IV. OCCURRENCE, COMPOSITION, AND SOURCES OF PARTICULATE MATTER AND ITS PRECURSORS
The particulate matter in air consists of tiny bits of airborne liquid or solid matter that are either: 1) emitted directly into the air (primary particles), or 2) are formed in the atmosphere (secondary particles) by a wide variety of photochemical, condensation, and other atmospheric processes. The primary and secondary particles have a wide variety of environmental effects that range from direct impacts on human and animal health to regional haze that decreases visibility at airports and scenic vistas in wilderness areas.
SOS was selected by EPA to implement the Agency's first PM Program field research effort. The Atlanta Supersite Project consisted of a one-month intensive field program to compare advanced methods for measurement of PM2.5 mass, chemical composition (including single particle composition) in real time, and aerosol precursor species. The Project was funded by EPA through a cooperative agreement with SOS. It included intercomparisons of results from filter-based time-integrated aerosol measurements, and continuous or semi-continuous measurement of mass and PM components and precursors. Special attention was paid to the semi-volatile PM constituents because of the analytical problems their on-filter volatilization posed.
The Atlanta Supersite Project took place during the month of August 1999 at the Jefferson Street Site near downtown Atlanta. This same site was operated since 1998 as part of the Southeastern Aerosol Research and Characterization Study (SEARCH), the Aerosol Research Inhalation Epidemiology Study (ARIES), and the Assessment of Spatial Aerosol Composition in Atlanta (ASACA) - all three of which were affiliated with SOS but funded by EPRI, the Southern Company, and the Georgia Power Company.
The photochemical processes that lead to formation of the secondary aerosols within PM2.5 are essentially the same as those that produce ozone, except that they also include photooxidation of SO2 and VOC into condensable products. Thus, the photochemical processes that lead to accumulation of ozone are very closely related to those that form PM2.5. Unfortunately, however, air quality management approaches aimed at decreasing ozone accumulation in the air sometimes lead to increased accumulation of PM2.5. Furthermore, management approaches aimed at decreasing PM2.5 sometimes lead to increased accumulation of ozone. This strategy-conflict problem is not confined to the ozone and PM2.5 problems, it exists among all photochemical pollution problems - namely, ozone, NO2, PM, acid deposition, greenhouse effects, stratospheric ozone depletion, and secondary toxic pollution.
PM occurrence and characterization efforts, using methods well characterized during the Atlanta Supersite Program, also were conducted in Anderson, South Carolina, during SOS' Nashville '99 field research program, and in connection with both the Texas 2000 Air Quality Study in the Houston-Galveston area, and in southeastern Texas through the Texas Supersite Program.
A. Occurrence and Composition of Ambient PM (PMC)
PMC1. In Atlanta, hourly PM2.5 data from the 1999 Atlanta Supersite Program indicated two types of events - morning peaks dominated by carbonaceous material and afternoon events dominated by sulfate. Carbon and sulfate accounted for ~75% of aerosol mass during these peak events. Nitrate concentrations were generally low. However, the hourly data clearly indicated the temporal nature of nitrate, with nitrate concentrations peaking in the early morning before sunrise, when temperature was at its minimum (Weber et al., 2003b).
PMC2. In Atlanta, significant semi-volatile organic material was present in PM2.5 particles collected during the Atlanta Supersite Experiment (Solomon et al., 2003b).
PMC3. In Atlanta, the composition of the ultrafine particles less than 100 nm particles was dominated by carbon compounds. The major composition classes (expressed as percentage of particle mass) were: organic carbon (~74%), potassium (~8%), iron (~3%), calcium (~2%), nitrate (~2%), elemental carbon (~1.5%), and sodium (~1%) (Solomon et al., 2003b).
PMC4. In Atlanta, the total mass of ultrafine particles (<10 nm) was higher during traffic rush hours, while larger diameter particles (10-100 nm and 100-2000 nm) had higher concentrations at night than during the day, while also reaching their highest concentrations during traffic rush hours (Rhoads et al., 2003).
PMC5. In Atlanta, organic matter and elemental carbon comprised ~40% and ~8%, respectively, of PM2.5 mass on average during August 1999 (Lim and Turpin, 2002).
PMC6. In Atlanta, the average air concentration of PM2.5 mass during August 1999 was 31.3 µg m-3, with a peak value of 47.2 µg m-3. Thus, the 24-hour PM2.5 standard was not exceeded. Interestingly, the 1-hr ozone standard was exceeded for multiple hours on several days during the Atlanta Supersite Study in August 1999. Sulfate and ammonium ion concentrations were well correlated with PM2.5 mass; but organic carbon and elemental carbon concentrations were not very well correlated (Solomon et al., 2003b).
