Organic Fine Particles

Sources of Organic Fine Particles Using Carbon-14 Measurements

Introduction

Particulate matter originates from a variety of both natural and human-induced sources. Fine particulate matter (less than 2.5 micrometers in diameter or PM2.5) may have considerable health and/or environmental consequences. Also, PM2.5 is a major factor in the impairment of visibility.

Largely because of health concerns, the U.S. Environmental Protection Agency adopted stringent new standards for particulate matter in 1997. If and when these standards are enforced, they will be quite difficult to attain in much of the TVA region.

Organic Fraction of PM2.5

A significant portion of PM_2.5 mass consists of material containing carbon (C). These fine particles, i.e., organic aerosols, are a complex mixture formed from a variety of sources that are dynamically changing in time and space. In the southeastern United States, typically 25 to 40 percent of the fine-particle mass consists of organic carbon. Much of the remaining PM_2.5 mass (up to 50 percent) consists of sulfate particles.

Determining Sources of Organic Particles

Determining the sources of carbon-containing aerosols has been hampered by the complex nature of these substances and by humankind's incomplete understanding of the transformation processes that occur after the aerosols (or their gaseous precursors) are emitted. The tremendous number of samples needed to trace contributions from specific emission sources make such research expensive and time-consuming.

TVA found that one underutilized way of addressing this challenge is by measuring the carbon-14 (¹⁴C) content of organic aerosol particles. Organic aerosol carbon will consist of either: (1) "modern" carbon or (2) "fossil" carbon.

"Modern" carbon contains a predictable concentration of ¹⁴carbon dioxide (¹⁴CO₂) because it was fixed by plants using air that has current natural concentrations of atmospheric ¹⁴CO₂. "Fossil" carbon, on the other hand, greatly exceeds the half-life of ¹⁴C (circa 5,580 years), and contains essentially no ¹⁴C. Therefore, by measuring the ¹⁴C content of ambient aerosol samples, one can determine the fraction of "modern" C in the samples. Then, by subtracting the determined amount of "modern" ¹⁴C from the total C, one can determine that the remainder is "fossil" carbon derived from fossil-fuel combustion.

Determining the fractions of carbon aerosols that come from controllable sources is complex. For example, products of fossil-fuel combustion, which scientific data show are largely from mobile sources (gasoline- and diesel-powered vehicles), clearly comprise most of the "fossil"-derived organic carbon in ambient aerosols.

In contrast, natural emissions of vegetation contribute greatly to "modern" organic aerosols. These emissions are converted from gases to fine particles by photochemical oxidation. Also, some contribution is derived from re-suspended plant debris. However, "modern" aerosol organic carbon also is derived from the burning of wood and agricultural wastes. These latter (human) activities are potentially controllable.

Seasonal Changes in Sources

The ¹⁴C content has now been measured in aerosol samples collected at TVA's enhanced monitoring facility at Look Rock, Tennessee, on the western edge of the Great Smoky Mountains National Park. Results reported here are from samples collected during three seasons: spring of 2000, the summers of 2000 and 2001, and the fall of 2001.

These measurements suggest that, at this southeastern rural location, the concentration of organic fine particles from "fossil" fuels remains relatively constant and low throughout all seasons—in the range of 1-2 micrograms per cubic meter (µg/m³). "Modern" carbon—largely originating from vegetation emissions and combustion of wood and agricultural wastes—varies from 2 to 6 µg/m³ and is highest in summer (Figure 1). Thus, "modern," plant-derived carbon appears to make up 50 to 90 percent of the organic fraction of fine particles.

Since organic fine particles ("fossil" and "modern") make up, on average, about 30 percent of the total fine particle pool, the "modern," vegetation-derived fine particles account for 10 to 25 percent of the total fine particle mass.

"Modern" carbon fractions of fine particles observed at Look Rock during the spring season increased from about 55 percent in mid-April to about 90 percent in mid-May (Figure 2). This increase occurred during the same period that the leaves were emerging in the mixed-tree deciduous forest surrounding the sampling site. These data imply that as the forest canopy develops in spring, "natural" plant emissions, e.g., terpenes, are increasingly converted into particles by photochemical processes. This conversion thereby increases the "modern" carbon portion to about 30 percent of the total mass of fine particles.

Lower and relatively constant "modern" fractions of organic fine particles (40± 10 percent) were observed during the late summer period (mid-August to early September) in 2000. In contrast, samples taken in the summer of 2001, including the entire month of July, had much higher "modern" carbon fractions, varying from 73 to 88 percent. This occurred even though fine-mass concentrations varied widely, from less than 10 to as high as 40 µg/m³. The reasons for this wide variation in biogenic carbon from season to season and year to year are not yet clear. However, one supposition offered is that it may be related to soil moisture conditions or other plant growth factors.

Look Rock samples from October 2001 also had high fractions of "modern" carbon, ranging from 60 to 80 percent. However, the leaves of the surrounding deciduous tree forest had either dropped or become inactive by this time of year. Therefore, it is likely that the most significant sources of carbon-containing fine particles in the fall are the burning of wood for heating, the burning of agricultural wastes, and forest fires. Large seasonal variations in these sources are to be expected.

Even with complexities in interpreting data on the fraction of "fossil" carbon, scientists can conclude from this study that the amount of fossil-derived carbon described here gives lower limits to the fraction of fine organic aerosols that could be targeted for emissions reductions. Carbon-containing aerosols clearly will be legitimate targets in any necessary future control strategies for fine particles.

Summary

Fine particles of organic origin constitute a significant component (on average, about 30 percent) of the fine particle (PM_2.5) mass in the atmosphere of the southeastern United States. But determining the sources of these fine particles is a difficult task. However, TVA has found that by comparing the ¹⁴C content of fine-particle C with the known ¹⁴C content currently in the atmosphere, scientists can calculate the fraction of "modern" carbon in the fine-particle mass. Using this methodology can enable scientists to distinguish the amount of carbon-containing particles coming from fossil fuels and the amount derived from other sources. Measurements at Look Rock, Tennessee, over three seasons and during two years, indicate that the concentration of fossil fuel-derived organic particles is relatively low and constant (30± 20 percent of aerosol organic carbon). On the other hand, the modern plant-derived particle fractions vary greatly, from 40 percent up to 90 percent of organic aerosol particle mass. This modern, plant-derived fraction varies widely from season to season. During autumn, and presumably winter as well, the burning of wood and other plant materials may be a significant source of aerosols and therefore could become a target for fine particle control strategies.

Information Contacts

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

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

Inquiries and comments should be sent to wjparkhurst@tva.gov.