Introduction

1. Relevance of dew formation and stomatal conductance on mercury fluxes A.D. Converse and T.M. ScanlonDepartment of Environmental Science, University of Virginia, Charlottesville, Virginia 22904 Results: Stomatal conductance and mercury fluxes Mercury fluxes were measured using the aerodynamic method in the summer and fall of 2008. A diurnal pattern is visible in the summer Hg fluxesbut no pattern was seen in the fall (Figure 3). Stomatal conductance was estimated using the Penman-Monteith equation. It is compared to absolute value of Hg fluxes since gaseous elemental Hg is believed to enter and leave through stomatal openings (deposition and emission, respectively). A positive correlation to Hg fluxes is present with temperature, UV radiation (Figure 5), photosynthetically active radiation (PAR), and incident shortwave radiation in the summer. No significant correlation between Hg fluxes and other parameters are present in the fall. Conclusions Dew modeling The surface energy budget model to predict dew depth better matched the collected dew samples and onsite observations in comparison to the eddy covariance technique. This suggests that the model is more appropriate for our site. In similar sites with low-lying vegetation it could provide a good estimate of dew depth without the difficulties associated with early morning dew collection. The mass of Hg deposited in dew can be compared to precipitation data. A ‘back-of-the envelope’ calculation indicates that dew only contributes a small portion of total deposition in comparison to rainfall in the summer (Table 2). In regions or seasons with less rainfall, dew might provide a larger portion of the wet Hg deposition budget. Stomatal conductance Stomatal conductance is not correlated with absolute Hg fluxes. This does not disprove the idea of gaseous Hg entering and leaving stomatal cavities, but it certainly is a curious observation. Significant correlations between temperature, UV radiation, PAR and short wave radiation with Hg fluxes are present in the summer. Two more collection campaigns for Hg fluxes, dew and stomatal conductance will take place in the winter and spring of 2009. Introduction Atmospheric mercury (Hg) deposition is a growing global concern, even in pristine regions far from point sources. Once deposited, it can be transformed into the potent neurotoxin methylmercury, which is hazardous to humans and wildlife. It is necessary to quantify the total Hg deposition to ecosystems in order to better understand Hg cycling and to develop appropriate legislation to reduce Hg pollution. Previous studies suggest that wet surfaces enhance surficial Hg deposition. Regular dew formation could therefore result in increased Hg deposition, but only one dew study has been previously reported. Plants accumulate Hg over the course of the growing season and stomatal conductance has been suggested to play a role. If stomata are open (high conductance), more gaseous Hg could enter or leave the plant. This study has two major goals: 1) To quantify the contribution of dew to Hg deposition using an energy balance technique combined with dew sampling. 2) Compare Hg fluxes to stomatal conductance using the Penman-Monteith equation. Results: Dew collection and modeling Dew depth (dew volume/surface area) is determined through micrometeorological techniques by summing the negative evapotranspiration (E) indicative of dew formation. The calculated depth is representative of dew forming on natural surfaces, rather than an artificial collection table. Using eddy covariance to determine E tends to underestimate dew formation, particularly in evenings when there are low wind speeds, wet conditions, and/or a stable atmosphere. Jacobs et al. (2006) reported a dew deposition model that does not incorporate eddy covariance equipment and is based on the surface energy budget: Combining this equation with free water evaporation, then followed by Penman’s substitution results in: The surface energy budget model, eddy covariance, and physical sampling were used to determine dew depth in the summer and fall. Figure 1 displays a single evening/morning of dew formation and Figure 2 displays the entire summer period (fall data not shown). Table 2: Summerwet Hg deposition from dew and rainfall.Deposition from dew determined from average dew depth and concentration of 6 summer dew samples. Rainfall data are from the Mercury Deposition Network (MDN) site at Big Meadows (average of 2003-2007). Figure 3: Mercury fluxes measured in the August and November. Vertical dashed lines indicate 12pm. Mercury fluxes ranged from -75.6 to 106.6 ng m-2 h-1 over the two seasons. The average Hg flux was 2.9 ng m-2 h-1 in the summer, and -0.12 ng m-2 h-1 in the fall. Materials and methods Our field site is in Big Meadows, a high elevation wetland and the largest open area within Shenandoah National Park, VA. Vegetation consists of non-woody grasses with a summer canopy height of approximately 60 cm. Samples were collected during 1-weeklong campaigns in August (summer) and November (fall), 2008. Eddy covariance equipment was mounted on a portable flux tower at a reference height of 2.4 m and measurements were averaged over 20 minute intervals. Gaseous atmospheric Hg was alternated sampled at two inlet heights (Tekran 2537) and fluxes were determined using the aerodynamic method. Table 1 lists all the equipment used. Dew samples were collected using trace-metal techniques following a modified protocol of Malcolm et al. (2002). Mercury analysis was conducted at the University of Virginia using cold vapor atomic fluorescence (Tekran 2600). Literature Cited Jacobs, A. F. G., Heusinkveld, B. G., Kruit, R. J. W., & Berkowicz, S. M. (2006). Contribution of dew to the water budget of a grassland area in the Netherlands. Water Resources Research, 42(3). Malcolm, E. G., & Keeler, G. J. (2002). Measurements of mercury in dew: Atmospheric removal of mercury species to a wetted surface. Environmental Science & Technology, 36(13). Figure 1: One night of dew formation (August 8th). Eddy covariance underestimates dew formation in comparison to other methods. Figure 4: The magnitude of daytime mercury flux as a function of stomatal conductance. This is no significant correlation was between Hg fluxes and stomatal conductance in the summer or fall (p> 0.05). (a) Table 1.Equipment used in field campaigns. The photo on the left displays the flux tower during set-up and the Big Meadows site. Acknowledgments Ami Riscassi diligently helped in all field work, including the early morning dew collections. Others offered assistance in set-up and tear down: Clara Funk, Leslie Piper, Kelly Hokanson , Thushara Gunda, and Karen Vandecar. Liz Garcia (NPS) fielded all of our questions concerning the NDAP and MDN. Finally, we’d like to thank the National Park Service and the staff at the Big Meadows Ranger station for graciously allowing us to set-up camp in the park. This project is funded by grants from the NSF and GAANN (Graduate Assistance in Areas of National Need). (b) For further information Please contact adc9x@svirginia.edu. More information on this and related projects can be obtained at www.hydrology.evsc.virginia.edu/Scanlon_Lab. A link to the pdf of this poster is also available. Figure 2: Summer dew collection and modeling. The upper panel (a) indicates the dew depths determined using three separate techniques. The lower panel (b) displays Hg concentrations in the collected dew samples. The [Hg] was non-detect on August 11th and a dew sample was not collected August 12th due to technical difficulties. Figure 5: Daytime mercury fluxes as a function of ultraviolet (UV) light. There is a significant correlation between UV radiation in the summer (p<0.05) but not in the fall.