Atmospheric deposition is the main input of mercury (Hg) to the ocean, even in remote locations such as the Arctic. Furthermore, evasion of elemental Hg (Hg degrees) is the major sink for oceanic Hg. As a result, air-sea exchange is an important part of the oceanic Hg cycle. To examine the air-sea exchange of Hg in the Arctic Ocean we made high resolution measurements of Hg degrees in surface waters and Hg speciation in the atmosphere using continuous sampling systems during the 2015 U.S. Arctic GEOTRACES cruise from August 9 to October 12. Additionally, samples were obtained for measurement of total Hg and methybriercury (CH3Hg) in wet deposition and bulk aerosols as well as surface snow and sea ice. We used these measurements made in the water and the atmosphere to estimate fluxes of Hg degrees from the ocean to the atmosphere. Concentrations of dissolved Hg (Hedi,) were near saturation in ice-free waters (32 +/- 30 fM) and resultant fluxes were low; however, Hg was highly enriched under contiguous ice (101 +/- 98 fM, up to 544 fM) suggesting the continual formation of Hg in waters even when ice covered. Predicted evasion fluxes in these regions (these being potential rates for locations under ice) were as high as 492 pmol m(-2) h(-1). Atmospheric Hg concentrations averaged 1.2 +/- 0.1 ng m-3 with little variation over the course of the cruise even above waters with elevated Hg degrees(diss) indicating that sea ice acts as a barrier to air-sea exchange. Measurements of Hg in precipitation and aerosols were lower than have been found in more coastal regions of the Arctic. We used these concentrations to estimate deposition of Hg and CH3Hg during the time of the cruise. Overall, wet deposition represented 88% of the CH3Hg flux and 38% of the HgT flux. Our flux estimates confirm the importance of air-sea exchange in Hg cycling in the Arctic and suggest that evasion was greater than deposition, indicating a net loss of Hg from the Arctic during this period or the presence of other sources not measured during this study. Additionally, our results suggest that fluxes for offshore waters are lower than found in coastal regions of the Arctic. From these estimates, we predict how Hg concentrations may respond to future changes in ice cover and other potential impacts of climate change on Hg dynamics and food web bioaccumulation in this important ocean region.

Atmospheric chemistry and transport of mercury play a key role in the global mercury cycle. However, there are still considerable knowledge gaps concerning the fate of mercury in the atmosphere. This is the second part of a model intercomparison study investigating the impact of atmospheric chemistry and emissions on mercury in the atmosphere. While the first study focused on ground-based observations of mercury concentration and deposition, here we investigate the vertical and interhemispheric distribution and speciation of mercury from the planetary boundary layer to the lower stratosphere. So far, there have been few model studies investigating the vertical distribution of mercury, mostly focusing on single aircraft campaigns. Here, we present a first comprehensive analysis based on various aircraft observations in Europe, North America, and on intercontinental flights. The investigated models proved to be able to reproduce the distribution of total and elemental mercury concentrations in the troposphere including interhemispheric trends. One key aspect of the study is the investigation of mercury oxidation in the troposphere. We found that different chemistry schemes were better at reproducing observed oxidized mercury patterns depending on altitude. High concentrations of oxidized mercury in the upper troposphere could be reproduced with oxidation by bromine while elevated concentrations in the lower troposphere were better reproduced by OH and ozone chemistry. However, the results were not always conclusive as the physical and chemical parameterizations in the chemistry transport models also proved to have a substantial impact on model results.

The first phase of an atmospheric tracer experiment program, designated Project Sagebrush, was conducted at the Idaho National Laboratory in October 2013. The purpose was to reevaluate the results of classical field experiments in short-range plume dispersion (e.g., Project Prairie Grass) using the newer technologies that are available for measuring both turbulence levels and tracer concentrations. All releases were conducted during the daytime with atmospheric conditions ranging from neutral to unstable. The key finding was that the values of the horizontal plume spread parameter sigma(y) tended to be larger, by up to a factor of similar to 2, than those measured in many previous field studies. The discrepancies tended to increase with downwind distance. The values of the ratio sigma(y)/sigma(theta), where sigma(theta) is the standard deviation of the horizontal wind direction, also trend near the upper limit or above the range of values determined in earlier studies. There was also evidence to suggest that the value of sigma(y) began to be independent of sigma(theta) for sigma(theta) greater than 18 degrees. It was also found that the commonly accepted range of values for sigma(theta) in different stability conditions might be limiting, at best, and might possibly be unrealistically low, especially at night in low wind speeds. The results raise questions about the commonly accepted magnitudes of sigma(y) derived from older studies. These values are used in the parameterization and validation of both older stability-class dispersion models as well as newer models that are based on Taylor's equation and modern PBL theory.

