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).     

Dry deposition and foliar leaching of mercury and selected trace elements in deciduous forest throughfall

The estimated annual throughfall deposition flux of Hg in a northern mixed-hardwood forest in the Lake Huron Watershed was 10.5±1.0 μg m⁻² compared to an annual precipitation Hg flux of 8.7±0.5 μg m⁻² (June 1996–June 1997). The source of this additional Hg in throughfall is often attributed to wash-off of dry deposition, but foliar leaching of Hg may also be important. To determine the influence of both dry deposition and foliar leaching of Hg and other elements in throughfall, we measured a suite of trace elements (Hg, Al, Mg, V, Mn, Cu, Zn, As, Rb, Sr, Cd, Ba, La, Ce, and Pb) in throughfall, precipitation, and ambient air samples from a northern mixed-hardwood forest. Based on a multiple linear regression model, dry deposition had the most important influence on Hg, Al, La, Ce, V, As, Cu, Zn, Cd, and Pb fluxes while foliar leaching strongly influenced Mg, Mn, Rb, Sr, and Ba fluxes in net throughfall. The Hg dry deposition flux was estimated using gaseous and aerosol Hg measurements and modeled deposition velocities. The calculated dry deposition flux (∼12–14 μg m⁻²) of Hg to the canopy indicated that atmospheric deposition of Hg could easily account for all of the Hg deposited in net throughfall (1.9±0.1 μg m⁻²). Although there is a large uncertainty associated with these techniques, the modeling estimates indicate that atmospheric Hg may account for all of the Hg deposited in litterfall (11.4±2.8 μg m⁻²).     

Estimating contribution of precipitation scavenging of atmospheric particulate mercury to mercury wet deposition in Japan

Mercury wet deposition is dependent on both the scavenging of divalent reactive gaseous mercury (RGM) and atmospheric particulate mercury (Hg(p)) by precipitation. Estimating the contribution of precipitation scavenging of RGM and Hg(p) is important for better understanding the causes of the regional and seasonal variations in mercury wet deposition. In this study, the contribution of Hg(p) scavenging was estimated on the basis of the scavenging ratios of other trace elements (i.e., Cd, Cu, Mn, Ni, Pb and V) existing entirely in particulate form. Their wet deposition fluxes and concentrations in air, which were measured concurrently from April 2004 to March 2005 at 10 sites in Japan, were used in this estimation. The monthly wet deposition flux of mercury at each site correlated with the amount of monthly precipitation, whereas the Hg(p) concentrations in air tended to decrease during summer. There was a significant correlation (P<0.001) among the calculated monthly average scavenging ratios of trace elements, and the values in each month at each site were similar. Therefore, it is assumed the monthly scavenging ratio of Hg(p) is equivalent to the mean value of other trace elements. Using this scavenging ratio (W), the wet deposition flux (F) due to Hg(p) scavenging in each month was calculated by F=WKP, where K and P are the Hg(p) concentration and amount of precipitation, respectively. Relatively large fluxes due to Hg(p) scavenging were observed at a highly industrial site and at sites on the Japan Sea coast, which are strongly affected by the local sources and the long-range transport from the Asian continent, respectively. However, on average, at the 10 sites, the contribution of Hg(p) scavenging to the annual mercury deposition flux was 26%, suggesting that mercury wet deposition in Japan is dominated by RGM scavenging. This RGM should originate mainly from the in situ oxidation of Hg⁰ in the atmosphere.     

Reduced mercury deposition in New Hampshire from 1996 to 2002 due to changes in local sources

Changes in deposition of gaseous divalent mercury (Hg(II)) and particulate mercury (Hg(p)) in New Hampshire due to changes in local sources from 1996 to 2002 were assessed using the Industrial Source Complex Short Term (ISCST3) model (regional and global sources and Hg atmospheric reactions were not considered). Mercury (Hg) emissions in New Hampshire and adjacent areas decreased significantly (from 1540 to 880 kg yr⁻¹) during this period, and the average annual modeled deposition of total Hg also declined from 17 to 7.0 μg m⁻² yr⁻¹ for the same period. In 2002, the maximum amount of Hg deposition was modeled to be in southern New Hampshire, while for 1996 the maximum deposition occurred farther north and east. The ISCST3 was also used to evaluate two future scenarios. The average percent difference in deposition across all cells was 5% for the 50% reduction scenario and 9% for the 90% reduction scenario.     

