Environ. Sci. Technol. 2011, 45, 569–575
Climate Sensitivity of Gaseous Elemental Mercury Dry Deposition to Plants: Impacts of Temperature, Light Intensity, and Plant Species †
ANDREW P. RUTTER, J A M E S J . S C H A U E R , * ,†,‡ M A R T I N M . S H A F E R , †,‡ J O E L C R E S W E L L , † MICHAEL R. OLSON,† ALOIS CLARY,‡ MICHAEL ROBINSON,† ANDREW M. PARMAN,† AND TANYA L. KATZMAN† Environmental Chemistry and Technology Program, 660 N. Park Street, Madison, Wisconsin 53705, United States, and Wisconsin State Laboratory of Hygiene, 2601 Agriculture Drive, Madison, Wisconsin 53718, United States
Received August 6, 2010. Revised manuscript received November 11, 2010. Accepted November 17, 2010.
Foliar accumulations of gaseous elemental mercury (GEM) were measured in three plant species between nominal temperatures of 10 and 30 °C and nominal irradiances of 0, 80, and 170 W m-2 (300 nm-700 nm) in a 19 m3 controlled environment chamber. The plants exposed were as follows: White Ash (Fraxinus americana; WA); White Spruce (Picea glauca; WS); and Kentucky Bluegrass (Poa partensis; KYBG). Foliar enrichments in the mercury stable isotope (198Hg) were used to measure mercury accumulation. Exposures lasted for 1 day after which the leaves were digested in hot acid and the extracted mercury was analyzed with ICPMS. Resistances to accumulative uptake by leaves were observed to be dependent on both light and temperature, reaching minima at optimal growing conditions (20 °C; 170 W m-2 irradiance between 300-700 nm). Resistances typically increased at lower (10 °C) and higher (30 °C) temperatures and decreased with higher intensities of irradiance. Published models were modified and used to interpret the trends in stomatal and leaf interior resistances to GEM observed in WA. The model captured the experimental trends well and revealed that stomatal and internal resistances were both important across much of the temperature range. At high temperatures, however, stomatal resistance dominated due to increased water vapor pressure deficits. The resistances measured in this study were used to model foliar accumulations of GEM at a northern US deciduous forest using atmospheric mercury and climate measurements made over the 2003 growing season. The results were compared to modeled accumulations for GEM, RGM, and PHg using published deposition velocities. Predictions of foliar GEM accumulation were observed to be a factor of 5-10 lower when the temperature and irradiance dependent resistances determined in this study were used in place of previously published data. GEM uptake by leaves over the growing season was shown to be an important deposition pathway (2.3-3.7 µg m-2 * Corresponding author e-mail:
[email protected]. † Environmental Chemistry and Technology Program. ‡ Wisconsin State Laboratory of Hygiene. 10.1021/es102687b
2011 American Chemical Society
Published on Web 12/13/2010
of one-sided leaf area; OSLA) when compared to total mercury wet deposition (1.2 µg m-2 OSLA) and estimates of reactive mercury dry deposition (0.1-6 µg m-2 OSLA). ResistanceTemperature-Irradiance relationships are provided for use in models.
Introduction The atmospheric residence time of mercury is largely determined by the emission and removal rates of the gaseous elemental mercury species (GEM; Hg0(g)), due to its low reactivity and slow removal rates compared to the oxidized forms. The magnitude of these emission and removal pathways must be well understood for models to effectively represent the atmospheric dynamics of mercury. In locations uninfluenced by point sources, accumulation of GEM in foliage is more important than the dry deposition of oxidized mercury (1-3). As litterfall decomposes, the sequestered mercury may partition to the soil, be methylated, or evade back to the atmosphere (1, 4). There are two complementary reservoirs of mercury in foliage: i) the exchangeable fraction; and, ii) the accumulated or “fixed” fraction (5). The accumulative uptake of Hg0(g) to interior of a leaf progresses by a two-step process. First, Hg0(g) passes through the stomata and is adsorbed into the mesophyll at the leaf interior, and then the adsorbed mercury is oxidized and biochemically fixed by catalytic enzymes (6). Until oxidation, the adsorbed Hg0 can desorb back into the gas phase and is therefore exchangeable. Several studies have observed this exchangeable fraction of foliar mercury using dynamic flux chambers which completely enclose the plant being studied ( 5, 7, 8). In contrast, the method used in this study only measured the accumulated, nonexchangeable mercury, by making direct measurements of mercury retained in the leaves. Atmospheric mercury accumulates in foliage over the course of a growing season as a result of exposure to: GEM; oxidized mercury in dry aerosol, also known as reactive gaseous mercury (RGM) and particulate mercury (PHg); and oxidized mercury in rainwater (3, 5, 8-10). Foliar mercury concentrations are primarily controlled by atmospheric mercury concentrations, with soil-water mercury concentrations having a minor effect (
