Abstract
Abstract. Mercury is released to the atmosphere from natural and anthropogenic sources. Due to its persistence in the atmosphere, mercury is subject to long range transport and is thus a pollutant of global concern. Mercury emitted to the atmosphere enters terrestrial and aquatic ecosystems which act as sinks but also as sources of previously emitted and deposited mercury when the accumulated mercury is emitted back to the atmosphere. Studying the factors and processes that influence the behaviour of mercury from terrestrial sources is thus important for a better understanding of the role of natural ecosystems in the mercury cycling and emission budget. A study was conducted over ten months (November 2006 to August 2007) at Elora, Ontario, Canada to measure gaseous elemental mercury (GEM), reactive gaseous mercury (RGM) and particulate bound mercury (HgP) as well as GEM fluxes over different ground cover spanning the four seasons typical of a temperate climate zone. GEM concentrations were measured using a mercury vapour analyzer (Tekran 2537A) while RGM and HgP were measured with the Tekran 1130/1135 speciation unit coupled to another mercury vapour analyzer. A micrometeorological approach was used for GEM flux determination using a continuous two-level sampling system for GEM concentration gradient measurement above the soil surface and crop canopy. The turbulent transfer coefficients were derived from meteorological parameters measured on site. A net GEM volatilization (6.31 ± 33.98 ng mM−2 hr−1, study average) to the atmosphere was observed. Average GEM concentrations and GEM fluxes showed significant seasonal differences and distinct diurnal patterns while no trends were observed for HgP or RGM. Highest GEM concentrations, recorded in late spring and fall, were due to meteorological changes such as increases in net radiation and air temperature in spring and lower atmospheric mixing height in fall. Highest GEM fluxes (18.1 ng m−2 hr−1, monthly average) were recorded in late spring but also during specific events in winter and fall. The main factors influencing the GEM flux were soil moisture content, soil temperature, precipitation events and ground cover. These trends indicate that the soil surface could be a significant mercury source in spring and summer seasons but also under specific meteorological conditions during the winter and fall.
Highlights
Mercury is a priority pollutant due of its ability to accumulate in the food chain in the form of methylmercury, a neurotoxin to living organisms and human beings (Satoh, 2000)
One factor that may cause an increase in the gaseous elemental mercury (GEM) flux is, as the surface soil moisture is replenished from values less than 20% to approximately 40% during the spring thaw, air within the soil matrix must be vented to allow the increase in soil moisture to occur
The average GEM, reactive gaseous mercury (RGM) and HgP concentrations measured during the study over agricultural land were 1.17 ± 0.51 ng m−3, 15.10 ± 10.02 pg m−3 and 16.35 ± 9.54 pg m−3 while the average annual GEM flux was 6.31 ± 33.98 ng m−2 hr−1
Summary
Mercury is a priority pollutant due of its ability to accumulate in the food chain in the form of methylmercury, a neurotoxin to living organisms and human beings (Satoh, 2000). A few studies have spanned periods of up to a year looking at GEM fluxes above controlled tallgrass praire monoliths (Obrist et al, 2005) and European sub-alpine grasslands (Fritsche et al, 2008) but represent a limited number of terrestrial surfaces. Longer-term GEM flux and GEM, RGM and HgP concentration measurements above varying terrestrial surfaces are needed to better understand the Hg cycle and calculate the mass balance for Hg. In addition, the longer-term studies can provide a more complete picture of the biogeochemical cycle of Hg in the environment including the interactions between environmental parameters (e.g. soil moisture, soil temperature, radiation), chemical factors and surface characteristics (e.g. snow or crop cover). The main objective was to assess the seasonal behaviour of the main Hg species and to quantify the seasonal flux of elemental mercury over different agricultural ground covers
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