PMC7. In Atlanta, light scattering by PM was dependent on a wide range of chemical components of the aerosols. Light absorption was most strongly linked to the elemental carbon component (Carrico et al, 2003).
PMC8. The average direct aerosol radiative forcing properties estimated in the Atlanta Supersite Experiment was about minus 11 + 6 watts m-2; this value is about 10 times larger than global mean estimates for aerosols (Carrico et al., 2003).
PMC9. The composition of particles measured during the Atlanta Supersite Study was generally internally mixed, with components of organic matter, sulfate, nitrate, ammonium and other constituents (Lee et al., 2002).
PMC10. During the ASACA study, annual PM2.5 mass concentrations measured from March 1999 to February 2000 exceeded the annual NAAQS of 15 µg m-3 at all four monitoring sites, with annual averages ranging from 19.3 to 21.2 µg m-3. One site violated the daily PM2.5 NAAQS of 65 µg m-3 (Butler et al., 2003).
PMC11. ASACA data in Atlanta showed that most PM2.5 constituents peaked during summer months; but nitrate, metals, and elemental carbon usually showed some enhancement during winter due mainly to lower inversion heights. Diurnally, there were discernible early morning and late night peaks that corresponded to rush-hour traffic patterns and inversion heights, respectively (Butler et al., 2003).
PMC12. At a rural site near Anderson, SC, the average PM2.5 mass during July 2001 was 20.9 µg m-3, with a high of 41.2 µg m-3 on July 18 and a low of 4.4 µg m-3 on July 25. The overall average in January 2002 was 9.4 µg m-3, with a high of 18.2 µg m-3 on January 18 and a low of 3.7 µg m-3 on January 25. Across all sampling events, the average annual mass concentration was 15.1 µg m-3, just above the new NAAQS annual standard of 15 µg m-3 (Husain and Christoforou, 2003).
PMC13. At a rural site near Anderson, SC, winter and summer data showed higher mass concentrations in summer than in winter. Sulfate ion and ammonium ion concentrations increased in summer, but nitrate ion concentrations decreased in summer. Comparison of these SC air concentration data with those for similar rural sites in GA and NC showed that the NC sites generally had higher and the GA sites generally had lower air concentrations of PM2.5 mass during late 2001 and early 2002 (Husain and Christoforou, 2003).
PMC14. Across southeast Texas, sulfate, ammonium ion (which neutralizes the sulfate ion), organic carbon, and elemental carbon are the major constituents of PM2.5; the annual average concentrations of these major components were generally spatially homogeneous although localized events with high mass fractions of sulfate or carbon occurred frequently at many monitors in this region. When averaged over long time periods, PM2.5 mass concentrations were spatially homogeneous throughout southeast Texas (Russell and Allen, 2004; Russell et al., 2004).
PMC15. Throughout southeast Texas, a consistent and strong morning peak in PM2.5 mass concentrations is observed and a weaker and slightly less consistent peak in mass concentration is observed in the late afternoon to early evening (Russell et al., 2004).
PMC16. In southeast Texas, concentrations of sulfate were slightly higher in the spring and late fall than in the summer; carbon concentrations were highest in the late fall (Russell and Allen, 2004; Russell et al., 2004).
PMC17. In southeast Texas, high organic-carbon to elemental-carbon ratios suggest that much of the carbonaceous material in PM2.5 is not emitted directly, but is formed in the air through reactions involving both gaseous biogenic and anthropogenic VOC emissions (Russell and Allen, 2004; Russell et al., 2004).
PMC18. Over wide regions of eastern and southeast Texas, annual average mass concentrations of PM2.5 ranged from about 10 µg m-3 to 15 µg m-3, which is close to the annual average NAAQS of 15 µg m-3 (Russell et al., 2004).
PMC19. Data from both the Atlanta and Houston Supersite Programs indicate that secondary formation of organic aerosols tended to be large compared to primary emissions (Dechapanya et al., 2002; Lemire et al., 2002; Lim and Turpin, 2002). In some suburban and rural locations in SE Texas secondary aerosol formation is dominated by biogenic VOC reactions (Lemire et al, 2002).
Scientific findings PMC1-PMC19 indicate that PM2.5 in the SOS region consists of directly emitted primary particles and secondary formation of aerosols produced from atmospheric chemical reactions involving VOC, SO2, NOx, and NH3. Thus, management strategies aimed at decreasing air emissions of VOC, SO2, NOx, and NH3 will be necessary to decrease both regional haze and human exposures to PM2.5.