Factors contributing to seasonal variations in wet deposition fluxes of trace elements at sites along Japan Sea coast

In this study, we measured the wet deposition fluxes of ten trace elements (As, Cd, Cr, Cu, Mn, Ni, Pb, Sb, V and Zn) from December 2002 to March 2006 at three sites along the Japan Sea coast, which have been strongly affected by the long-range transport of air pollutants from the Asian continent. Also, factors, contributing to their seasonal variations were investigated. At the northern and central sites, the monthly wet deposition fluxes of all or most trace elements greatly increased during the cold season (typically, November–April), along with their monthly average (volume-weighted) concentrations in the precipitation. The cold/warm season ratios for the average concentrations of trace elements in precipitation were within the range of 2.7–5.1 at the northern site and 1.8–5.9 at the central site, which were similar to the average scavenging ratios (= concentration in precipitation/concentration in air) at each site. However, there were small differences (0.47–1.2 at the northern site and 0.73–1.7 at the central site) in the ratios of average concentrations in air between the two seasons. These suggest that the increase in the wet deposition fluxes of trace elements during the cold season is due to increases in their scavenging ratios. On the other hand, the result for the southern site was different from those at the other sites. The number of days when the daily maximum wind speed exceeded 10 m s^?1 at the meteorological observatories near the study sites increased markedly during the cold season at the northern and central sites, showing that strong winds usually blow during the cold season at those sites, but not at the southern site. Higher wind speed transports larger amounts of constituents into the cloud system, which can result in their increased concentrations in precipitation. Thus, high scavenging ratios of trace elements during the cold season may be caused by the increase in their amounts of discharge into the cloud system owing to high wind speed, suggesting that wind speed is an important factor in the seasonal variations in the wet deposition fluxes.     

Polycyclic aromatic hydrocarbon (PAH) deposition to and exchange at the air-water interface of Luhu, an urban lake in Guangzhou, China

Urban lakes are vulnerable to the accumulation of semivolatile organic compounds, such as PAHs from wet and dry atmospheric deposition. Little was reported on the seasonal patterns of atmospheric deposition of PAHs under Asian monsoon climate. Bulk (dry + wet) particle deposition, air-water diffusion exchange, and vapour wet deposition of PAHs in a small urban lake in Guangzhou were estimated based on a year-round monitoring. The total PAH particle deposition fluxes observed were 0.44-3.46 μg m⁻² day⁻¹. The mean air-water diffusive exchange flux was 20.7 μg m⁻² day⁻¹. The vapour deposition fluxes of PAHs ranged 0.15-8.26 μg m⁻² day⁻¹. Remarkable seasonal variations of particulate PAH deposition, air-water exchange fluxes and vapour wet deposition were influenced by seasonal changes in meteorological parameters. The deposition fluxes were predominantly controlled by the precipitation intensity in wet season whereas by atmospheric concentration in dry season.     

Characteristics and impacts of trace elements in atmospheric deposition at a high-elevation site,southern China

To investigate the regional background trace element (TE) level in atmospheric deposition (dry and wet), TEs (Fe, Al, V, Cr, Mn, Ni, Cu, Zn, As, Se, Mo, Cd, Ba, and Pb) in 52 rainwater samples and 73 total suspended particles (TSP) samples collected in Mt. Lushan, Southern China, were analyzed using inductively coupled plasma-mass spectrometry (ICP-MS). The results showed that TEs in wet and dry deposition of the target area were significantly elevated compared within and outside China and the volume weight mean pH of rainwater was 4.43. The relative contributions of wet and dry depositions of TEs vary significantly among elements. The wet deposition fluxes of V, As, Cr, Se, Zn, and Cd exceeded considerably their dry deposition fluxes while dry deposition dominated the removal of pollution elements such as Mo, Cu, Ni, Mn, and Al. The summed dry deposition flux was four times higher than the summed wet deposition flux. Prediction results based on a simple accumulation model found that the content of seven toxic elements (Cr, Ni, Cu, Zn, As, Cd, and Pb) in soils could increase rapidly due to the impact of annual atmospheric deposition, and the increasing amounts of them reached 0.063, 0.012, 0.026, 0.459, 0.076, 0.004, and 0.145 mg kg^?1, respectively. In addition, the annual increasing rates ranged from 0.05% (Cr and Ni) to 2.08% (Cd). It was also predicted that atmospheric deposition induced the accumulation of Cr and Cd in surface soils. Cd was the critical element with the greatest potential ecological risk among all the elements in atmospheric deposition.     

