UNCERTAINTIES IN MASS BALANCE OF U.S. ATMOSPHERIC MERCURY EMISSIONS

Leonard Levin*, Mary Ann Allan, Paul Chu (Electric Power Research Institute, 3412 Hillview Avenue, Palo Alto, California 94303 USA). Corresponding author: llevin@epri.com

ABSTRACT

Mercury sources to air within the continental United States are thought to total about 150 t/yr (US EPA, 1997). The primary source of current mercury input to many U.S. water bodies is believed to be atmospheric deposition (Engstrom & Swain, 1998). Ionic state of mercury at emission is critical to deposition patterns; covalent mercury (Hg(II)) is water soluble with an atmospheric lifetime of hours to days, elemental mercury, Hg(0), has a lifetime of 1-2 years before oxidizing and dissolving in precipitation.

Current measurement data at background U.S. and Canadian sites for wet deposition are not yet sufficiently long or dense enough to act as a control on the emissions inventory. Modeling exercises combining regional and local scales spatially can be well-matched in general to these monitoring data (Seigneur et al., 1996), but still are too uncertain to act as a test of either model dynamics or inventories.

Additional data are also required on the distribution of background sources geographically, and the nature of the mercury emissions from them: how episodic, whether covalent or elemental, etc. Although rough calculations scaled by land area indicate background emissions might make up some 120 t/y additional input to the atmosphere, local deposition may account for 50% or more of this, so that only 50-60 t/y need be considered in the continental mass balance.

INTRODUCTION

Mercury emissions to air from industrial activiities in the United States total about 150 t/y (EPA, 1997); most of this, perhaps 90%, is emitted at elevations tens to hundreds of meters above ground level, but still in the atmospheric boundary layer.

Recent measurements in Ontario, California, Nevada, and Tennessee have begun to quantify background emissions as an additional source of mercury into the North American balance. This emission category has been long included in global mass balances, but to date not incorporated in regional mercury inventories (Rasmussen, 1997; Gustin, 1998; Lindberg, 1999). The background sources can be considered in two classes: natural background, and areas previously subject to anthropogenic activity, such as mining sites or mineral processing faciliites. These latter “legacy” sites may be widely distributed. In addition, re-emission of deposited mercury may occur anywhere, including the subcategories making up background areas.

Field measurements of emission rates from natural and anthropogenically-impacted background areas show a wide variability in rates of emission of total gaseous mercury with terrain characteristics, the occurrence of precipitation onto the surface, and other factor. Measurements in west-central Nevada in 1997 (Gustin et al., 1999) showed increased outgassing of total mercury following precipitation events (Poissant et al., 1999). It is still uncertain whether this is simple replacement of pore-space mercury gas by liquid water, or a more complex surficial action with hydrophilic Hg(II) remaining behind while hydrophobic Hg (0) is released.

Extension of these point measurements of background mercury in time and space is highly speculative at this time, since no general scheme for assigning outgassing rates to terrain characteristics is yet available. One extrapolation to the area of the continental U.S. yields background emission rates roughly equal to the total of current U.S. industrial emissions. Another extrapolation from Ontario measurements of mercury emissions from black shale yielded rates sufficient to explain a good portion of station-monitored atmospheric concentrations in northeastern states (Pai et al., 1999).

The primary source of current mercury input to many U.S. water bodies is believed to be atmospheric deposition (Engstrom & Swain, 1998). Of this deposition, an unknown portion is made up of regional and local atmospheric emissions from U.S. point sources, and the balance from globally-circulating mercury from both U.S. and international atmospheric sources. These proportions may be about 60 and 40% respectively, at least for northern tier U.S. states. Speciation of industrially emitted mercury is similarly uncertain; a common default assumption is that 50% is in each ionic form when emitted from combustion sources (U.S. EPA, 1998). That speciation is likely to impact the fraction that is transported beyond local scale. Calculations indicate that, for a combustion source stack height of 300m, less than 20% of the emitted mass of mercury will deposit within a radius of 50km even under the assumption that it is all ionic.

RESULTS AND DISCUSSION,

The control on a mass balance of U.S.-emitted mercury is the set of data on ground-level atmospheric concentrations and deposition nationally. Monitoring and sampling networks are only now beginning to reach spatial density and time extent that allow patterns and trends to begin to be discerned (NADP, 1999). These are still inadequate to provide good closure on goodness-of-fit for regional or local models of atmospheric mercury transport and deposition, and so these tools remain highly uncertain for purposes of assessing source-receptor relationships.

Nonetheless, inclusion of these new point readings in regional and continental inventories is not yet possible. In addition to their low density of coverage, basic characteristics of these emissions are unknown, including their ionic form and the mechanism for their transport. Diffusive vertical mixing is possibly dominant, but episodic large-volume vertical transport by wildfires and meteorological events may also play a role. Current research is investigating these potential mechanisms in order to allow inclusion of the background terms in future inventories.

When these numbers are totaled, a rough balance for U.S. mercury emissions and deposition can be derived. West of 90 W longitude, a demarcation line based on annual average precipitation figures (US EPA 1997), background emissions are expected to play a significant role in wet deposition. East of 90 W longitude, however, greater rates of precipitation and more sources of Hg(II) to the atmosphere imply that background emissions from the entire U.S. would play less of a role in that sector’s wet deposition totals, accounting for perhaps 4-10% of the total, and therefore being less evident within the uncertainties of deposition measurements.

REFERENCES

Allan M A, Levin L, Porcella D, Yager J, Wyzga R, Chang R, Chu P, Nott B, Toole-O’Neil B (1996), Mercury in the Environment — A Research Update. Palo Alto, EPRI.

Gustin M S, Lindberg S L, Allan M A (1999), J. Geophys. Res. 104: 21829-21830.

Poissant L, Pilote M, Casimir A (1999), J. Geophys. Res. 104: 21845-21858.

US EPA (1997), Mercury Study Report to Congress. Washington DC.