Seasonal contribution of dewfall to mercury deposition determined using a micrometeorological technique and dew chemistry

Abstract

[1] Deposition of mercury (Hg) to dew is thought to influence the diurnal dynamics of Hg species in the near-surface air, yet this depositional pathway is not well characterized and is seldom accounted for in Hg budgets. This study quantifies Hg deposition to dew on a seasonal basis at a high-elevation meadow in Shenandoah National Park, Virginia, and compares these rates with those for wet deposition and other forms of dry deposition. Dew samples were collected on a surrogate surface and analyzed for Hg concentrations (ng L À1 ), and dew depths (mm) were calculated using a micrometeorological approach based on the surface energy budget over 1 week period during each of the four seasons. Concentrations of Hg in dew (mean: 5.57 ng L À1 ) were lower than those observed in precipitation (mean weekly composite samples: 8.80 ng L À1 ). When dew deposition was scaled to the seasonal and annual levels, Hg deposition in dew was estimated to be 2-3 orders of magnitude smaller than wet deposition (estimated annual Hg deposition to dew/frost:~120 ng m À2 ; 2008 annual precipitation deposition:~12400 ng m À2 ). This disparity is attributed to the much lower dewfall amounts. Rates of Hg accumulation in dew were not related to gaseous elemental mercury (GEM) concentrations in the atmosphere nor were they related to measured GEM fluxes. They were, however, found to be similar to independently estimated rates of gaseous oxidized mercury (GOM) and particulate mercury (Hg p ) deposition during the nighttime, suggesting that dew scavenges GOM and Hg P from the atmosphere at the study site.Citation: Converse, A. D., A. L. Riscassi, and T. M. Scanlon (2014), Seasonal contribution of dewfall to mercury deposition determined using a micrometeorological technique and dew chemistry,

Mercury fluxes over an Australian alpine grassland and observation of nocturnal atmospheric mercury depletion events

“…Malcolm and Keeler (2002), from the first such measurements, calculated values of mercury deposition that ranged between 1.2 and 9.6 ng m −2 per dew event. THg concentrations similar to those from Malcolm and Keeler have since been reported by Engle et al (2010) and Converse et al (2014). Engle et al (2010) reported GEM depletion events coincident with their dew mercury measurements and concluded, based on a mass balance approach, that mercury uptake to dew could explain < 1 % of observed depletion.…”

“…Engle et al (2010) reported GEM depletion events coincident with their dew mercury measurements and concluded, based on a mass balance approach, that mercury uptake to dew could explain < 1 % of observed depletion. From the combined dew measurements of Malcolm and Keeler (2002), Engle et al (2010) and Converse et al (2014) a reasonable estimate of dew mercury concentration is 6.3 ± 4.9 ng L −1 (n = 27). Applying this concentration to the median modelled dew depth (0.13 mm) gives a cumulative nocturnal deposition to dew of 0.82 ng m −2 .…”

“…We followed the method of Converse et al (2014) and modelled dew depth on the basis of the surface energy balance methodology described in Jacobs et al (2006). For each time step i of length t, the depth of dew (D i+1 ) was calculated using…”

Atmospheric mercury deposition over the land surfaces and the associated uncertainties in observations and simulations: a critical review

“…For PBM dry deposition, resistance models regarding both fine and coarse particles are more and more widely applied based on the theory that v d for atmospheric particles strongly depends on particle size (Dastoor and Larocque, 2004;Zhang et al, 2009;Zhang and He, 2014). Many independent studies (Fang et al, 2012b;Zhu et al, 2014) showed that Hg in coarse particles constitutes a large mass fraction of the total PBM, which was previously neglected.…”

Mercury Accumulation in Tree Rings: Observed Trends in Quantity and Isotopic Composition in Shenandoah National Park, Virginia

“…Overall, SHEN is 95% forested, with oak and oak hickory forests (65%) and cove hardwood forests (13%) as the major components of the landscape (Young et al, ). Mercury in biological (Bank et al, ; Eagles‐Smith et al, ), atmospheric (Converse et al, , ; Kolker et al, ), terrestrial (Gunda & Scanlon, ; Risch et al, ), and aqueous (Riscassi & Scanlon, , ) samples from SHEN has been evaluated, and each of the corresponding studies has indicated that current Hg levels in this ecosystem are within the range measured in natural settings at other U.S. sites.…”