Temporal distribution and potential sources of atmospheric mercury measured at a high-elevation background station in Taiwan

Measurements of gaseous elemental mercury (GEM), reactive gaseous mercury (RGM), and particulate mercury (PHg) have been conducted at Lulin Atmospheric Background Station (LABS) in Taiwan since April 2006. This was the first long-term free tropospheric atmospheric Hg monitoring program in the downwind region of East Asia, which is a major Hg emission source region. Between April 13, 2006 and December 31, 2007, the mean concentrations of GEM, RGM, and PHg were 1.73 ng m^?3, 12.1 pg m^?3, and 2.3 pg m^?3, respectively. A diurnal pattern was observed for GEM with afternoon peaks and nighttime lows, whereas the diurnal pattern of RGM was opposite to that of GEM. Spikes of RGM were frequently observed between midnight and early morning with concurrent decreases in GEM and relative humidity and increases in O₃, suggesting the oxidation of GEM and formation of RGM in free troposphere (FT). Upslope movement of boundary layer (BL) air in daytime and subsidence of FT air at night resulted in these diurnal patterns. Considering only the nighttime data, which were more representative of FT air, the composite monthly mean GEM concentrations ranged between 1.06 and 2.06 ng m^?3. Seasonal variation in nighttime GEM was evident, with lower concentrations usually occurring in summer when clean marine air masses prevailed. Between fall and spring, air masses passed the East Asian continent prior to reaching LABS, contributing to the elevated GEM concentrations. Analysis of GEM/CO correlation tends to support the argument. Good GEM/CO correlations were observed in fall, winter, and spring, suggesting influence of anthropogenic emission sources. Our results demonstrate the significance of East Asian Hg emissions, including both anthropogenic and biomass burning emissions, and their long-range transport in the FT. Because of the pronounced seasonal monsoon activity and the seasonal variation in regional wind field, export of the Asian Hg emissions to Taiwan occurs mainly during fall, winter, and spring.     

Spatial distribution and seasonal variation of atmospheric bulk deposition of polycyclic aromatic hydrocarbons in Beijing-Tianjin region, North China

Bulk deposition samples were collected in remote, rural village and urban areas of Beijing-Tianjin region, North China in spring, summer, fall and winter from 2007 to 2008. The annually averaged PAHs concentration and deposition flux were 11.81 ± 4.61 μg/g and 5.2 ± 3.89 μg/m²/day respectively. PHE and FLA had the highest deposition flux, accounting for 35.3% and 20.7% of total deposition flux, respectively. More exposure risk from deposition existed in the fall for the local inhabitants. In addition, the PAHs deposition flux in rural villages (3.91 μg/m²/day) and urban areas (8.28 μg/m²/day) was 3.8 and 9.1 times higher than in background area (0.82 μg/m²/day), respectively. This spatial variation of deposition fluxes of PAHs was related to the PAHs emission sources, local population density and air concentration of PAHs, and the PAHs emission sources alone can explain 36%, 49%, 21% and 30% of the spatial variation in spring, summer, fall and winter, respectively.     

Soil surface Hg emission flux in coalfield in Wuda,Inner Mongolia,China

Hg emission flux from various land covers, such as forests, wetlands, and urban areas, have been investigated. China has the largest area of coalfield in the world, but data of Hg flux of coalfields, especially, those with coal fires, are seriously limited. In this study, Hg fluxes of a coalfield were measured using the dynamic flux chamber (DFC) method, coupled with a Lumex multifunctional Hg analyzer RA-915+ (Lumex Ltd., Russia). The results show that the Hg flux in Wuda coalfield ranged from 4 to 318 ng m^?2 h^?1, and the average value for different areas varied, e.g., coal-fire area 99 and 177 ng m^?2 h^?1; no coal-fire area 19 and 32 ng m^?2 h^?1; and backfilling area 53 ng m^?2 h^?1. Hg continued to be emitted from an underground coal seam, even if there were no phenomena, such as vents, cracks, and smog, of coal fire on the soil surface. This phenomenon occurred in all area types, i.e., coal-fire area, no coal-fire area, and backfilling area, which is universal in Wuda coalfield. Considering that many coalfields in northern China are similar to Wuda coalfield, they may be large sources of atmospheric Hg. The correlations of Hg emission flux with influence factors, such as sunlight intensity, soil surface temperature, and atmospheric Hg content, were also investigated for Wuda coalfield.