Dry deposition of atmospheric mercury (Hg) to various land covers surrounding 24 sites in North America was estimated for the years 2009 to 2014. Depending on location, multiyear mean annual Hg dry deposition was estimated to range from 5.1 to 23.8 μg m-2 yr-1 to forested canopies, 2.6 to 20.8 μg m-2 yr-1 to nonforest vegetated canopies, 2.4 to 11.2 μg m-2 yr-1 to urban and built up land covers, and 1.0 to 3.2 μg m-2 yr-1 to water surfaces. In the rural or remote environment in North America, annual Hg dry deposition to vegetated surfaces is dominated by leaf uptake of gaseous elemental mercury (GEM), contrary to what was commonly assumed in earlier studies which frequently omitted GEM dry deposition as an important process. Dry deposition exceeded wet deposition by a large margin in all of the seasons except in the summer at the majority of the sites. GEM dry deposition over vegetated surfaces will not decrease at the same pace, and sometimes may even increase with decreasing anthropogenic emissions, suggesting that Hg emission reductions should be a long-term policy sustained by global cooperation.

Abstract. We perform global-scale inverse modeling to constrain present-day atmospheric mercury emissions and relevant physio-chemical parameters in the GEOS-Chem chemical transport model. We use Bayesian inversion methods combining simulations with GEOS-Chem and ground-based Hg0 observations from regional monitoring networks and individual sites in recent years. Using optimized emissions/parameters, GEOS-Chem better reproduces these ground-based observations, and also matches regional over-water Hg0 and wet deposition measurements. The optimized global mercury emission to the atmosphere is ~5.8 Gg yr−1. The ocean accounts for 3.2 Gg yr−1 (55% of the total), and the terrestrial ecosystem is neither a net source nor a net sink of Hg0. The optimized Asian anthropogenic emission of Hg0 (gas elemental mercury) is 650–1770 Mg yr−1, higher than its bottom-up estimates (550–800 Mg yr−1). The ocean parameter inversions suggest that dark oxidation of aqueous elemental mercury is faster, and less mercury is removed from the mixed layer through particle sinking, when compared with current simulations. Parameter changes affect the simulated global ocean mercury budget, particularly mass exchange between the mixed layer and subsurface waters. Based on our inversion results, we re-evaluate the long-term global biogeochemical cycle of mercury, and show that legacy mercury becomes more likely to reside in the terrestrial ecosystem than in the ocean. We estimate that primary anthropogenic mercury contributes up to 23% of present-day atmospheric deposition.

