Laurier Poissant

Water-air and soil-air exchange rate of total gaseous mercury measured at background sites

Abstract In order to evaluate and understand the processes of water-air and soil-air exchanges involved at background sites, an intensive field measurement campaign has been achieved during the summer of 1995 using high-time resolution techniques (10 min) at two sites (land and water) in southern Quebec (Canada). Mercury flux was measured using a dynamic flux chamber technique coupled with an automatic mercury vapour-phase analyser (namely, Tekran®). The flux chamber shows that the rural grassy site acted primarily as a source of atmospheric mercury, its flux mimicked the solar radiation, with a maximum daytime value of ∼ 8.3 ng m−2 h−1 of TGM. The water surface location (St. Lawrence River site located about 3 km from the land site) shows deposition and evasion fluxes almost in the same order of magnitude (−0.5 vs 1.0 ng m−2 h−1).The latter is influenced to some extent by solar radiation but primarily by the formation of a layer of stable air over the water surface in which some redox reactions might promote evasion processes over the water surface. This process does not appear over the soil surface. As a whole, soil-air exchange rate is about 6–8 fold greater than the water-air exchange.

Nevada STORMS project: Measurement of mercury emissions from naturally enriched surfaces

Diffuse anthropogenic and naturally mercury-enriched areas represent long- lived sources of elemental mercury to the atmosphere. The Nevada Study and Tests of the Release of Mercury From Soils (STORMS) project focused on the measurement of mercury emissions from anaturally enriched area. During the project, concurrent measurements of mercury fluxes from naturally mercury-enriched substrate were made September 1-4, 1997, using four micrometeorological methods and seven field flux chambers. Ambient air mercury concentrations ranged from 2 to nearly 200 ng m- 3 indicating that the field site is a source of atmospheric mercury. The mean day time mercury fluxes, durin p conditions of no precipitation, measured with field chambers were 50 to 360 ng m -2 h - , and with the micrometeorological methods we re 230 to 600 ng m- 2 h -1. This wide range in mercury emission rates reflects differences in method experimental designs and local source strengths. Mercury fluxes measured by many field chambers were significantly different (p < 0.05) but linearly correlated. This indicates that field chambers responded similarly to environmental conditions, but differences in experimental design and site heterogeneity had a significant influence on the magnitude of mercury fluxes. Data developed during the field study demonstrated that field flux chambers are ideal for assessment of the physicochemical processes driving mercury flux and development of an understanding of the magnitude of the influence of individual factors on flux. In general, mean mercury fluxes measured with micrometeorological methods during day time periods were nearly 3 times higher than me an fluxes measured with field flux chambers. Micrometeorological methods allow for derivation of a representative mercury flux occurring from an unconstrained system and provide an assessment of the actual magnitude and variability of fluxes occurring from an area.

The fate of mercury species in a sub-arctic snowpack during snowmelt

An extensive mercury study was conducted in April 2002 prior to and during the annual melting of a snowpack in a sub-arctic site along the Hudson Bay (Canada). Gas-phase measurements show that the snowmelt coincides with an elemental mercury (Hg°) pulse in the snowpack air far above ambient levels. Additional measurements of inorganic mercury (Hg2+) and methylmercury (MeHg+) in snow pits, in surface snow and in a meltwater sample clearly reveal that most of Hg is removed from the snow during the first days of snowmelt. We estimate that gas-phase exchanges contribute poorly to remove Hg from the snowpack; consequently during a snowmelt day more than 90% of Hg present in the snow surface is likely released with the meltwater. In arctic areas, where Hg accumulates at an accelerated rate in the snow surfaces [Lu et al., 2001] during mercury depletion events (MDE), the discharge of this toxic and bio-accumulating pollutant in water systems could be a threat to ecosystems and local indigenous populations.

