Mercury in Smoke from Biomass Fires  

Science Sep01

Hans R. Friedli¹, Lawrence F. Radke¹ and Julia Y. Lu²

¹National Center for Atmospheric Research, Boulder, CO
²Meteorological Service of Canada, Downsview, Ontario, Canada

Abstract. Litter and green vegetation were collected in 7 locations in the contiguous United States, analyzed for mercury, and burned under controlled conditions at the US Forest Service Fire Science laboratory in Missoula, MT. Among fuels, leaf and 3needle litter contained the highest concentration (up to 71ng/g on dry weight) of mercury. The combustion of litter and green vegetation resulted in essentially complete release of mercury stored in the fuel. Mercury is emitted primarily as elemental mercury, >95% for most burns, with particulate mercury (TPM) accounting for the remainder. From the laboratory experiments we project that mercury emitted from temperate/boreal forest fires and from all biomass burning is an important source components for the atmospheric mercury budget.

1. Introduction

Environmental toxicologists are focusing increasing attention to mercury compounds in the environment because of their toxicity and ubiquitous presence. Mercury in the form of methyl mercury is highly bio accumulated [Downs et al., 1998] and national and international agencies have targeted mercury for possible emission control [Lindberg, 1998]. Mercury can move through the environment by atmospheric transport [Poissant, 1999, Cheng and Schroeder, 2000], facilitated by chemical [Lin and Pehkonen, 1999] and biological [Downs et al., 1998] transformations. Mercury accumulates through the food chain and ends up in the tissues of top predatory species including fish [Rolfhus and Fitzgerald, 1995] and carnivores [Gnamus, et al., 2000]. In some arctic lakes the methyl mercury concentration in fish has reached levels that threaten human health [Clarkson et al., 1998]. Gaseous elemental Hg (0) has an atmospheric lifetime of about one year. Mercury enters the atmosphere from natural and anthropogenic sources [Schroeder and Munthe, 1998]. In the troposphere, mercury is present as elemental Hg (0) and ionic Hg (+2), and is also contained in particulates. Water, soil and vegetation take up these mercury species following wet and dry deposition. In vegetation, mercury is predominately contained in leaves and needles and its concentration is increasing with plant mass during the growing season. [Salt et al., 1998] and is species-specific. [Rea et al., 2000]. Upon senescence, the leaves and needles falling to the ground generally contains the highest mercury concentration found in plant material. Minor amounts of mercury are taken up through the xylem from mercury contained in the soil [Bishop et al., 1998].

We hypothesized that the properties of mercury would favor liberation and redistribution of mercury contained in vegetation and plant litter under wildfire conditions. Indeed, previous work by Veiga et al., [1994] on Brazilian biomass burning of fuel containing 40ng/g mercury resulted in unburned material containing 32ng/g mercury indicating at least partial mercury release. The postulated re-suspension of mercury is a corollary to the observation by Hegg et al., [1987] that urban pollutants deposited on soil or foliage are efficiently liberated by fire.

The objective of this work was 1. To quantify, in laboratory experiments, the fire mediated mercury emission process from vegetation, 2. To quantify mercury release from vegetation, both litter and live, collected in different regions of the contiguous United States, and 3. To estimate a preliminary budget for mercury emissions from biomass burning.

2. Experimental

2.1. Burn Experiments and Analytical

The experiment was carried out at the burn facility of the Fire Science Laboratory of the US Forest Service in Missoula, MT in April 2000. Samples weighing 500-1000 g were evenly spread in a 1x1m square on a burn table covered with a heavy aluminum foil. Dry fuel was ignited by four or five spot ignitions using a match. Less easily ignited fuels were lit with a propane torch. Combustion- resistant green fuels were mixed with excelsior (made from generic poplar) to obtain successful burns. Data from burns using excelsior were corrected for its small contribution to ash formation and mercury release. The burn table was placed on an electronic balance under a fume hood (3.6m bottom cone, tubular stack 1.6 m diameter and 20+m height). Analytical measurements were made at 15m above the burn table. Mass loss, volume flow and temperature of the combustion gases and the concentration of CO (Thermo Environmental Instruments, Inc., TECO, Model 48) and CO2 (Li-Cor 6262 CO2/H2O analyzer were monitored at 2 second time resolution during the burns. The CO and CO2 instruments were calibrated before and after each burn.