Observations of mercury wet deposition in Mexico

We provide a longer-term record of Hg wet deposition at two tropical latitude monitoring sites in Mexico, selected to provide regionally representative data. Weekly wet deposition samples were collected over 2 years, from September 2003 to November 2005. Based on this data set, we discuss the magnitude and seasonal variation of Hg in wet deposition and compare the results to other measurement sites and to several model estimates. With precipitation-weighted mean (PWM) concentrations of 8.2 and 7.9 ng L^?1, respectively, during the sampling period from Sep 30 2003 to Oct 11 2005, and median weekly concentrations of 9.4?±?1 ng L^?1 for both sites, the wet Hg concentrations and deposition at HD01 were much lower than those observed at the US Gulf Coast MDN sites while the wet Hg deposition at OA02 was much lower than most MDN sites, but somewhat similar to US MDN sites along the Pacific Coast. Based on the limited available data, we conclude that the approximately 30 % higher average precipitation at HD01 and roughly equal PWM concentrations lead to the higher deposition at HD01 versus OA02. We believe that these observations may offer scientists and modelers additional understanding of the depositional fluxes in the lower latitudes of North America.     

Elevated atmospheric deposition and dynamics of mercury in a remote upland forest of southwestern China

Mt. Gongga area in southwest China was impacted by Hg emissions from industrial activities and coal combustion, and annual means of atmospheric TGM and PHg concentrations at a regional background station were 3.98 ng m⁻³ and 30.7 pg m⁻³, respectively. This work presents a mass balance study of Hg in an upland forest in this area. Atmospheric deposition was highly elevated in the study area, with the annual mean THg deposition flux of 92.5 μg m⁻² yr⁻¹. Total deposition was dominated by dry deposition (71.8%), and wet deposition accounted for the remaining 28.2%. Forest was a large pool of atmospheric Hg, and nearly 76% of the atmospheric input was stored in forest soil. Volatilization and stream outflow were identified as the two major pathways for THg losses from the forest, which yielded mean output fluxes of 14.0 and 8.6 μg m⁻² yr⁻¹, respectively.     

Historical deposition of mercury and selected trace elements to high-elevation National Parks in the Western U.S. inferred from lake-sediment cores

Atmospheric deposition of Hg and selected trace elements was reconstructed over the past 150 years using sediment cores collected from nine remote, high-elevation lakes in Rocky Mountain National Park in Colorado and Glacier National Park in Montana. Cores were age dated by ²¹⁰Pb, and sedimentation rates were determined using the constant rate of supply model. Hg concentrations in most of the cores began to increase around 1900, reaching a peak sometime after 1980. Other trace elements, particularly Pb and Cd, showed similar post-industrial increases in lake sediments, confirming that anthropogenic contaminants are reaching remote areas of the Rocky Mountains via atmospheric transport and deposition. Preindustrial (pre-1875) Hg fluxes in the sediment ranged from 5.7 to 42 μg m^?2 yr^?1 and modern (post-1985) fluxes ranged from 17.7 to 141 μg m^?2 yr^?1. The average ratio of modern to preindustrial fluxes was 3.2, which is similar to remote lakes elsewhere in North America. Estimates of net atmospheric deposition based on the cores were 3.1 μg m^?2 yr^?1 for preindustrial and 11.7 μg m^?2 yr^?1 for modern times. Current-day measurements of wet deposition range from 5.0 to 8.6 μg m^?2 yr^?1, which are lower than the modern sediment-based estimate of 11.7 μg m^?2 yr^?1, perhaps owing to inputs of dry-deposited Hg to the lakes.     