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Gaseous mercury fluxes from the forest floor of the Adirondacks

The flux of gaseous elemental mercury (Hg⁰) from the forest floor of the Adirondack Mountains in New York (USA) was measured numerous times throughout 2005 and 2006 using a polycarbonate dynamic flux chamber (DFC). The Hg flux ranged between −2.5 and 27.2 ng m⁻² h⁻¹ and was positively correlated with temperature and solar radiation. The measured Hg emission flux was highest in spring, and summer, and lowest in winter. During leaf-off periods, the Hg emission flux was highly dependent on solar radiation and less dependent on temperature. During leaf-on periods, the Hg emission flux was fairly constant because the forest canopy was shading the forest floor. Two empirical models were developed to estimate yearly Hg⁰ emissions, one for the leaf-off period and one for the leaf-on period. Using the U.S. EPA's CASTNET meteorological data, the cumulative estimated emission flux was approx. 7.0 μg Hg⁰ m⁻² year⁻¹.     

Evidence for short-range transport of atmospheric mercury to a rural,inland site

Atmospheric mercury (Hg) species, including gaseous elemental mercury (GEM), reactive gaseous mercury (RGM) and particulate-bound mercury (Hg_p), were monitored near three sites, including a cement plant (monitored in 2007 and 2008), an urban site and a rural site (both monitored in 2005 and 2008). Although the cement plant was a significant source of Hg emissions (for 2008, GEM: 2.20 ± 1.39 ng m^?3, RGM: 25.2 ± 52.8 pg m^?3, Hg_p 80.8 ± 283 pg m^?3), average GEM levels and daytime average dry depositional RGM flux were highest at the rural site, when all three sites were monitored sequentially in 2008 (rural site, GEM: 2.37 ± 1.26 ng m^?3, daytime RGM flux: 29 ± 40 ng m^?2 day^?1). Photochemical conversion of GEM was not the primary RGM source, as highest net RGM gains (75.9 pg m^?3, 99.0 pg m^?3, 149 m^?3) occurred within 3.0–5.3 h, while the theoretical time required was 14–23 h. Instead, simultaneous peaks in RGM, Hg_p, ozone (O₃), nitrogen oxides, and sulfur dioxide in the late afternoon suggested short-range transport of RGM from the urban center to the rural site. The rural site was located more inland, where the average water vapor mixing ratio was lower compared to the other two sites (in 2008, rural: 5.6 ± 1.4 g kg^?1, urban: 9.0 ± 1.1 g kg^?1, cement plant: 8.3 ± 2.2 g kg^?1). Together, these findings suggested short-range transport of O₃ from an urban area contributed to higher RGM deposition at the rural site, while drier conditions helped sustain elevated RGM levels. Results suggested less urbanized environments may be equally or perhaps more impacted by industrial atmospheric Hg emissions, compared to the urban areas from where Hg emissions originated.     

Transpiration of gaseous elemental mercury through vegetation in a subtropical wetland in Florida

Four seasonal sampling campaigns were carried out in the Florida Everglades to measure elemental Hg vapor (Hg°) fluxes over emergent macrophytes using a modified Bowen ratio gradient approach. The predominant flux of Hg° over both invasive cattail and native sawgrass stands was emission; mean day time fluxes over cattail ranged from ∼20 (winter) to ∼40 (summer) ng m⁻² h⁻¹. Sawgrass fluxes were about half those over cattail during comparable periods. Emission from vegetation significantly exceeded evasion of Hg° from the underlying water surface (∼1–2 ng m⁻² h⁻¹) measured simultaneously using floating chambers. Among several environmental factors (e.g. CO₂ flux, water vapor flux, wind speed, water, air and leaf temperature, and solar radiation), water vapor exhibited the strongest correlation with Hg° flux, and transpiration is suggested as an appropriate term to describe this phenomenon. The lack of significant Hg° emissions from a live, but uprooted (floating) cattail stand suggests that a likely source of the transpired Hg° is the underlying sediments. The pattern of Hg° fluxes typically measured indicated a diel cycle with two peaks, possibly related to different gas exchange dynamics: one in early morning related to lacunal gas release, and a second at midday related to transpiration; nighttime fluxes approached zero.     