In this paper we are using the common term TGM (total gaseous mercury) and TPM (total particulate mercury). We recognize that there remain uncertainties about the interpretation of such measurements, e.g. how much of the reactive gaseous mercury (RGM) is analyzed by the Tekran analyzer as part of TGM. No independent RGM analyses were performed during these laboratory experiments. TGM was collected from the center of the chimney through a 0.1um Teflon filter and analyzed using a Tekran Mercury Vapor Analyzer (Tekran, Model 2537A, cold vapor atomic fluorescence spectrometry) with an collection and analysis time of 2.5 minutes. TPM was collected in a fraction of the plume by the procedure described by Lu et al., [1998] using a 25mm quartz fiber filter. The collected particulates were combusted in air at 900 °C and the liberated mercury was measured in a second Tekran instrument. Frontier Geoscience of Seattle, WA assayed mercury in fuels and ash. 

2.2. Fuel Sample Collection and Handling:

Forest Service professionals and volunteers collected fuel samples in MT, ID, CA, CT, SC, WA and FL using a uniform protocol and supplied collection tools to avoid contamination and assure sampling consistency. To compare dissimilar fuels (green, litter of different degrees of dryness) the mercury content was expressed on a dry weight base. (48 hours @ 78-79°C in nitrogen) but the fuels were analyzed and burned in the same un-dried state.

3. Results and Discussion

3.1. The Burn And Emission Process

The mercury release process was investigated in 5 replicate burns of Montana ponderosa pine litter (needles) at 500-1000g loads. The weight loss (see Table 1) during the burn replicated between 94.1-96.2% and the mercury emissions based on the difference between mercury in fuel and in ash ranged from 97.5-99.8%. The burn and mercury emission behavior is illustrated in (Figure 1) for Burn 2 as an example. Figure 1 illustrates the evolution of CO and CO2 during the burn and the fractional mass loss and TGM release as a function of time after ignition, as well as the transition from flaming to smoldering. The linear correlation between TGM release and weight loss is R2= 0.99. At the end of the flaming phase, 86-91 % of TGM were released from 1000 g fuel loads in contrast to 55-67% for 500g loads, presumably the result of the higher radiant heat flux to the unburned fuel. Mercury was fully released independent of fuel load. The observation that TGM emission continues under smoldering (reducing) conditions suggests that a reduction step from Hg (2+) to Hg (0) may be part of the release mechanism.

Figure 1.  MT Ponderosa dry needle burn as a function of the time after ignition. Left axis: evolution of CO2 and CO; right axis: fraction of mass and TGM loss.

The second form released of mercury is total particulate mercury (TPM), the mercury species associated with particulates. For the replicates, during the full length of the burn, only 0.12-0.71% of the emitted mercury was in form of TPM. For green coniferous needles and deciduous vegetation the TPM contribution covered a range up to 11%, indicating that the evolution of TPM is fuel-specific.

3.2. Emission of Mercury from Regionally Collected Vegetation

Montana samples originated from near Superior, MT (Ponderosa Pine) and near Moscow, ID (Western White Pine). Longleaf Pine samples are from the Oceola National Forest, Baker County, FL, and the Loblolly Pine samples from the Savanna River Forest Service Station in Aiken, SC; both are courtesy of D. Wade. The samples from WA are from Port Angeles, WA [C. Radke] and the CT samples were collected near Colliersville, CT. [H. Lyons]. The CA samples originated from the San Dimos Forest Service Experimental Forest, north of Los Angeles, CA [R. Lockwood]. All samples were collected in March or April 2000 with the exception of the MT samples, which were stock items at the Missoula Fire Science laboratory.

The results from 18 burns are given in Table 1, subdivided into coniferous and deciduous vegetation. Litter from conifers contained from 21.7-27.4ng/g mercury for different species grown in different locations.

Table 1.  Summary of burns of regionally collected fuels: % fuel burned, total mercury in fuel and ash, and % mercury emitted.

   *Burned as mixture with excelsior.

The mercury content of MT Western White Pine was unexpectedly high, 43.7ng/g, which may reflect the proximity to a mining or smelting operation. All samples burned efficiently to 93 to 96% completion and mercury release was 94-99%. Live coniferous vegetation required excelsior admixture to sustain effective combustion at 88 to 89%. For litter and green samples of the same species grown at the same location, the difference in mercury content between green and litter vegetation is clearly evident, confirming the observations by Salt et al., [1998] (FL Longleaf Pine, and SC Loblolly Pine). The coniferous WA samples showed higher mercury contents than the FL or SC samples coincident with known higher dry/wet deposition rates for the Pacific Northwest.