Mercury flux measurements in a naturally enriched area: Correlation with environmental conditions during the Nevada Study and Tests of the Release of Mercury From Soils (STORMS)

An international intercomparison of micrometerological techniques and dynamic flux chamber methods applied to measure mercury fluxes was conducted from September 1 to 4, 1997, during the Nevada Study and Tests of the Release of Mercury From Soils (STORMS) in Reno, Nevada. Nine research groups from four countries met in the Steamboat Springs, Nevada Geothermal Area, to participate in the first international flux intercomparison ever attempted for mercury. The highly heterogeneous soil Hg concentrations and complex landscape within the study area (4 ha) were unfavorable for spatial intercomparison of Hg fluxes between the research groups. However, reliable and correlated Hg fluxes were measured between our micrometerological technique and a dynamic flux chamber method (r2 = 0.29), run side by side (5 m). Hg fluxes and their relationships with environmental factors were complex. After ∼90 days of dry condition, a series of storm events impacted the site and increased the soil moisture from <0.5 to 6.6%. This appeared to promote strong Hg evasion during the transition period from dry to wet soil conditions. However, the subsequent relationship between soil moisture and Hg flux was significantly negatively correlated. Multivariate analysis was applied to extract the principal components (principal component analysis). Three principal components were extracted (explained up to 79% of the total variance) and discussed with respect to their environmental signification. Environmental conditions under southern wind sectors were optimal to promote Hg fluxes. Turbulence rather than Hg air concentrations seemed to be the main factor promoting the determined Hg fluxes during this study.

Trace inorganic elements in rainfall in the Montreal Island

Abstract Using inductively coupled plasma-mass spectrometry (ICP-MS), 24 inorganic elements were analysed for about 65 station-events during the summer of 1989, at Montreal Island (Canada). Enrichment factors indicate that numerous elements are enriched as compared to the Earths crust, since only six elements (titanium, iron, rubidium, yttrium, zirconium, and cerium) can be considered natural. A comparison with results from a rural area indicates that most elements are affected by urbanization. For the majority of the elements, scavenging below the cloud (wash-out) seems to be a significant atmosphere cleansing mechanism, although one element, lead, appears to be rather governed by long-range transportation and rain-out (in-cloud scavenging). Correlation between elements indicates that some elements have similar behaviour, which can be partly explained by sorption phenomena or chemical bonding in the same compound.

Potential sources of atmospheric total gaseous mercury in the St. Lawrence River valley

The potential source contribution function (PSCF) has been used to study the source–receptor relationships for total gaseous mercury (TGM) found in air collected at two sites along the St. Lawrence River valley, namely at St. Anicet and Mingan. TGM concentrations have been measured with high time-resolution analysers (Tekran instrument). The source–receptor analyses have been applied with regards to the seasonality of TGM. Median TGM concentrations are significantly less (χ2: α=0.01) during the summertime than other periods at both sites. A total of 12 225 trajectory end-points for St. Anicet and 4480 trajectory end points for Mingan have been used to create potential source area maps. This study identifies preferred potential sources of TGM at St. Anicet during wintertime with strongest probability stretching from the Gulf of Mexico to the southern tip of Greenland. This pattern mimics, the North American anthropogenic Hg emission inventory. Furthermore, some Eurasian mercury air mass intrusions are suggested at Mingan during wintertime. The summertime period at Mingan points out some potential sources stretching from the american mid-west to the St. Lawrence River valley as well as areas around the southern tip of the Hudson Bay.

Assessment of modeled mercury dry deposition over the Great Lakes region

Three sets of model predicted values for speciated mercury concentrations and dry deposition fluxes over the Great Lakes region were assessed using field measurements and model intercomparisons. The model predicted values were produced by the Community Multiscale Air Quality Modeling System for the year 2002 (CMAQ2002) and for the year 2005 (CMAQ2005) and by the Global/Regional Atmospheric Heavy Metals Model for the year 2005 (GRAHM2005). Median values of the surface layer ambient concentration of gaseous elemental mercury (GEM) from all three models were generally within 30% of measurements. However, all three models overpredicted surface-layer concentrations of gaseous oxidized mercury (GOM) and particulate bound mercury (PBM) by a factor of 2-10 at the majority of the 15 monitoring locations. For dry deposition of GOM plus PBM, CMAQ2005 showed a clear gradient with the highest deposition in Pennsylvania and its surrounding areas while GRAHM2005 showed no such gradient in this region; however, GRAHM2005 had more hot spots than those of CMAQ2005. Predicted dry deposition of GOM plus PBM from these models should be treated as upper-end estimates over some land surfaces in this region based on the tendencies of all the models to overpredict GOM and PBM concentrations when compared to field measurements. Model predicted GEM dry deposition was found to be as important as GOM plus PBM dry deposition as a contributor to total dry deposition. Predicted total annual mercury dry deposition were mostly lower than 5 μg m(-2) to the surface of the Great lakes, between 5 and 15 μg m(-2) to the land surface north of the US/Canada border, and between 5 and 40 μg m(-2) to the land surface south of the US/Canada border. Predicted dry deposition from different models differed from each other by as much as a factor of 2 at regional scales and by a greater extent at local scales.