Wet and dry deposition of sulfur and nitrogen compounds in Ontario

Measurements have been made of sulfur and nitrogen compounds in precipitation since 1980 and in air since 1981 in Ontario. This paper presents results of the atmospheric deposition measurement program to the end of 1985. As is to be expected from the distribution of emission sources, annual concentrations of SO₄²⁻ andNO₃⁻ in precipitation, and of SO₂,SO₄²⁻ andNO₃⁻ in air are higher in southern Ontario than in northern Ontario. The corresponding distribution pattern for deposition is similar to that of concentration. A wet SO₄²⁻ deposition rate of 20 kg ha¹⁻ y¹⁻, a value considered critical for the acidification of sensitive water bodies, is exceeded in all of central and southern Ontario. On a province-wide basis, sulfur wet deposition is about four times higher than sulfur dry deposition. For nitrogen, wet and dry deposition are more comparable, though the former is still higher. The S- and N-species display different seasonal trends in concentration and deposition reflecting a dependence on meteorological factors, and on the associated chemical transformation rates. On the other hand, year to year variations are small.     

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.     

Scientific uncertainties in atmospheric mercury models II: Sensitivity analysis in the CONUS domain

In this study, we present the response of model results to different scientific treatments in an effort to quantify the uncertainties caused by the incomplete understanding of mercury science and by model assumptions in atmospheric mercury models. Two sets of sensitivity simulations were performed to assess the uncertainties using modified versions of CMAQ-Hg in a 36-km Continental United States domain. From Set 1 Experiments, it is found that the simulated mercury dry deposition is most sensitive to the gaseous elemental mercury (GEM) oxidation product assignment, and to the implemented dry deposition scheme for GEM and reactive gaseous mercury (RGM). The simulated wet deposition is sensitive to the aqueous Hg(II) sorption scheme, and to the GEM oxidation product assignment. The inclusion of natural mercury emission causes a small increase in GEM concentration but has little impact on deposition. From Set 2 Experiments, it is found that both dry and wet depositions are sensitive to mercury chemistry. Change in model mercury chemistry has a greater impact on simulated wet deposition than on dry deposition. The kinetic uncertainty of GEM oxidation by O₃ and mechanistic uncertainty of Hg(II) reduction by aqueous HO₂ pose the greatest impact. Using the upper-limit kinetics of GEM–O₃ reaction or eliminating aqueous Hg(II)–HO₂ reaction results in unreasonably high deposition and depletion of gaseous mercury in the domain. Removing GEM–OH reaction is not sufficient to balance the excessive mercury removal caused by eliminating the HO₂ mechanism. Field measurements of mercury dry deposition, better quantification of mercury air-surface exchange and further investigation of mercury redox chemistry are needed for reducing model uncertainties and for improving the performance of atmospheric mercury models.     

Simultaneous measurements of total gaseous mercury at four sites on a 800 km transect: spatial distribution and short-time variability of total gaseous mercury over central Europe

Over a distance of approximately 800 km simultaneous measurements of total gaseous mercury (TGM) were performed at four sampling sites between Stockholm and Berlin. The time resolution of mercury concentration measurements was 5 min. During the sampling period from 26 June to 7 July 1995, event sampling for precipitation was carried out using bulk samplers. In addition to the mercury data, the most common meteorological and air-quality parameters were determined. Comparing the TGM background concentrations at the four sites, a weak but statistically significant south-to-north declining TGM gradient was found. From the most southern sampling site to the northern most site median values of 1.93, 1.78, 1.53 and 1.54 ng m^-3 TGM were detected. Compared with the median TGM concentration observed at the two Swedish sites, the regional background concentration near Berlin was elevated by about 25%. Whereas the 0.5 h average TGM concentration varies at the Swedish sampling sites in a very narrow range of only 0.69 ng m^-3, a much broader range of 3.28 ng m^-3 was observed at the southern sites. The increased TGM variability near Berlin was characterised by a diurnal cycle with a distinct maximum in the early morning hours (about 4:00 a.m.) before sun rise. The short time variability of the TGM concentration measured at the four sites on the south-to-north transect shows regional differences. A decreasing variability from the most southern to the most northern site became evident.     

