Dust in the Wind: The Challenge of Fine Particles

Particulate matter (PM), consisting of small liquid or solid “dust” particles, is one of the six original National Ambient Air Quality Standard (NAAQS) air pollutants targeted for cleanup in 1971. Evolutionary changes in our understanding of the environmental and health effects of particulate matter reflect greater and greater concern about smaller and smaller particles. The original PM standard considered levels of “total suspended particulates” (TSP), i.e., particles 100 micrometers (_m) in diameter or less–about the diameter of a human hair. In 1987, the standard was revised to consider particles 10 _m in diameter or less (PM₁₀) and in 1997, the TSP standard was discontinued, and the fine particle standard (particles 2.5 _m in diameter or less [PM_2.5]) was added. Although long-term trends of both suspended and inhalable PM reveal consistent and significant improvements in national and regional air quality, fine particulates will pose a significant new environmental challenge.

Particulate matter originates from many different natural (e.g. wind-blown soil, volcanoes, pollen, wild fires, etc.) and human-made (e.g. fossil fuel combustion, agricultural burning, industry, transportation, demolition, petrochemical production and storage, etc.) sources. Generally speaking, particles created by physical processes–such as weathering, grinding, or cutting–are relatively large and, although they can be a nuisance, are of lesser concern from health or environmental perspectives. Directly-emitted fine particles, such as soot (carbon black) or fine particles created by the transformation of particle-forming gases, such as sulfur dioxide (SO₂) or volatile organic compounds (VOCs), are of greater health concern. This is because they can enter the respiratory system. In addition to their potential to affect health, fine particles also play a role in visibility impairment, acid rain, and toxic air pollution.

Although the current PM standards (PM₁₀ and PM_2.5) essentially treat all particles within these size ranges as being “equal” in terms of their potential ability to influence health, recent research indicates that particle composition matters a great deal. Health research suggests that some fine particles are relatively benign whereas others are not. Urban fine particles, for example, with their more complex composition appear to have more health impacts than rural particles. Studies also reveal differing levels of association−positive, negative, or no effect−between particle levels and various measures of community health. Clearly, all particles are not created equal relative to their consequences.

Historic Fine Particle Monitoring

TVA has long been in the forefront of fine particle monitoring. Beginning in 1978, TVA assisted the U.S. Environmental Protection Agency (EPA) in a fine particle research study in the Great Smoky Mountains National Park (GSMNP). Two years later, with the cooperation of the National Park Service, TVA began operation of a visibility/fine particle monitoring station at Look Rock on the western edge of the GSMNP. In August 1982, to gain perspective, TVA added two additional fine particle monitoring stations in Memphis, TN and Sand Mountain, AL. Ten years, five more stations, and 900 samples later, the final analysis of TVA’s first fine particle network revealed annual average PM_2.5 levels for the region ranging from 12.6 to 21.3 µg/m3, an overall mean of 15.7 µg/m3, and maximum 24-hour values ranging from 39 to 78 µg/m3.

Five years after this effort−in 1997−EPA revised the PM standard by discontinuing the TSP standard and adding two new fine particle standard metrics, a 24-hour limit of 65 micrograms per cubic meter (µg/m3) and an annual limit of 15 µg/m3. Data collected prior to 1992 indicated that the new EPA annual PM_2.5 standard was exceeded for five of 13 station-years in the TVA region. On Earth Day (April 22) 1997, TVA partnered with state and local air regulatory organizations to initiate a new eight-station, urban-oriented, regional PM_2.5 monitoring network. Based on more than 1,500 samples collected through 1999 at various locations in the Valley, annual average PM_2.5 concentrations ranged from 14.1-20.5 µg/m3 with a system-wide average of 16.8 µg/m3. Annual concentrations exceeded the level of the annual PM_2.5 standard at all but one station. However, no sample exceeded the level of the 24-hour PM_2.5 standard.

Particle Matter Air Quality Trends

Figure 1. South-central U.S. annual averages for total suspended particulates (TSP, 1979-2003), particulates < 10 _m (PM₁₀, 1987-2003) and (fine) particulates < 2.5 _m (PM_2.5, 1999-2003).