Snow-to-air exchanges of mercury in an Arctic seasonal snow pack in Ny-Ålesund,Svalbard

The study of mercury (Hg) cycle in Arctic regions is a major subject of concern due to the dramatic increases of Hg concentrations in ecosystem in the last few decades. The causes of such increases are still in debate, and an important way to improve our knowledge on the subject is to study the exchanges of Hg between atmosphere and snow during springtime. We organized an international study from 10 April to 10 May 2003 in Ny-Ålesund, Svalbard, in order to assess these fluxes through measurements and derived calculations.Snow-to-air emission fluxes of Hg were measured using the flux chamber technique between ∼0 and 50 ng m⁻² h⁻¹. A peak in Gaseous Elemental Mercury (GEM) emission flux from the snow to the atmosphere has been measured just few hours after an Atmospheric Mercury Depletion Event (AMDE) recorded on 22 April 2004. Surprisingly, this peak in GEM emitted after this AMDE did not correspond to any increase in Hg concentration in snow surface. A peak in GEM flux after an AMDE was observed only for this single event but not for the four other AMDEs recorded during this spring period.In the snow pack which is seasonal and about 40 cm depth above permafrost, Hg is involved in both production and incorporation processes. The incorporation was evaluated to ∼5–40 pg m² h. Outside of AMDE periods, Hg flux from the snow surface to the atmosphere was the consequence of GEM production in the air of snow and was about ∼15–50 ng m⁻² h⁻¹, with a contribution of deeper snow layers evaluated to ∼0.3–6.5 ng m⁻² h⁻¹. The major part of GEM production is then mainly a surface phenomenon. The internal production of GEM was largely increasing when snow temperatures were close to melting, indicating a chemical process occurring in the quasi-liquid layer at the surface of snow grains.     

Measurements of GEM fluxes and atmospheric mercury concentrations (GEM,RGM and Hgp) from an agricultural field amended with biosolids in Southern Ont., Canada (October 2004–November 2004)

Five weeks of gaseous elemental mercury (GEM), reactive gaseous mercury (RGM) and particle bound mercury (Hg^p) concentrations as well as fluxes of GEM were measured at Maryhill, Ontario, Canada above a biosolids amended field. The study occurred during the autumn of 2004 (October–November) to capture the effects of cool weather conditions on the behaviour of mercury in the atmosphere. The initial concentration of total mercury (Hg) in the amended soil was relatively low (0.4 μg g⁻¹±10%).A micrometeorological approach was used to infer the flux of GEM using a continuous two-level sampling system with inlets at 0.40 and 1.25 m above the soil surface to measure the GEM concentration gradient. The required turbulent transfer coefficients were derived from meteorological parameters measured on site. The average GEM flux over the study was 0.1±0.2 ng m⁻² h⁻¹(±one standard deviation). The highest averaged hourly GEM fluxes occurred when the averaged net radiation was highest, although the slight diurnal patterns observed were not statistically significant for the complete flux data series. GEM emission fluxes responded to various local events including the passage of a cold front when the flux increased to 2 ng m⁻² h⁻¹ and during a biosolids application event at an adjacent field when depositional fluxes peaked at −3 ng m⁻² h⁻¹. Three substantial rain events during the study kept the surface soil moisture near field capacity and only slightly increased the GEM flux. Average concentrations of RGM (2.3±3.0 pg m⁻³), Hg^p (3.0±6.2 pg m⁻³) and GEM (1.8±0.2 ng m⁻³) remained relatively constant throughout the study except when specific local events resulted in elevated concentrations. The application of biosolids to an adjacent field produced large increases in Hg^p (25.8 pg m⁻³) and RGM (21.7 pg m⁻³) concentrations only when the wind aligned to impact the experimental equipment. Harvest events (corn) in adjacent fields also corresponded to higher concentrations of GEM and Hg^p but with no elevated peaks in RGM concentrations. Diurnal patterns were not statistically significant for RGM and Hg^p at Maryhill.     