Sources of polycyclic hydrocarbons and pesticides in soluble fraction of deposition samples in Kocaeli,Turkey

A wet–dry deposition sampler was located at The Scientific and Technological Research Council of Turkey-National Metrology Institute (TUBITAK-UME) station, and a bulk deposition sampler was placed at the Kad?ll? village to determine the atmospheric deposition flux of polycyclic aromatic hydrocarbons (PAHs) and pesticides (organochlorine and organophosphorus) in soluble fraction of samples in Kocaeli, Turkey. The 28 samples for each wet, dry, and total deposition were collected weekly from March 2006 to March 2007. Gas chromatography-tandem mass spectrometry was used to analyze the samples which were prepared by using solid-phase extraction (SPE) method. The sum of volume weighted mean of deposition fluxes was obtained as 7.43 μg m^?2 day^?1 for wet deposition, 0.28 μg m^?2 day^?1 for dry deposition and 0.54 μg m^?2 day^?1 for bulk deposition samples for PAHs and 9.88 μg m^?2 day^?1 for wet deposition, 4.49 μg m^?2 day^?1 for dry deposition, and 3.29 μg m^?2 day^?1 for bulk deposition samples for pesticides. While benzo(a)anthracene had the highest fluxes among PAH compounds for all types of depositions, guthion and phosphamidon had the highest deposition flux compared with the other pesticides. Benzo(ghi)perylene, dibenz(a,h)anthracene, indeno(1,2,3-c,d)pyrene, benzo(a)pyrene, and acenaphthene were not detected in any of the samples. Beta-HCH, gamma-HCH, and endrin aldehyde were the only compounds among 18 organochlorine pesticides to be detected in all deposition samples. The main sources of pesticides were the high number of greenhouses around the sampling stations. However, all of the organophosphorus pesticides were detected in all deposition samples. The pollution sources were identified as coal and natural gas combustion, petrogenic sources, and traffic for TUBITAK-UME station whereas coal and natural gas combustion and traffic were the main sources for Kad?ll? station by considering the results of factor analysis, ratios, and wind sector analysis.     

Atmospheric phosphorus deposition at a montane site: Size distribution,effects of wildfire,and ecological implications

The dry deposition of atmospheric particulate matter can be a significant source of phosphorus (P) to oligotrophic aquatic ecosystems, including high-elevation lakes. In this study, measurements of the mass concentration and size distribution of aerosol particles and associated particulate P are reported for the southern Sierra Nevada, California, for the period July–October, 2008. Coarse and fine particle samples were collected with Stacked Filter Units and analyzed for Total P (TP) and inorganic P (IP) using a digestion-extraction procedure, with organic P (OP) calculated by difference. Particle size-resolved mass and TP distributions were determined concurrently using a MOUDI cascade impactor. Aerosol mass concentrations were significantly elevated at the study site, primarily due to transport from offsite and emissions from local and regional wildfires. Atmospheric TP concentrations ranged from 11 to 75 ng m^?3 (mean = 37 ± 16 ng m^?3), and were typically dominated by IP. Phosphorus was concentrated in the coarse (>1 μm diameter) particle fraction and was particularly enriched in the 1.0–3.2 μm size range, which accounted for 30–60% of the atmospheric TP load. Wildfire emissions varied widely in P content, and may be related to fire intensity. The estimated dry depositional flux of TP for each daily sampling period ranged between 7 and 118 μg m^?2 d^?1, with a mean value of 40 ± 27 μg m^?2 d^?1. Relative rates of dry deposition of N and P in the Sierra Nevada are consistent with increasing incidence of N limitation of phytoplankton growth and previously observed long-term eutrophication of lakes.     