PreviousNext No AccessNear-Surface Asia Pacific Conference, Waikoloa, Hawaii, 7-10 July 2015Aircraft-based thermal imaging and remote sensing to assess initial conditions leading to thermal convection and thunderstorm developmentAuthors: Steve Brooks*Tilden MeyersEd DumasBruce BakerChris VogelWill PendergrassRick EckmanShuyan LiuSteve Brooks*The University of Tennessee Space InstituteSearch for more papers by this author, Tilden MeyersNational Oceanic and Atmospheric Administration, Atmospheric Turbulence and Diffusion DivisionSearch for more papers by this author, Ed DumasNational Oceanic and Atmospheric Administration, Atmospheric Turbulence and Diffusion DivisionSearch for more papers by this author, Bruce BakerNational Oceanic and Atmospheric Administration, Atmospheric Turbulence and Diffusion DivisionSearch for more papers by this author, Chris VogelNational Oceanic and Atmospheric Administration, Atmospheric Turbulence and Diffusion DivisionSearch for more papers by this author, Will PendergrassNational Oceanic and Atmospheric Administration, Atmospheric Turbulence and Diffusion DivisionSearch for more papers by this author, Rick EckmanNational Oceanic and Atmospheric Administration, Field Research DivisionSearch for more papers by this author, and Shuyan LiuNational Oceanic and Atmospheric Administration, Field Research DivisionSearch for more papers by this authorhttps://doi.org/10.1190/nsapc2015-041 SectionsAboutPDF/ePub ToolsAdd to favoritesDownload CitationsTrack CitationsPermissions ShareFacebookTwitterLinked InRedditEmail Abstract During the summer months of 2014 we conducted aircraft-based thermal imaging and surface temperature measurements over an area (~25 km2) centered over the Auburn University's Tennessee Valley Research and Extension Center near Belle Mina, Alabama. The purpose of this research was to assess the surface and meteorological conditions that initiate the atmospheric convection that potentially develops thunderstorms, and increase the severity of storms advected into the local area. Further, these measurements will be compared to current weather models and used to improve the determination of initial convection conditions that favor the formation of severe storms. Keywords: imaging, remote sensing, weatherPermalink: https://doi.org/10.1190/nsapc2015-041FiguresReferencesRelatedDetails Near-Surface Asia Pacific Conference, Waikoloa, Hawaii, 7-10 July 2015ISSN (online):2159-6832Copyright: 2015 Pages: 501 publication data© 2015 Published in electronic format with permission by the Society of Exploration Geophysicists, Australian Society of Exploration Geophysicists, Chinese Geophysical Society, Korean Society of Earth and Exploration Geophysicists, and Society of Exploration Geophysicists of JapanPublisher:Society of Exploration Geophysicists HistoryPublished Online: 10 Jul 2015 CITATION INFORMATION Steve Brooks*, Tilden Meyers, Ed Dumas, Bruce Baker, Chris Vogel, Will Pendergrass, Rick Eckman, and Shuyan Liu, (2015), "Aircraft-based thermal imaging and remote sensing to assess initial conditions leading to thermal convection and thunderstorm development," SEG Global Meeting Abstracts : 156-157. https://doi.org/10.1190/nsapc2015-041 Plain-Language Summary Keywordsimagingremote sensingweatherPDF DownloadLoading ...

Atmospheric transport and in situ oxidation are important factors influencing mercury concentrations at the surface and wet and dry deposition rates. Contributions of both natural and anthropogenic processes can significantly impact burdens of mercury on local, regional and global scales. To address these key issues in atmospheric mercury research, airborne measurements of mercury speciation and ancillary parameters were conducted over a region near Tullahoma, Tennessee, USA, from August 2012 to June 2013. Here, for the first time, we present vertical profiles of Hg speciation from aircraft for an annual cycle over the same location. These airborne measurements included gaseous elemental mercury (GEM), gaseous oxidized mercury (GOM) and particulate bound mercury (PBM), as well as ozone (O3), sulfur dioxide (SO2), condensation nuclei (CN) and meteorological parameters. The flights, each lasting ~3 h, were conducted typically one week out of each month to characterize seasonality in mercury concentrations. Data obtained from 0 to 6 km altitudes show that GEM exhibited a relatively constant vertical profile for all seasons with an average concentration of 1.38 ± 0.17 ng∙m−3. A pronounced seasonality of GOM was observed, with the highest GOM concentrations up to 120 pg∙m−3 in the summer flights and lowest (0–20 pg∙m−3) in the winter flights. Vertical profiles of GOM show the maximum levels at altitudes between 2 and 4 km. Limited PBM measurements exhibit similar levels to GOM at all altitudes. HYSPLIT back trajectories showed that the trajectories for elevated GOM (>70 pg∙m−3) or PBM concentrations (>30 pg∙m−3) were largely associated with air masses coming from west/northwest, while events with low GOM (<20 pg∙m−3) or PBM concentrations (<5 pg∙m−3) were generally associated with winds from a wider range of wind directions. This is the first set of speciated mercury vertical profiles collected in a single location over the course of a year. Even though there are current concerns that the KCl denuders used in this study may under-collect GOM, especially in the presence of elevated ozone, the collected data in this region shows the strong seasonality of oxidized mercury concentrations throughout the low to middle free troposphere.