NOX fluxes from several typical agricultural fields during summer-autumn in the Yangtze Delta,China

NO_X fluxes from three kinds of vegetable lands and a rice field were measured during summer–autumn in the Yangtze Delta, China. The average NO fluxes from the rice fields (RF), celery field (CE), maize field (MA) and cowpea field (CP) were 4.1, 30.8, 54 and 32.2 ng N m^?2 s^?1, respectively; and the average NO₂ fluxes were ?2.12, 0.68, 1.33 and 0.5 ng N m^?2 s^?1, respectively. The liquid N fertilizer (the mixture of swine excrement and urine) which is widely applied to vegetable lands by Chinese farmers was found to quickly stimulate NO emission, and have significant contribution to NO emission from the investigated vegetable lands. Apparent linearity correlations were found between NO₂ fluxes and the ambient concentrations of the rice fields, with a compensation point of about 2.84 μg m^?3. Total emissions of NO during summer–autumn time from this area were roughly estimated to be 4.1 and 8.4 Gg N for rice field and vegetable lands, respectively.     

Wet deposition mercury fluxes in the Canadian sub-Arctic and southern Alberta,measured using an automated precipitation collector adapted to cold regions

This paper reports mercury (Hg) concentrations and fluxes in precipitation that was collected from 2006 to 2008 at three sites in Canada: sub-Arctic boreal forest, sub-Arctic coast, and southern Alberta, using cold-adapted precipitation collectors which operated reliably at temperatures below ?30 °C during the study. The southern Alberta site (Crossfield) may be influenced by Calgary urban air, whereas the sub-Arctic coastal (Churchill, Manitoba) and boreal forest (Fort Vermilion, Alberta) sites are in more remote northern areas. Annual mean Hg concentrations in precipitation (5.0–9.2 ng L^?1) at the study sites were in the lower half of the range reported for southern Canada and the USA by the Mercury Deposition Network (MDN). But owing to typically low precipitation rates, gross wet Hg fluxes (0.54–2.0 μg m^?2 yr^?1) were among the lowest reported by MDN, with Crossfield having about twice the flux in 2007 of the other two sites. Flux was significantly correlated with precipitation, and thus was highest in summer (June–August) and lowest during winter, a pattern typical of other temperate continental locations. There was no evidence of higher wet Hg fluxes or concentrations in springtime at Churchill where atmospheric mercury depletion events (AMDEs) occur. Measured gross deposition fluxes at the study locations were ～2–8 times lower than estimated by GEOS-Chem and GRAHM atmospheric models. The largest discrepancy occurred for Churchill, which raises the question of how well Hg deposition from AMDEs is described by current models. Better agreement between measurements and models was obtained from MDN stations in Alberta and Alaska, where wet Hg fluxes were 2–10 times higher than the study sites either because of power plant emissions (Alberta), or because of high precipitation rates (Alaska).     

Study on dissolved organic carbon in precipitation in Northern China

Dissolved organic carbon (DOC) was measured in 483 precipitation samples collected at 10 sites in Northern China from December 2007 to November 2008. The annual volume-weighted mean (VWM) concentrations and wet deposition fluxes of DOC for 10 sites ranged from 2.4 to 3.9 mg C/L and 1.4 to 2.7 g C m^?2 yr^?1, respectively. The proportion of DOC to total organic carbon (TOC) was 79% on average, suggesting that a significant fraction of TOC was present as insoluble particulate organic carbon. Due to intensive domestic coal use for house heating and smaller dilution of scavenged organic carbon, higher VWM concentrations of DOC were observed during winter and spring than during summer and autumn. When precipitation events were classified via air mass back-trajectories, the mixed trajectories from SE and NW always corresponded to significantly higher DOC than those from SE or NW alone, coinciding with the centre of a low pressure system moved eastward and the wind direction changed from southeast to northwest. The results also showed that each site had a similar seasonal variation for DOC wet deposition flux. The largest flux occurred during the rainy season, and the lowest flux appeared during winter months. The product of the TC/DOC ratio and the DOC flux yielded an average TC wet deposition flux of 3.2 g C m^?2 yr^?1 in Northern China, accounting for 8.6% and 22% of the carbon sink magnitude (37 g C m^?2 yr^?1) in terrestrial ecosystems and anthropogenic carbon emissions (14 g C m^?2 yr^?1), respectively. This indicates that atmospheric wet deposition of TC is a significant carbon flux that cannot be neglected in regional models of the carbon cycle, and should be considered along with dry deposition in the removal mechanism for carbon from regional atmosphere.     