Isolation of Streptomyces sp. PCB7, the first microorganism demonstrating high-affinity uptake of tropospheric H2.

Microbial-mediated soil uptake accounts for ∼80% of the global tropospheric dihydrogen (H2) sinks. Studies conducted over the last three decades provide indirect evidences that H2 soil uptake is mediated by free soil hydrogenases or by unknown microorganisms that have a high affinity for H2. The exact nature of these hypothetical free soil enzymes or of H2-consuming microorganisms remains elusive because the activity has never been observed in pure culture. Here, we present the first aerobic microorganism able to consume tropospheric H2 at ambient levels. A dynamic microcosm chamber was developed to enrich a microbial consortium with a high affinity for H2, from which selected bacterial and fungal strains were isolated and tested for H2 uptake. Strain PCB7 had a H2 consumption activity that followed a Michaelis–Menten kinetics, with an apparent Km of 11 p.p.m.v. and a H2 threshold concentration <0.100 p.p.m.v., corresponding to the high-affinity uptake of tropospheric H2 observed in soil. 16S ribosomal RNA gene sequences showed that strain PCB7 is highly related to several Streptomyces species. H2 consumption occurred during the sporulation period of the bacterium. Addition of nickel increased the activity, suggesting that the enzymes involved in H2 consumption belong to the NiFe uptake class of hydrogenases. Because this is the first microorganism showing a high-affinity uptake of tropospheric H2, we anticipate that Streptomyces sp. PCB7 will become a model organism for the understanding of the environmental factors influencing H2 soil uptake.

Tropospheric H2 budget and the response of its soil uptake under the changing environment

Molecular hydrogen (H(2)) is an indirect greenhouse gas present at the trace level in the atmosphere. So far, the sum of its sources and sinks is close to equilibrium, but its large-scale utilization as an alternative energy carrier would alter its atmospheric burden. The magnitude of the emissions associated with a future H(2)-based economy is difficult to predict and remains a matter of debate. Previous attempts to predict the impact that a future H(2)-based economy would exert on tropospheric chemistry were realized by considering a steady rate of microbial-mediated soil uptake, which is currently responsible of ~80% of the tropospheric H(2) losses. Although soil uptake, also known as dry deposition is the most important sink for tropospheric H(2), microorganisms involved in the activity remain elusive. Given that microbial-mediated H(2) soil uptake is influenced by several environmental factors, global change should exert a significant effect on the activity and then, assuming a steady H(2) soil uptake rate for the future may be mistaken. Here, we present an overview of tropospheric H(2) sources and sinks with an emphasis on microbial-mediated soil uptake process. Future researches are proposed to investigate the influence that global change would exert on H(2) dry deposition and to identify microorganisms involved H(2) soil uptake activity.

Field observations of total gaseous mercury behaviour: Interactions with ozone concentration and water vapour mixing ratio in air at a rural site

Total Gaseous Mercury (TGM) data (n = 5125) measured from September 9 to November 30, 1994 in Southern Québec with an automatic TGM analyser (Tekran®) are used to study the behaviour of TGM with ozone and the water vapour mixing ratio. Results show, as a whole, no straight-forward correlation between TGM and ozone concentrations. A significant correlation between TGM and water vapour mixing ratio was measured. Some back reduction of Hg(II) to Hgo in the aqueous phase and subsequent evaporation of Hg0 according to Henrys Law is expected. The latter correlation seems to be partly ozone dependent since under ‘clean conditions’ (ozone > 30 ppbv) the slope of TGM vs the water vapour mixing ratio is twice that during ‘polluted conditions’ (ozone > 30 ppbv) which suggests some compensation effects under polluted conditions (counteracting the back-reduction of Hg(II) to Hgo in the aqueous phase). Oxidation of TGM by ozone during high moisture conditions (water vapour mixing ratio ∼14g/kg) seems to appear at the threshold ozone concentration ⩾ 30 ppbv.