The highest mercury contents in fuels were observed in deciduous plants, again higher values for litter material as also observed for coniferous plants. The deciduous fuels (leaves, twigs < 0.5cm) burned less efficiently (65-85%) because of their higher water content, but mercury again was released at 97.5-99.7%. The mixture of green and litter CA chamise and mountain lilac lost 95.9% weight upon burning and released 99.5% of the contained mercury.

The limited amount of data does not allow for conclusions about the presence of regional mercury levels in vegetation, although the vegetation from CT and WA appeared highest. This coincides with the highest wet/dry deposition rates reported for the US. The main observation is that irrespective of mercury content and vegetation type, fires released mercury at >95%.

3.3. Importance To Global Mercury Budgets

Lobert et al., [1999] estimated global carbon release at 265 Tg C/y from fires in temperate/boreal forests, and 3716 Tg/y for all biomass burning. Assuming 50% carbon on dry weight fuel mass and 100% mercury release during fires, we can estimate mercury release budgets for the high (deciduous litter, 71ng/g) and low (green conifers, 14ng/g) mercury concentration in the fuels burnt in the Missoula experiments. The estimates for temperate/boreal forests are 7.4 and 37.7 t/y (1 t/y = 103 kg/y), respectively, for the high and low mercury content cases. Extrapolating these emission values to all biomass burning, 104 and 526 t/y, respectively, would be expected as upper limits since bulk fuel, which also burned to some extent, generally contains less mercury.

Schroeder and Munthe [1996] gave 3000 and 6560 t/y as estimates for global mercury emissions from natural and all sources, respectively. Our extrapolated estimates for mercury release from biomass fires bracket the range of 3.5-17.5% of the natural emissions and 1.6-8.0% of all total emissions.

Where does the liberated mercury end up? Since it is almost exclusively present as elemental mercury with a one-year lifetime, it becomes part of the global pool and undergoes chemical transformation in clouds and in the free troposphere and subsequent wet or dry deposition. The direction of transport of regional anthropogenic mercury emissions from the eastern US has been traced to arctic regions by Chen and Schroeder [2000] but the transport can also proceed in the opposite direction as indicated by carbon monoxide plumes from wildfires in the Canadian Northwest observed near Nashville, TN, [Wotawa and Trainer, 2000] in which elemental mercury would be included.

The only minor partition into particulate mercury is one of the unexpected results of this work. We expected much larger partition into the particulate phase, which would have favored local deposition because of the excellent CCN properties of biomass fire aerosols and frequent rain out. The consequence would be local rather than regional or global redistribution of the mercury released from biomass burning.

4. Summary and Conclusions

The combustion of litter and green vegetation under controlled burn conditions resulted in essentially complete release of mercury contained in fuel. This is different and higher than releases reported for some coal and biomass burning. Highest mercury concentrations were found in litter, reflecting accumulation of dry and wet deposition mercury over growing seasons. A suggested regional difference in mercury concentrations in vegetation coincides with the known highest dry/wet deposition rates in the US northeast and northwest. Mercury is emitted almost exclusively as elemental mercury (TGM, >95%) and thus joins the global pool. TPM emission is fuel-specific: it is mostly <5% but was measured to reach 11% for green coniferous fuel. From the laboratory experiments we can project that fire-emitted mercury is an important and not yet recognized source of atmospheric mercury. Measurements on wildfires are needed to verify and quantify balances as inputs to regional and global models. In a future publication we will present a detailed account of the experiments and verification of the results by an aircraft measurement of speciated mercury from on a wildfire.

Acknowledgments. Our thanks go to the management and staff at the Fire Science Burn Facility in Missoula, MT, for letting us uses their excellent facility. Special thanks go to I. Bertschi, R. Susott and R. Yokelson for guidance and process and analytical data. Thanks to the collectors of the regional samples and to S. McNamara for computational assistance. We are indebted to the Meteorological Services of Canada for making mercury analysis equipment available and to E. Prestbo of Frontier Geoscience for advice on sample handling and analysis. This work is funded by EPRI contract P 2044. The National Center for Atmospheric Research is sponsored by the National Science Foundation.

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(Received: November 29, 2000; revised: May 25, 2001; accepted: May 26, 2001.)