Growing season total gaseous mercury (TGM) flux measurements over an Acer rubrum L. stand

Relaxed eddy accumulation (REA) measurements of the total gaseous mercury (TGM) flux measurements were taken over a deciduous forest predominantly composed of Red Maple (Acer rubrum L.) during the growing season of 2004 and the second half of the growing season of 2005. The magnitudes of the flux estimates were in the range of published results from other micrometeorological mercury fluxes taken above a tall canopy and larger than estimates from flux chambers. The magnitude and direction of the flux were not static during the growing season. There was a significant trend (p < 0.001), from net deposition of TGM in early summer to net evasion in the late summer and early fall before complete senescence. A growing season atmosphere-canopy total mercury (TGM) compensation point during unstable daytime conditions was estimated at background ambient concentrations (1.41 ng m^?3). The trend in the seasonal net TGM flux indicates that long term dry deposition monitoring is needed to accurately estimate mercury loading over a forest ecosystem.     

Dynamic exchange of gaseous elemental mercury during polar night and day

From February 29 until June 15 2008 gaseous elemental mercury (GEM) fluxes above a snow covered surface was measured in Ny-Ålesund, Svalbard using a GEM flux gradient method. A clear seasonal pattern in the meteorological variables associated with the GEM flux was observed. For the first time in Ny-Ålesund a net deposition of GEM was recorded during polar night, despite the lack of Atmospheric Mercury Depletion Events (AMDE). 7500 ng m⁻² GEM was emitted from the surface snow to the air during the entire study. The depositions of GEM and reactive gaseous mercury (RGM) were calculated to be 1500 and 1000 ng m⁻², respectively, during the same time period. The GEM fluxes reported in this study were found to be comparable to GEM fluxes measured at other Arctic locations (i.e. Alert and Barrow), suggesting that GEM acts in a similar way throughout the Arctic. An assessment of the GEM flux gradient method used discovered a non-linear GEM concentration profile. The nonlinearity was explained by a non-stationary turbulence regime. The GEM flux calculated was not found to be representative for the entire surface boundary layer.     

Characteristics of mercury exchange flux between soil and air in the heavily air-polluted area,eastern Guizhou,China

To investigate the characteristics of mercury exchange between soil and air in the heavily air-polluted area, total gaseous mercury (TGM) concentration in air and Hg exchange flux were measured in Wanshan Hg mining area (WMMA) in November, 2002 and July–August, 2004. The results showed that the average TGM concentrations in the ambient air (17.8–1101.8 ng m⁻³), average Hg emission flux (162–27827 ng m⁻² h⁻¹) and average Hg dry deposition flux (0–9434 ng m⁻² h⁻¹) in WMMA were 1–4 orders of magnitude higher than those in the background area. It is said that mercury-enriched soil is a significant Hg source of the atmosphere in WMMA. It was also found that widely distributed roasted cinnabar banks are net Hg sources of the atmosphere in WMMA. Relationships between mercury exchange flux and environmental parameters were investigated. The results indicated that the rate of mercury emission from soil could be accelerated by high total soil mercury concentration and solar irradiation. Whereas, highly elevated TGM concentrations in the ambient air can restrain Hg emission from soil and even lead to strongly atmospheric Hg deposition to soil surface. A great amount of gaseous mercury in the heavily polluted atmosphere may cycle between soil and air quickly and locally. Vegetation can inhibit mercury emission from soil and are important sinks of atmospheric mercury in heavily air-polluted area.     