Mercury speciation in Pacific coastal rainwater,Monterey Bay,California

We measured mercury speciation in coastal rainwater samples from Monterey Bay, California in 2007–2008 to investigate the source of monomethylmercury (MMHg) in rainwater and determine the relative importance of wet atmospheric deposition of MMHg to coastal waters compared to other sources on the central Pacific coast. Total mercury (HgT) ranged from 10 to 88 pM, with a sample mean ± standard deviation of 33 ± 22 pM (volume-weighted average 29 pM). MMHg concentrations ranged from 0.12 to 2.3 pM with a sample mean of 0.7 ± 0.5 pM (volume-weighted average 0.68 pM). Reactive mercury (HgR) concentrations ranged from 0.87 to 47 pM, sample mean 7.8 ± 8.3 pM (volume-weighted average 6.1 pM). Acetate concentration in rainwater, measured in a subset of samples, ranged from 0.34 to 3.1 μM, and averaged 1.6 ± 0.9 μM (volume-weighted average 1.3 μM). Dimethylmercury (DMHg) concentrations were below the limit of detection in air (<0.01 ng m^?3) and rainwater (<0.05 pM). Despite previous suggestions that DMHg in upwelled ocean waters is a potential source of MMHg in coastal rainwater, MMHg in rain was not related to coastal upwelling seasons or surface water DMHg concentrations. Instead, a multiple linear regression analysis demonstrated that MMHg concentrations were positively and significantly correlated (p = 0.002, adjusted R² = 0.88) with those of acetate and HgR. These data appear to support previous suggestions that the aqueous phase methylation of Hg(II) by acetate may be the source of MMHg in rainwater, but imply that acetate concentrations in rainwater play a more important role relative to HgR than previously hypothesized. However, the calculated chemical speciation of Hg(II) in rainwater and the minimal predicted complexation of Hg(II) by acetate suggest that the aqueous phase methylation of Hg(II) by acetate is unlikely to account for the MMHg found in precipitation, or that the mechanism of this reaction in the atmosphere differs from that previously reported (Gardfeldt et al., 2003).     

Influence of usage and chemical–physical properties on the atmospheric transport and deposition of pesticides to agricultural regions of Manitoba,Canada

This study quantified the masses of 14 pesticides deposited as wet (precipitation) versus dry (gaseous and particle) atmospheric deposition at a research farm in southwestern Manitoba, Canada. The concentration in air of these pesticides was also measured. Total bulk deposition amounts (wet + dry) ranged from 0.009 to 2.3 μg m^?2 for the 12 pesticides detected, and for the six pesticides with both wet and dry deposition detections, dry deposition contributed 12–51% of the total deposition over the crop growing season. Although not applied at the site, eight herbicides registered for use in Canada, as well as lindane (γ-HCH), were all frequently detected (92–100%) in the 12 air samples analyzed during the crop growing season, with by-product isomer α-HCH (75%), clopyralid (50%) and atrazine (8%) detected to a lesser extent. The chemical’s physicochemical properties and the relative mean mass of each agricultural pesticide applied in the province of Manitoba and in a 13 km radius were significant parameters in explaining the trends in the concentrations of pesticides detected in our samples. The important contribution of dry deposition to total pesticide deposition warrants greater attention in arid and semi-arid areas such as the Prairie Region of Canada, also because under a changing climate this region is estimated to experience more severe droughts while the more favorable conditions predicted for pest infestations could lead to increased pesticide applications in agricultural and urban areas.     

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.     

Wet deposition of mercury in the U.S. and Canada, 1996–2005: Results and analysis of the NADP mercury deposition network (MDN)

One of the most critical measurements needed to understand the biogeochemical cycle of mercury, and to verify atmospheric models, is the rate of mercury wet-deposition. The Mercury Deposition Network (MDN) operates sites across North America to monitor total mercury in wet-deposition. MDN's primary goal is to provide both spatial and temporal continental-scale observations of mercury wet-deposition fluxes to support researchers, modelers, policy-makers and the public interest. MDN represents the only continental-scale mercury deposition database with a >10-year record of continuous values. This study provides analysis and interpretation of MDN observations at 10 years (1996–2005) with an emphasis on investigating whether rigorous, statistically-significant temporal trends and spatial patterns were present and where they occurred. Wet deposition of mercury ranges from more than 25 μg m^?2 yr in south Florida to less than 3 μg m^?2 yr in northern California. Volume-weighted total mercury concentrations are statistically different between defined regions overall (Southeast ≈ Midwest > Ohio River > Northeast), with the highest in Florida, Minnesota, and several Southwest locations (10–16 ng L^?1). Total mercury wet-deposition is significantly different between defined regions (Southeast > Ohio River > Midwest > Northeast). Mercury deposition is strongly seasonal in eastern North America. The average mercury concentration is about two times higher in summer than in winter, and the average deposition is approximately more than three times greater in summer than in winter. Forty-eight sites with validated datasets of five years or more were tested for trends using the non-parametric seasonal Kendall trend test. Significant decreasing mercury wet-deposition concentration trends were found at about half of the sites, particularly across Pennsylvania and extending up through the Northeast.