Assessing the trends and effects of environmental parameters on the behavior of mercury in the lower atmosphere over cropped land over four seasons
School of Engineering, University of Guelph University, Guelph, ON, Canada
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. The terrestrial ecosystem is an important atmospheric mercury sink as a significant portion of the mercury emitted can be accumulated on soil surfaces making terrestrial surfaces an important source of previously emitted and deposited mercury. Studying the factors and processes that influence the behavior 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 one year study (July 2006–August 2007) was conducted at Elora, Ontario, Canada to measure total gaseous mercury (TGM), reactive gaseous mercury (RGM) and particulate bound mercury (HgP) as well as TGM fluxes over different ground cover spanning the four seasons typical of a temperate climate zone. TGM 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 TGM flux determination using a continuous two-level sampling system for TGM concentration gradient measurement above the soil surface and crop canopy. The turbulent transfer coefficients were derived from meteorological parameters measured on site.
A net TGM volatilization (6.31±33.98 ng m−2 h−1, annual average) to the atmosphere was observed during the study. Average TGM concentrations and TGM fluxes showed significant seasonal differences and distinct diurnal patterns while no trends were observed for HgP or RGM. Highest TGM 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 TGM fluxes (18.1 ng m−2 h−1, monthly average) were recorded in late spring but also during specific events in winter and fall. The main factors influencing TGM 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 in winter or fall.