B. Sources and Emissions of Primary PM and Precursors of Secondary PM2.5 (PMS)
The SOS program on emissions of PM constituents and precursors focused on identification of natural and human sources of primary PM and precursors of secondary PM and on source allocation of precursors of different sizes of particles.
PMS1. In Atlanta, based on data collected between 1998 and 2000, airborne soil was the largest source of primary PM10 mass and sulfate-rich secondary aerosol was the primary contributor to Atlanta PM2.5 mass (Kim et al., 2003).
PMS2. In Atlanta, eight types of sources were identified as major emission sources for PM2.5 constituents: 1) SO42--rich secondary aerosol sources (56 percent), 2) motor vehicle sources (22 percent), 3) wood smoke sources (11 percent), 4) NO3--rich secondary aerosol sources (7 percent), 5) mixed cement kiln and organic carbon sources (2 percent), 6) airborne soil sources (1 percent), 7) metal recycling facilities (0.5 percent), and 8) a miscellaneous source that includes bus stations and metal processing facilities (0.3 percent). Invariably, NH4+ (presumably mainly from agricultural sources) was associated with both the SO42--rich and NO3--rich secondary aerosols (Kim et al., 2003).
PMS3. In Atlanta, five types of sources were identified as major emission sources for PM10: 1) airborne soil sources (60 percent), 2) NO3--rich secondary aerosol sources (16 percent), 3) SO42--rich secondary aerosol sources (12 percent), 4) cement kiln facilities (11 percent), and 5) metal recycling facilities (1 percent) (Kim et al., 2003).
PMS4. Point sources of primary PM10 particles are significant, but point-sources of primary PM2.5 particles have not yet been quantified. Thus, additional research is needed to determine the importance, size distributions, and chemical compositions of these PM2.5 primary emissions Texas (Allen, 2002; Brock et al., 2003; NOAA, 2003).
PMS5. In Atlanta, using carbon monoxide as a tracer, motor vehicles were indicated as a primary emission source of elemental carbon. Elemental carbon concentrations tended to peak at 0600-0900 EST and had a much smaller peak during evening hours. These temporal patterns of variability are also indicative of motor vehicle emissions (Lim and Turpin, 2002).
PMS6. In southeast Texas, wind-blown soil was a relatively minor source of PM2.5 mass (Allen, 2002; Brock et al., 2003; NOAA, 2003).
PMS7. In southeast Texas, when high concentrations of PM2.5 mass, sulfate and organic carbon were observed throughout this region, back-trajectory analyses of these air parcels often indicated high concentrations of background sulfate and organic carbon in PM2.5 that extend far upwind in an easterly direction. These observations suggest that much sulfate and carbonaceous aerosol is transported into southeast Texas elsewhere in eastern North America (Russell et al., 2004).
PMS8. In southeast Texas, high concentrations of PM2.5 mass and organic carbon sometimes are observed at isolated monitors. These observations suggest that local source contributions are important on some days (Russell et al., 2004).
PMS9. In southeast Texas, mobile-source emissions account for about 25-35 percent of the primary particles in PM2.5 mass. Sources of primary emissions of PM2.5 in this area are diesel engines in heavy duty trucks, trains, and farm or construction equipment; gasoline engines in cars, trucks, boats, and hand tools; and jet-fueled aircraft (Allen, 2002; Brock et al., 2003; NOAA, 2003).
PMS10. In southeast Texas, primary particle emissions from cooking of foods were significant in all urban areas. These emissions account for about 10-15 percent of PM2.5 mass in urban areas (Allen, 2002; Brock et al., 2003; NOAA, 2003).
PMS11. In southeast Texas, fires are a sporadic, but significant, source of primary PM2.5 emissions. On an annual average basis, they contribute about 1-2% of the total mass of PM2.5 particles in the Houston-Galveston area; but these emissions tend to be concentrated on specific days with fire events in Texas (Allen, 2002; Brock et al., 2003; NOAA, 2003).
PMS12. Regional sources of PM2.5 are primary contributors to fine PM mass amounts in the Southeast, but instances of long-distance transport of particles can have a noticeable influence on monthly PM mass concentration, as observed during a Central American fire event in 1998 (Tanner et al., 2001).
Given the identity of the PM2.5 precursors, one might assume at first glance that the photochemically produced part of PM2.5 could be controlled simply by decreasing emissions of all four precursors -- SO2, NOx, NH3, and VOC. In actuality, however, as in the case of ozone, formation of sulfate, nitrate, and organic-carbon particles does not depend linearly on their precursors. Minimum formation of secondary aerosols occurs when the ratios among NOx, VOC, and SO2 precursors are least favorable for photochemical interactions. Regrettably, however, the ratios least favorable for secondary aerosol formation are not necessarily optimal for control of ozone formation.