Figure 1 displays regional (south-central U.S.) averages for TSP (beginning in 1979), PM₁₀ (beginning in 1987), and PM_2.5 (beginning in 1999) through 2003 for all particle monitoring stations. Average annual TSP levels have improved by 47 percent, PM₁₀ levels by 34 percent and PM_2.5 levels by 14 percent. Except for a single unusual, source-specific exception, all TSP and PM₁₀ levels are well below (better than) their annual and 24-hour standards. The regional trends are entirely consistent with national, EPA-determined, air quality trends and indicate substantial improvement in particulate air quality over the past quarter century. Although it is encouraging that PM_2.5 levels have improved over the five years since comprehensive fine particle monitoring began to be conducted throughout our region, there has not yet been sufficient time to establish meaningful trends.

Figure 2. South-central U.S. PM_2.5 compliance map−red indicates non-compliance; green indicates compliance; white indicates lack of data based on 2001-2003 information. Because of allowances for averaging, the state of Tennessee has petitioned EPA to proclaim only its eastern counties as out of compliance at this time.

Figure 2 displays the potential regional PM_2.5 attainment/non-attainment map for 2001-2003 monitoring data based on the annual standard of 15 µg/m3. Actual attainment status, which should be issued in late 2004, may consider spatial averages to establish final status. For example, because of such averaging, within the state of Tennessee only the eastern counties might be declared out of compliance at this time (see Fig. 2 and figure heading). Background annual average PM_2.5 concentrations range from 11 to 14 µg/m3 and urban concentrations range from 14 to 21 µg/m3. For this region, a non-attainment finding will be the result of exceeding the annual standard. Maximum 24-hour concentrations rarely, if ever, exceed the 24-hour standard of 65 µg/m3. As it turns out, 2001 and 2003 were both “good” fine particle years because conditions conducive to the production and build-up of fine particles seldom occurred. While this fortuitous circumstance translates to fewer non-attainment areas than might otherwise be expected under more typical conditions, we should not become overconfident in our ability to maintain attainment levels.

Global PM_2.5 Background

Recently, EPA and EPRI sponsored research to examine the global burden of PM_2.5, with the goal of improving our understanding of background levels. Modeling work at Harvard University by Dr. Daniel Jacob provided estimates of both natural and man-made PM_2.5 levels entering the U.S. from Asia, Latin America and Africa. Asian dust and biomass burning in Central America and Mexico were found to contribute substantial PM_2.5 mass in the western U.S., while Saharan dust and biomass burning contributed to eastern PM_2.5.

Jacob estimates that background levels of carbonaceous particles in the eastern U.S. average about 1.2 μg/m3 on an annual basis. This is similar in magnitude to the default levels (~1.4 μg/m3) assumed by the EPA in setting natural background targets for visibility improvement. Background levels of ammonium sulfate and ammonium nitrate are substantially higher than previous estimates, however. The EPA estimates for the total of these two particle types is about 0.3 μg/m3 in the eastern U.S., whereas Dr. Jacob estimates an average of almost 0.8 μg/m3, or over a factor of two greater.

Jacob’s global modeling results suggest that, on average, eastern PM_2.5 levels include about 2.2 μg/m3 of carbonaceous, nitrate and sulfate mass from outside the U.S. With so large a fraction of the total PM in the U.S. coming from sources outside the country or from natural background sources, a real challenge may be created for meeting and maintaining the new annual standard in those areas that are at or above the attainment cutoff. Since only anthropogenic sources of fine particulates from within the U.S. can be regulated, a higher percentage reduction in anthropogenic emissions from within the U.S. will be needed to achieve the new standards set by EPA.

Conclusions

Major changes in the particulate matter clean air standards have occurred since their inception in 1971. Over the same period, regional improvements in the levels of total suspended particulates and PM₁₀ have been measured. Over the past quarter century there has been a 47 percent improvement in TSP levels and a 34 percent improvement in PM₁₀ levels since 1987. However, while preliminary indications are encouraging, our PM_2.5 data are too limited to draw any conclusions on trends. Fine particles (PM_2.5) present a substantial new regulatory challenge because of the multiplicity of potential sources, the stringency of the annual standard, controversy over assumptions about background levels, and regional climatology. The most recent three years of PM_2.5 data include two (2001 & 2003) “good” PM_2.5 years and, therefore, attainment designation is favorable at this time. However, most areas are close to or above the annual standard level and, additional controls notwithstanding, we might expect many places to move in and out of attainment.

Information Contacts

For more information on this and other air quality issues, please contact:

William J. Parkhurst 256 386-2793
Stephen F. Mueller 256 386-3643
Roger L. Tanner 256 386-2958