average.

[30] Spatially, metropolitan Atlanta was found to be homogeneous regarding PM2.5 mass and chemical composition. Thus the supersite location is representative of the area, though its location near downtown appears to lead to a slightly greater impact from morning rush-hour traffic.


[31] This project was funded by Georgia Power and the authors would like to thank John Jansen for his patience and assistance. The authors would also like to thank Karsten Baumann and his team, Eric Edgerton, Ben Hartsell, Jim Pearson, Bill Murphey, and G. Zhu for their tremendous support in various aspects of the project. The authors gratefully acknowledge the contributions of several current and former Georgia Tech students who assisted in sampling and analysis: Helena Park, Herman Holm, Melissa Antoine, Wesley Parkmond, Phillip Nyirabu, Jeff Crowley, and Sara Henry.