Urban–rural differences in atmospheric mercury speciation

Measurements of gaseous elemental mercury (GEM), particulate mercury (Hg_p), and reactive gaseous mercury (RGM) were concurrently recorded at an urban site in Detroit and a rural site in Dexter, both in Michigan for the calendar year 2004. Their average concentrations (±standard deviation) for the urban area were 2.5 ± 1.4 ng m^?3, 18.1 ± 61.0 pg m^?3, and 15.5 ± 54.9 pg m^?3, respectively, while their rural counterparts were 1.6 ± 0.6 ng m^?3, 6.1 ± 5.5 pg m^?3, and 3.8 ± 6.6 pg m^?3, respectively. The medians of urban-to-rural ratios of Hg concentrations indicate approximately 1-fold, 2-fold, and 3-fold gradients between Detroit and Dexter for GEM, Hg_p, and RGM, respectively. The urban–rural differences in Hg also varied considerably on different temporal scales and with wind flow patterns, which was most evident in RGM. Our results show that while Hg at both sites was impacted by regional sources, meteorological conditions, and photochemical transformations, the extent of variations in the observed urban-to-rural gradients, particularly in RGM, cannot be fully accounted for by these processes. Both analyses of the annual data and case studies indicate that the more variable and episodic nature of Hg, particularly RGM, seen in Detroit compared with Dexter, was the result of direct impact from local anthropogenic sources.     

Natural chloroform emissions from the blanket peat bogs in the vicinity of Mace Head,Ireland over a 14-year period

Simultaneous chloroform (CHCl₃) emission and ozone (O₃) deposition are regularly observed under nocturnal inversions during the summer months from and to the peat bogs in the vicinity of the Mace Head Atmospheric Research Station, Connemara, Co Galway, Ireland. Emissions were estimated using the nocturnal box model applied to routine atmospheric observations collected over a 14-year period from 1995 to 2008. Strict criteria were applied in the selection of events of low wind speed, under a stable night-time inversion layer in baseline air conditions, with no transport from Europe. The mean peatland CHCl₃ flux was 2.91 μg m^?2 h^?1 with highly variable fluxes ranging from 0.44 to 12.94 μg m^?2 h^?1. These fluxes are generally larger than those reported previously for similar biomes and if representative would make a significant contribution to the global estimated source of CHCl₃. Fluxes were not strongly correlated with either atmospheric temperature or the level of precipitation. Over the 14-year period there appears to have been a small increase in overall CHCl₃ emissions, although we stress that the nocturnal box model has a number of limitations and assumptions which should be taken into account.     

Dry and wet deposition of water-insoluble dust and water-soluble chemical species during spring 2007 in Tsukuba,Japan

Dry and wet depositions were sampled daily in Tsukuba, Japan, in spring 2007. Temporal variations in the dry and wet deposition fluxes of dust and water-soluble chemical species were controlled largely by air mass origin, the water vapor mixing ratio, and Asian dust events. The contribution of local sources to dry deposition of dust was large when the wind speed was high. Dry deposition fluxes of water-soluble chemical species were larger in humid air masses than in dry air masses. Wet deposition fluxes of dust and water-soluble chemical species indicated that air masses that passed over dust source regions and industrial regions became mixed with the maritime air masses over the coastal site of the Asian continent and western part of the Japanese islands. The total deposition of dust was 4220 mg m^?2 month^?1, and that of water-soluble chemical species ranged from 10 to 636 mg m^?2 month^?1. Wet deposition fluxes of the total deposition flux of dust accounted for 72% and those of water-soluble chemical species was for 72–96%. In particular, the largest wet deposition occurred during a single Asian dust event on 3 April. This event accounted for 23% (950 mg m^?2 month^?1) of the monthly dust deposition flux and for 2–28% (0.43–51 mg m^?2 month^?1) of the monthly deposition flux of water-soluble chemical species. This result implies that the wet deposition flux associated with even one sporadic Asian dust event can have extensive impacts on both terrestrial and oceanic ecosystems in East Asia.     

Spatial and temporal variations of nitrous oxide flux between coastal marsh and the atmosphere in the Yellow River estuary of China