We present two numerical models designed to reproduce the temperatures of the illuminated Saturn rings as measured by the CASSINI-CIRS instrument. Our models are constrained by all available temperature measurements performed on the illuminated rings since SOI. Both models reproduce well the variations of temperature under any illumination and observation geometry. One model is derived from a purely numerical data mining approach, relying on the implementation of a Neural Network that treats the data set globally. This model is used as a test of coverage completeness of the observational parameter space, driving our ability to characterize the rings thermal response. The second (analytical) model is derived using simple physical considerations, by treating the rings as a surface rather than as a collection of individual particles, combined with an empirical anisotropy function to describe the temperature resulting from the Sun's and Saturn's heating. The thermal response of this ring-surface is parameterized by its Bond albedo and emissivity, thermal relaxation time and a set of geometrical parameters quantifying the anisotropy of the temperature measurements depending on azimuth and elevation of the observer with respect to the ring plane, as well as on the solar elevation. Both models provide formulae to predict the ring temperature, that will ease the benchmarking of future physical models against data. The physical model is applied to fit the temperature of tens of different radial slices, allowing us to constrain the combined emissivity and albedo, thermal relaxation time and anisotropy parameters of the ring slabs with the highest radial resolution achieved so far with CIRS. Using for the first time all observation geometries available for illuminated rings, we are confident that our values are as unbiased as possible against observation geometry. The thermal relaxation time appears to be short, a few tens of minutes, and independent of the radial distance across the whole ring. A study of the temperature anisotropy suggests inter-particle shadowing is important in the B ring and in the outer A ring regions. (C) 2014 Elsevier Inc. All rights reserved.

During two intensive studies in summer 2010 and spring 2011, measurements of mercury species including gaseous elemental mercury (GEM), gaseous oxidized mercury (GOM), and particulate-bound mercury (PBM), trace chemical species including O3, SO2, CO, NO, NOY, and black carbon, and meteorological parameters were made at an Atmospheric Mercury Network (AMNet) site at the Grand Bay National Estuarine Research Reserve (NERR) in Moss Point, Mississippi. Surface measurements indicate that the mean mercury concentrations were 1.42 ± 0.12 ng∙m−3 for GEM, 5.4 ± 10.2 pg∙m−3 for GOM, and 3.1 ± 1.9 pg∙m−3 for PBM during the summer 2010 intensive and 1.53 ± 0.11 ng∙m−3 for GEM, 5.3 ± 10.2 pg∙m−3 for GOM, and 5.7 ± 6.2 pg∙m−3 for PBM during the spring 2011 intensive. Elevated daytime GOM levels (>20 pg∙m−3) were observed on a few days in each study and were usually associated with either elevated O3 (>50 ppbv), BrO, and solar radiation or elevated SO2 (>a few ppbv) but lower O3 (~20–40 ppbv). This behavior suggests two potential sources of GOM: photochemical oxidation of GEM and direct emissions of GOM from nearby local sources. Lack of correlation between GOM and Beryllium-7 (7Be) suggests little influence on surface GOM from downward mixing of GOM from the upper troposphere. These data were analyzed using the HYSPLIT back trajectory model and principal component analysis in order to develop source-receptor relationships for mercury species in this coastal environment. Trajectory frequency analysis shows that high GOM events were generally associated with high frequencies of the trajectories passing through the areas with high mercury emissions, while low GOM levels were largely associated the trajectories passing through relatively clean areas. Principal component analysis also reveals two main factors: direct emission and photochemical processes that were clustered with high GOM and PBM. This study indicates that the receptor site, which is located in a coastal environment of the Gulf of Mexico, experienced impacts from mercury sources that are both local and regional in nature.

Giant planets helped to shape the conditions we see in the Solar System today and they account for more than 99% of the mass of the Sun’s planetary system. They can be subdivided into the Ice Giants (Uranus and Neptune) and the Gas Giants (Jupiter and Saturn), which differ from each other in a number of fundamental ways. Uranus, in particular is the most challenging to our understanding of planetary formation and evolution, with its large obliquity, low self-luminosity, highly asymmetrical internal field, and puzzling internal structure. Uranus also has a rich planetary system consisting of a system of inner natural satellites and complex ring system, five major natural icy satellites, a system of irregular moons with varied dynamical histories, and a highly asymmetrical magnetosphere. Voyager 2 is the only spacecraft to have explored Uranus, with a flyby in 1986, and no mission is currently planned to this enigmatic system. However, a mission to the uranian system would open a new window on the origin and evolution of the Solar System and would provide crucial information on a wide variety of physicochemical processes in our Solar System. These have clear implications for understanding exoplanetary systems. In this paper we describe the science case for an orbital mission to Uranus with an atmospheric entry probe to sample the composition and atmospheric physics in Uranus’ atmosphere. The characteristics of such an orbiter and a strawman scientific payload are described and we discuss the technical challenges for such a mission. This paper is based on a white paper submitted to the European Space Agency’s call for science themes for its large-class mission programme in 2013.

