Chemical Composition of Fine Particles

The Chemical Composition of Fine Particles in Tennessee Valley Air—and Why It Matters

Background

TVA is a leader in air sampling efforts to determine the chemical composition of fine particles (those less than about 2.5 micrometers in diameter, PM2.5). Recent work has been motivated by concerns about the health effects of PM2.5 mass. These suspected health effects are the reason the U.S. Environmental Protection Agency (EPA) proposed new standards for fine particulate matter in 1997.

TVA’s initial work in this field was detailed PM2.5 measurements taken in Chattanooga (1984-86) and near Knoxville (1990-91). This work was conducted to determine the levels of PM2.5 mass in urban areas. Also, TVA was interested in identifying the chemical composition and sources of PM2.5. Chemical mass balance (CMB) modeling was used to analyze Chattanooga and Knoxville PM2.5 data. This modeling approach attempts to reconcile the chemical composition at a monitoring site with primary sources of fine particulate matter based on known source profiles. However, CMB modeling does not work well with secondary pollutants such as sulfate, and it is only possible to identify the fraction of sulfate in particles and not the individual sources of the precursor sulfur dioxide (SO2).

More recently, in 1997, TVA began the Tennessee Valley PM2.5 Partnership Network in cooperation with state and local agencies. This followed the announcement by EPA that it intended to regulate PM2.5. Results from the Valley network have been consistent with earlier data: Fine mass concentrations are now just below the 15 micrograms per cubic meter annual standard at two monitored rural sites, and above the proposed standard at all of the monitored urban sites (Figure 1). With more than 2,000 samples analyzed to date, no sample had a mass concentration greater than the 24-hour standard of 65 micrograms per cubic meter (mg/m3).

Composition data from the Partnership Network consistently show that the two major constituents of PM2.5 mass in the Valley are organic carbon and sulfate, each constituting about one-third of the mass (Figure 2). All other measured constituents—nitrates, trace elements (including crustal material), and elemental carbon (soot) constitute the remaining one-third. Ammonium nitrate and ammonium sulfate percentages are calculated assuming that all of the measured nitrate is present as ammonium nitrate and that all of the sulfate is neutralized by ammonia, respectively. The percentage of PM2.5 mass not chemically identified “undetermined” in Figure 2) is highly variable, but usually it is 10% or less. It contains all the measurement errors and any moisture retained by sample constituents during the weighing process. As a rule, the fraction of sulfate is highest at rural sites, whereas the organic carbon fraction is highest at urban sites.

This composition is consistent with earlier CMB modeling results which indicated that regional sulfate and particulate matter related to mobile sources (vehicles) and biomass combustion sources (high organic carbon content) accounted for most of the fine mass. Nitrates, soil dust, and other materials made only small contributions to total mass. Recent data indicate that PM2.5 mass concentrations in the Valley have not changed much in the past 20 years, when they were near or above EPA’s proposed annual standard but consistently below EPA’s proposed 24-hour standard.

PM2.5 Issues

Why does the chemical composition of fine particles matter? TVA’s knowledge of fine particle composition in the Valley can assist state and local air quality managers in developing state implementation plans for the new PM2.5 standard, should these plans be needed. Some remaining questions, which will require additional research into the nature of PM2.5, need to be addressed.
Does it make any sense to vary control strategies for fine particles by season? Atmospheric chemists know that during summer months in the Southeast, the conversion of SO2 to sulfate proceeds faster, air masses travel more slowly, and temperatures rise higher. These factors mean that sulfate concentrations are usually higher in summer. And summer concentrations of organic carbon particles may also be higher, but the evidence for this is less clear. TVA data suggest that, in general, sulfate tends to be a larger fraction of fine particles in summer than in other seasons. However, climatological variability is great and summer-like meteorological conditions, with longer transport distances, may exist from as early as March through much of autumn. This means that seasonal controls on SO2 emissions might reduce fine particle loadings at times, but not consistently. Not enough is known about organic aerosols that are derived from human activities to predict how effective seasonal controls on these would be.
Do day-night (diurnal) changes in fine particle mass and composition affect human exposure? At a mobile-source-impacted site in Chattanooga, TVA hourly data indicated morning rush-hour levels of fine mass and soot carbon were enhanced. No noticeable enhancement occurred in late afternoons. Concentrations always were higher at night due to the development of a stable atmosphere at the surface. These results indicate that people driving in the morning rush hour and at night are exposed to higher levels of PM2.5 particles. Thus, personal driving habits and daily activities affect exposure. Also, because of diurnal variation, human exposure to ambient PM2.5 in urban areas may not be well characterized by a small number of 24-hour Federal Reference Method (FRM) monitors in a community.
Does the EPA’s FRM accurately measure exposure to the chemical constituents of fine mass? There is now evidence that, while FRM samplers accurately measure sulfate, soil dust, trace metals, and soot carbon, they may underestimate the contributions from nitrates and some organic carbon. These “semi-volatile” compounds, collected by PM2.5 samplers, can evaporate during the mandated 24-hour sampling period under some conditions. This potential error source is important, because control strategies will be developed based on the measured chemical nature of fine particles. The tendency of the FRM to underestimate the contributions of organic and nitrate aerosols could result in unjustified reliance on reductions in sulfate by implementing SO2 controls. Such reliance on SO2 controls would omit needed strategies based on reductions of precursor and primary emissions for organic and nitrate particles, which result primarily from vehicle emissions.

What can be learned from further research?

TVA continues to support research into the nature of PM2.5 mass. Measurements are being made at a “Supersite” monitoring station in the Great Smoky Mountains as part of a cooperative venture with the Department of Energy and EPRI. Measurements of PM2.5 mass and composition, gaseous pollutants, and visibility-related parameters are being made at this site. And state-of-the-art instrumentation is being deployed to examine short-term fluctuations in PM2.5-sulfate. Also, carbon isotopes are being measured in an effort to identify the potential contribution of fossil sources to organic particles. Finally, data collections by TVA and others are testing models that simulate the formation and transport of PM2.5.

What policy directions or control strategy approaches does this research suggest?

The need to focus more on emissions controls to reduce organic carbon aerosols, especially from transportation and other controllable combustion sources.
Targeting emissions controls to the warmer months of the year to more effectively achieve reductions in the sulfate fraction of PM2.5 mass.
Consideration by EPA to modify its current regulatory monitoring method for PM2.5 mass to more accurately account for “semi-volatile” compounds. Such modifications could result in reduced emphasis on the contribution of SO2 as a percursor of PM2.5.
Modifications to address the difficulty of sparse community-based monitors to accurately characterize the average exposure of humans to constituents of PM2.5 mass because of spatial and diurnal variation in concentrations and chemical composition.

Information Contacts

Roger Tanner
(256) 386-2958
rltanner@tva.gov

Stephen F. Mueller
(256) 386-3643
sfmueller@tva.gov

William J. Parkhurst
(256) 386-2793
wjparkhurst@tva.gov

Last updated on 1-07-2002.
Inquiries and comments should be sent to wjparkhurst@tva.gov.