To investigate the spatial and seasonal variations of nitrous oxide (N₂O) fluxes and understand the key controlling factors, we explored N₂O fluxes and environmental variables in high marsh (HM), middle marsh (MM), low marsh (LM), and mudflat (MF) in the Yellow River estuary throughout a year. Fluxes of N₂O differed significantly between sampling periods as well as between sampling positions. During all times of day and the seasons measured, N₂O fluxes ranged from ?0.0051 to 0.0805 mg N₂O m^?2 h^?1, and high N₂O emissions occurred during spring (0.0278 mg N₂O m^?2 h^?1) and winter (0.0139 mg N₂O m^?2 h^?1) while low fluxes were observed during summer (0.0065 mg N₂O m^?2 h^?1) and autumn (0.0060 mg N₂O m^?2 h^?1). The annual average N₂O flux from the intertidal zone was 0.0117 mg N₂O m^?2 h^?1, and the cumulative N₂O emission throughout a year was 113.03 mg N₂O m^?2, indicating that coastal marsh acted as N₂O source. Over all seasons, N₂O fluxes from the four marshes were significantly different (p?<?0.05), in the order of HM (0.0256?±?0.0040 mg N₂O m^?2 h^?1)?>?MF (0.0107?±?0.0027 mg N₂O m^?2 h^?1)?>?LM (0.0073?±?0.0020 mg N₂O m^?2 h^?1)?>?MM (0.0026?±?0.0011 mg N₂O m^?2 h^?1). Temporal variations of N₂O emissions were related to the vegetations (Suaeda salsa, Phragmites australis, and Tamarix chinensis) and the limited C and mineral N in soils during summer and autumn and the frequent freeze/thaw cycles in soils during spring and winter, while spatial variations were mainly affected by tidal fluctuation and plant composition at spatial scale. This study indicated the importance of seasonal N₂O contributions (particularly during non-growing season) to the estimation of local N₂O inventory, and highlighted both the large spatial variation of N₂O fluxes across the coastal marsh (CV?=?158.31 %) and the potential effect of exogenous nitrogen loading to the Yellow River estuary on N₂O emission should be considered before the annual or local N₂O inventory was evaluated accurately.     

Estimates of air-sea exchange of mercury in the Baltic Sea

The concentrations of total gaseous mercury (TGM) in air over the southern Baltic Sea and dissolved gaseous mercury (DGM) in the surface seawater were measured during summer and winter. The summer expedition was performed on 02–15 July 1997, and the winter expedition on 02–15 March 1998. Average TGM and DGM values obtained were 1.70 and 17.6 ng m⁻³ in the summer and 1.39 and 17.4 ng m⁻³ in the winter, respectively. Based on the TGM and DGM data, surface water saturation and air-water fluxes were calculated. The results indicate that the seawater was supersaturated with gaseous mercury during both seasons, with the highest values occurring in the summer. Flux estimates were made using the thin film gas-exchange model. The average Hg fluxes obtained for the summer and winter measurements were 38 and 20 ng m⁻² d⁻¹, respectively. The annual mercury flux from this area was estimated by a combination of the TGM and DGM data with monthly average water temperatures and wind velocities, resulting in an annual flux of 9.5 μg m⁻² yr⁻¹. This flux is of the same order of magnitude as the average wet deposition input of mercury in this area. This indicates that reemissions from the water surface need to be considered when making mass-balance estimates of mercury in the Baltic Sea as well as modelling calculations of long-range transboundary transport of mercury in northern Europe.     

Carbon dioxide fluxes over an urban park area

From September 2006 to October 2007 turbulent fluxes of carbon dioxide were measured at an urban tower station (26 m above ground level, z/z_h = 1.73) in Essen, Germany, using the eddy covariance technique. The site was located at the border between a public park area (70 ha) in the south–west of the station and suburban/urban residential as well as light commercial areas in the north and east of the tower. Depending on the land-use two different sectors (park and urban) were identified showing distinct differences in the temporal evolution of the surface-atmosphere exchange of CO₂. While urban fluxes appear to be governed by anthropogenic emissions from domestic heating and traffic (average flux 9.3 μmol m^?2 s^?1), the exchange of CO₂ was steered by biological processes when the park contributed to the flux footprint. The diurnal course during the vegetation period exhibited negative daytime fluxes up to ?10 μmol m^?2 s^?1 on average in summer. Nevertheless, with a mean of 0.8 μmol m^?2 s^?1 park sector fluxes were slightly positive, thus no net carbon uptake by the surface occurred throughout the year.In order to sum the transport of CO₂ a gap-filling procedure was performed by means of artificial neural network generalisation. Using additional meteorological inputs the daily exchange of CO₂ was reproduced using radial basis function networks (RBF). The resulting yearly sum of 6031 g m^?2 a^?1 indicates the entire study site to be a considerable source of CO₂.