From 10 May through 17 June 2007 and 6 June through 9 July 2008 intensive sampling campaigns at Summit, Greenland confirmed that active bromine chemistry is occurring in and above the snow pack at the highest part of the Greenland ice sheet (72°36´ N, 38°25´ W and 3.2 km above sea level). Direct measurements found BrO and soluble gas phase Br− mixing ratios in the low pptv range on many days (maxima < 10 pptv). Conversion of up to 200 pg m−3 of gaseous elemental mercury (GEM) to reactive gaseous mercury (RGM) and enhanced OH relative to HO2 plus RO2 confirm that active bromine chemistry is impacting chemical cycles even at such low abundances of reactive bromine species. However, it does not appear that Bry chemistry can fully account for observed perturbations to HOx partitioning, suggesting unknown additional chemical processes may be important in this unique environment, or that our understanding of coupled NOx-HOx-Bry chemistry above sunlit polar snow is incomplete. Rapid transport from the north Atlantic marine boundary layer occasionally caused enhanced BrO at Summit (just two such events observed during the 12 weeks of sampling over the two seasons). In general observed reactive bromine was linked to activation of bromide (Br−) in, and release of reactive bromine from, the snowpack. A coupled snow-atmosphere model simulated observed NO and BrO at Summit during a three day interval when winds were weak. The source of Br− in surface and near surface snow at Summit is not entirely clear, but concentrations were observed to increase when stronger vertical mixing brought free tropospheric air to the surface. Reactive Bry mixing ratios above the snow often increased in the day or two following increases in snow concentration, but this response was not consistent. On seasonal time scales concentrations of Br− in snow and reactive bromine in the air were directly related.

The purpose of this study was to directly measure the dry deposition of gaseous oxidized mercury (GOM) in western Maryland. Annual estimates were made using passive ion-exchange surrogate surfaces and a resistance model. Surrogate surfaces were deployed for seventeen weekly sampling periods between September 2009 and October 2010. Dry deposition rates from surrogate surfaces ranged from 80 to 1512pgm−2h−1. GOM dry deposition rates were strongly correlated (r2=0.75) with the weekly average atmospheric GOM concentrations, which ranged from 2.3 to 34.1pgm−3. Dry deposition of GOM could be predicted from the ambient air concentrations of GOM using this equation: GOM dry deposition (pgm−2h−1)=43.2×GOM concentration−80.3. Dry deposition velocities computed using GOM concentrations and surrogate surface GOM dry deposition rates, ranged from 0.2 to 1.7cms−1. Modeled dry deposition rates were highly correlated (r2=0.80) with surrogate surface dry deposition rates. Using the overall weekly average surrogate surface dry deposition rate (369±340pgm−2h−1), we estimated an annual GOM dry deposition rate of 3.2μgm−2year−1. Using the resistance model, we estimated an annual GOM dry deposition rate of 3.5μgm−2year−1. Our annual GOM dry deposition rates were similar to the dry deposition (3.3μgm−2h−1) of gaseous elemental mercury (GEM) at our site. In addition, annual GOM dry deposition was approximately 1/2 of the average annual wet deposition of total mercury (7.7±1.9μgm−2year−1) at our site. Total annual mercury deposition from dry deposition of GOM and GEM and wet deposition was approximately 14.4μgm−2year−1, which was similar to the average annual litterfall deposition (15±2.1μgm−2year−1) of mercury, which was also measured at our site.

