RAPID OXIDATION OF MERCURY VAPOUR IN ARCTIC AIR AFTER POLAR SUNRISE

W.H. Schroeder^*+, A. Steffen^*, J.Y. Lu^O

^* Environment Canada: Meteorological Service of Canada, 4905 Dufferin St., Toronto (Downsview), ON, Canada M3H 5T4 ⁺ bill.schroeder@ec.gc.ca

O 50 Milford Cr., London, ON, Canada N5X 1A8

Abstract

Mercury is a ubiquitous heavy metal which is commonly encountered in the environment, even in remote oceanic and continental locations. Continuous, automated measurements – since January 1995 – of Total Gaseous-Phase Mercury (TGM) concentrations in surface-level ambient air at Alert, Nunavut, Canada (82.5^o N; 62.5^o W) have revealed a consistently recurring annual pattern. Each season displays a typical atmospheric signature. In its various environmentally-significant physical and chemical forms, this air contaminant exhibits a remarkably broad range of atmos- pheric pathways and characteristics, including an ability for both short-range (< 10 km) and long-range atmospheric transport (> 1 000 km). The traditional, as well as the continuing, concerns associated with this persistent and potentially toxic inorganic pollutant, stem from the fact that, once it is released into (or mobilized in) the terrestrial or aquatic environment, this metal can be transformed – through biotic or abiotic processes – from its original physical/chemical form to other, often more toxic, species which are readily biomagnified in the food chain. The atmosphere plays a crucial role in dispersing, transporting, transforming and eventually depositing this element as well as its long-lived inorganic and organic compounds throughout the global environment.

INTRODUCTION

Up until the last two decades of the Twentieth Century, few reliable measurements of atmospheric mercury concentrations, especially at rural or remote sites, had been carried out. In ambient (outdoor) air this metallic element can exist in the gaseous phase and/or in the particulate phase. At most background sites ( i.e., locations remote from anthropogenic or natural mercury emission sources), the gaseous-phase mercury species are predominant: generally occurring at atmospheric concentrations which are about 2 orders of magnitude greater than those making up total particulate-phase mercury (TPM). The historical sparsity of reliable environmental data is due largely to the very low (trace or ultra-trace) levels of this metal and its derivatives which are normally present in environments not containing ‘natural hotspots’ or significant anthropogenic pollution sources, coupled with the unavailability of sufficiently sensitive and selective sampling and analytical methods based on affordable commercial instrumentation. This technological impediment has now been removed, at least in the case of atmospheric mercury measurements (Schroeder et al., 1995; Ebinghaus et al., 1999; Lu and Schroeder, 1999).

METHODS

The high-temporal-resolution TGM measurements initiated at Alert in January 1995, are being made with an automated analyzer (Tekran^® Inc., Model 2537A) operating with either a 5-minute or 30 minute sample integration time. Atmospheric aerosols are removed, with a 47 mm diameter micro-quartz fibre filter (Gelman), from the ambient air entering the heated Teflon^™ sampling line about 5 m above ground. Additional details on experimental methods are found in the Standard Operating Procedures (SOP) Manual (Steffen and Schroeder, 1999).

RESULTS AND DISCUSSION

Figure 1 shows the annual TGM concentrations (6-h means) observed at Alert during the 4-year period since measurements began in 1995. The distinctive seasonal pattern occurring each year is evident, with concentrations during the winter (Jan.-March) being similar in magnitude to those occurring in the fall (Oct.-Dec), but with more amplified excursions above and below the annual mean. Each year during spring (Apr.-June), TGM concentrations in near-surface air are much more variable than at other times of the year, with values frequently dropping below 0.5 ng/m³. However, whereas in springtime the concentrations typically exhibit a tendency towards values that are lower than the annual means, the TGM concentrations during the summer months (July-Aug.) are normally the highest of the year. It is interesting that other CAMNet sites further south exhibit a significantly different seasonal cycle of TGM concentrations (Kellerhals, 2000).

We have shown previously, on the basis of our 1995 data set of TGM concentrations observed at Alert (Schroeder et al., 1998) that, with the exception of summertime, the surface-level concentrations of tropospheric ozone (measured independently) at Alert display a very similar annual pattern: greatly enhanced variability of ozone concentrations during spring (after polar sunrise) compared with the remainder of the year. This puzzling springtime tropospheric ozone depletion phenomenon, first observed at Pt. Barrow, AK (Oltmans, 1981), has recently also been reported to occur at Svalbard/Spitsbergen in the Norwegian Arctic (Solberg et al., 1996) as well as in Antarctica (Wessel et al., 1998). Similarly, tropospheric mercury vapour depletion episodes have now also been documented at Pt. Barrow (Lindberg, 1999), Ny Ålesund, Svalbard (Berg, 1999). The excellent linear correlation between near-surface TGM and ozone concentrations, first discovered at Alert in 1995, is shown to also exist in subsequent years (Figure 2). While tropsospheric ozone depletion in the Arctic has been extensively and intensively investigated (Barrie et al., 1994) and its photo/chemical mechanism is now fairly well understood (Barrie and Platt, 1997), the physical processes and chemical/photochemical reactions responsible for the rapid depletion – via oxidation and deposition of the reaction product(s) – is still largely unknown.

Concurrent ambient air measurements of TGM and total particulate-phase mercury (TPM) concentrations have been carried out at Alert since 1995 (Schroeder et al., 1998). Results obtained there during the spring of 1998 are depicted in Figure 3. The figure clearly reveals that, when TGM concentrations decrease the concentrations of TPM increase. This conversion (an oxidation process) can, at times, result in TPM levels in excess of 1 ng/m³. Compare this to ambient air TPM concentrations in the range of 0.001 to 0.1ng/m³ reported for monitoring sites in the Great Lakes basin (Keeler, 1996), a region adjacent to many anthropogenic mercury emission sources.

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