We have developed a new nested-grid mercury (Hg) simulation over North America with a 1/2° latitude by 2/3° longitude horizontal resolution employing the GEOS-Chem global chemical transport model. Emissions, chemistry, deposition, and meteorology are self-consistent between the global and nested domains. Compared to the global model (4° latitude by 5° longitude), the nested model shows improved skill at capturing the high spatial and temporal variability of Hg wet deposition over North America observed by the Mercury Deposition Network (MDN) in 2008–2009. The nested simulation resolves features such as higher deposition due to orographic precipitation, land/ocean contrast and and predicts more efficient convective rain scavenging of Hg over the southeast United States. However, the nested model overestimates Hg wet deposition over the Ohio River Valley region (ORV) by 27%. We modify anthropogenic emission speciation profiles in the US EPA National Emission Inventory (NEI) to account for the rapid in-plume reduction of reactive to elemental Hg (IPR simulation). This leads to a decrease in the model bias to −2.3% over the ORV region. Over the contiguous US, the correlation coefficient (r) between MDN observations and our IPR simulation increases from 0.60 to 0.78. The IPR nested simulation generally reproduces the seasonal cycle in surface concentrations of speciated Hg from the Atmospheric Mercury Network (AMNet) and Canadian Atmospheric Mercury Network (CAMNet). In the IPR simulation, annual mean gaseous and particulate-bound Hg(II) are within 140% and 11% of observations, respectively. In contrast, the simulation with unmodified anthropogenic Hg speciation profiles overestimates these observations by factors of 4 and 2 for gaseous and particulate-bound Hg(II), respectively. The nested model shows improved skill at capturing the horizontal variability of Hg observed over California during the ARCTAS aircraft campaign. The nested model suggests that North American anthropogenic emissions account for 10–22% of Hg wet deposition flux over the US, depending on the anthropogenic emissions speciation profile assumed. The modeled percent contribution can be as high as 60% near large point sources in ORV. Our results indicate that the North American anthropogenic contribution to dry deposition is 13–20%.

The Greenland Summit Halogen-HOx (GSHOX) Campaign was performed in spring 2007 and summer 2008 to investigate the impact of halogens on HOx (= OH + HO2) cycling above the Greenland Ice Sheet. Chemical species including hydroxyl and peroxy radicals (OH and HO2 + RO2), ozone (O3), nitrogen oxide (NO), nitric acid (HNO3), nitrous acid (HONO), reactive gaseous mercury (RGM), and bromine oxide (BrO) were measured during the campaign. The median midday values of HO2 + RO2 and OH concentrations observed by chemical ionization mass spectrometry (CIMS) were 2.7 × 108 molec cm−3 and 3.0 × 106 molec cm−3 in spring 2007, and 4.2 × 108 molec cm−3 and 4.1 × 106 molec cm−3 in summer 2008. A basic photochemical 0-D box model highly constrained by observations of H2O, O3, CO, CH4, NO, and J values predicted HO2 + RO2 (R = 0.90, slope = 0.87 in 2007; R = 0.79, slope = 0.96 in 2008) reasonably well and under predicted OH (R = 0.83, slope = 0.72 in 2007; R = 0.76, slope = 0.54 in 2008). Constraining the model to HONO observations did not significantly improve the ratio of OH to HO2 + RO2 and the correlation between predictions and observations. Including bromine chemistry in the model constrained by observations of BrO improved the correlation between observed and predicted HO2 + RO2 and OH, and brought the average hourly OH and HO2 + RO2 predictions closer to the observations. These model comparisons confirmed our understanding of the dominant HOx sources and sinks in this environment and indicated that BrO impacted the OH levels at Summit. Although, significant discrepancies between observed and predicted OH could not be explained by the measured BrO. Finally, observations of enhanced RGM were found to be coincident with under prediction of OH.

The impact of rainfall on total gaseous mercury (TGM) flux from pavement and street dirt surfaces was investigated in an effort to determine the influence of wet weather events on mercury transport in urban watersheds. Street dirt and pavement are common urban ground surfaces that concentrate many substances (eroded soil, leaf and vegetation litter, automobile debris, industrial atmospheric fallout) which can contain elevated mercury concentrations. In this study, the primary analyses included (i) observing the time series flux of TGM from pavement and street dirt following surface wetting and (ii) determining if wet deposition provides a fresh source of mercury that is available for release (emission) when applied to these surfaces. Application of de-ionized water (DI) and rainwater both induced an immediate 65% increase in TGM emission from pavement (from 0.5 to 1.4ngm−2h−1 [based on averages]). For street dirt, an immediate 70% increase in emission was induced following DI water application (from 3.0 to 9.0ngm−2h−1 [based on averages]) and an immediate 30% increase in emission following rainwater application (from 4.5 to 6.5ngm−2h−1 [based on averages]). Both surfaces showed continuous elevated release of TGM following the initial water application stage. There was a decrease in emission as the pavement surface dried. Despite the difference in immediate TGM emission from street dirt using both solutions, statistical evaluation indicated there was no prolonged difference. This suggests that mercury in rainwater was not available for re-emission when applied to these surfaces, at least for the time frame studied (2h after water application). Therefore, it is likely that the elevated TGM emission following water application resulted primarily from pre-existing mercury. Removal of pre-existing mercury by water application followed a zero order process for both surfaces; however, removal rates were much different for each surface (k=0.26ngm−2min−1 for street dirt; k=0.03ngm−2min−1 for pavement). Results from laboratory surface washing experiments revealed only 0.1% of all available surface-bound mercury on pavement was removed by surface emission 90min after a simulated light rainfall event (0.13cm of rainfall).

Polar ecosystems are considered to be the last pristine environments of the earth relatively uninfluenced by human activities. Antarctica in particular, compared to the Arctic is considered to be even less affected by any kind of anthropogenic influences. Once contaminants reach the Polar Regions, their lifetime in the troposphere depends on local removal processes. Atmospheric mercury, in particular, has unique characteristics that include long-range transport to Polar Regions and the transformation to more toxic and water-soluble compounds that may potentially become bioavailable. These chemical-physical properties have placed mercury on the priority list of an increasing number of International, European and National conventions, and agreements, aimed at the protection of the ecosystems including human health (i.e. GEO, UNEP, AMAP, UNECE, HELCOM, OSPAR). This interest, in turn, stimulates a significant amount of research including measurements of gaseous elemental mercury reaction rate constant with atmospheric oxidants, experimental and modelling studies in order to understand the cycling of mercury in Polar Regions, and its impact to these ecosystems. Special attention in terms of contamination of Polar Regions is paid to the consequences of the springtime phenomena, referred to as "Atmospheric Mercury Depletion Events" (AMDEs), during which elemental mercury through a series of photochemically-initiated reactions involving halogens, may be converted to a reactive form that may accumulate in polar coastal, or sea ice, ecosystems. The discovery of the AMDEs, first noted in the Arctic, has also been observed at both poles and was initially considered to result in an important net input of atmospheric mercury into the polar surfaces. However, recent studies point out that complex processes take place after deposition that may result in less significant net-inputs from the atmosphere since a fraction, sometimes significant, of deposited mercury may be recycled. Therefore, the contribution of this unique reactivity occurring in polar atmospheres to the global budget of atmospheric mercury, and the role played by snow and ice surfaces of these regions, are important issues. This paper presents a review of atmospheric mercury studies conducted in the Antarctic troposphere, both at coastal locations and on the Antarctic Plateau since 1985. Our current understanding of atmospheric reactivity in this region is also presented.

Atmospheric mercury speciation was monitored within Houston, Texas, USA, August 6–October 14, 2006 as part of the TexAQS Radical and Aerosol Measurement Program (TRAMP). On average, all mercury levels were significantly elevated compared to a rural Gulf of Mexico coastal site. Concentrations varied from very clean to very dirty. Multi-day periods of stagnant or low-wind conditions brought elevated concentrations of all mercury species, whereas multi-day periods of strong winds, particularly southerly winds off the Gulf of Mexico, brought very low values of mercury species. Over the entire mercury measurement period, the daily averages of mercury species showed distinct and consistent relationships with the average planetary boundary layer dynamics, with gaseous elemental and particulate-bound mercury near-surface concentrations enhanced by a shallow nocturnal boundary layer, and reactive gaseous mercury concentration enhanced by midday convective boundary layer air entrainment transporting air aloft to the surface. Mercury concentrations were not significantly correlated with known products of combustion, likely indicating non-combustion mercury sources from the Houston area petrochemical complexes. On the morning of August 31, 2006 an observed emission event at a refinery complex on the Houston Ship Channel resulted in extremely high concentrations of aerosol mass and particulate-bound mercury at the TRAMP measurement site 20 km downwind.