Modelling chemical and physical processes of Hg compounds in the Marine Boundary Layer

Abstract

Only five years ago a chapter in a book such as this with the above title would either have been extremely short, or simply not included. The inclusion of this chapter is evidence of the progress made in fields as diverse as analytical methods and instrumentation, chemical kinetics and atmospheric chemistry modelling, as well as of the ever increasing interest in mercury (Hg) within both the scientific and environmental policy communities. The reason for the current interest in Marine Boundary Layer (MBL) processes and their influence on Hg was the discovery of higher than expected concentrations of Reactive Gaseous Mercury (RGM or RGHg or Hg11(g)) at coastal sites (Mason et al., 2001, Wangberg et al., 2001) and in the MBL of both seas (Sprovieri et al., 2003) and oceans (Mason et al., 2001, Laurier et al., 2003). The Hg11(g) concentration was also found to vary diurnally with a maximum occurring with maximum solar radiation intensity and a minimum at night. Hg11(g) is not a single compound, it is an operationally defined quantity which refers to the oxidised inorganic mercury compounds present in the gas phase which are collected on KCl denuders, see chapter 7. The most probable components of Hg11(g) are HgCl2and HgBr2, possibly with HgO and Hg(OH)2. Hg11(g) is fundamental to Hg cycling because its chemical and physical characteristics differ so greatly from those of Hg(g). Where Hg(g) is volatile and sparingly soluble, the compounds which make up Hg11(g)) are far less volatile and far more soluble, thus Hg deposition is almost totally dominated by Hg11(g) whilst Hg emission, even industrial emission, is predominantly Hg(g). Modelling studies had suggested that the sea salt aerosol could be important in Hg cycling in the MBL because of its ability to form a range of Hg11 complexes due to its high chloride ion content (Pirrone et al., 2000). Further modelling studies suggested that Hg11 produced in the gas phase (Hg from the reaction of Hg + O3) could be scavenged and then cycled via the sea salt aerosol to HgCl2, which is more volatile than HgO, thus providing an MBL source of Hg11(g), (Hedgecock and Pirrone, 2001). The development of our understanding of Hg chemical and physical processes in the MBL, is intertwined with the study of Hg in the Arctic, and has gained very much from the study, both in the field and modelling, of tropospheric O3 in remote areas, specifically the Arctic and the remote MBL. Taking things in chronological order, it was in the late 80s that ozone depletion events were first reported during polar spring, (Bottenheim et al, 1986), these events were linked to photochemical processes, (Barrie et al., 1988). In the years that followed there were a number of field and theoretical investigations into the mechanism behind the rapid O3 destruction, which pointed to the involvement of bromine containing radicals, (Fan and Jacob, 1992, Barrie and Platt, 1997, Foster et al, 2001). Measurements of Hg (g) over a number of years at Alert in Canada showed that when O 3 depletion events occurred, the concentration of Hg(g) also decreased rapidly, often to below the detection limit of the measurement techniques employed (Schroeder et al, 1998). This observation led to the suggestion that it was likely to be the Br radical compounds which were responsible for the Hg depletion events. At around the same time model studies were performed to see what effect, if any, the halogen radical chemistry seen in the Arctic may have in the MBL, (Vogt et al, 1996, Sander and Crutzen, 1996). Model predictions suggested that halogen compounds (Br2 and BrCl) would be released to the atmosphere as a result of the acidification of the sea salt aerosol, and that the radicals produced by their photolysis would destroy O3. Since then the phenomenon known as 'Sunrise Ozone Destruction' has been observed in the sub-tropical Pacific (Nagao et al., 1999), and a number of studies have observed depletion of Br containing species in supermicrometer sea-salt aerosol, and enrichment of the same species in submicron marine aerosol, (Sander et al., 2003). The link between O3 destruction at polar dawn with Hg(g) depletion and the measurement of elevated RGM concentrations in marine air masses hinted that Hg oxidation processes were also at work in the MBL. The recent measurement of the reaction rates of Hg (g) + OH(g) (Sommar et al., 2001) and subsequently of Hg(g) + Br(g) (Ariya et al, 2002), have been included in a box model, AMCOTS (Atmospheric Mercury Chemistry Over The Sea, Hedgecock and Pirrone, 2004)) of multiphase MBL photochemistry along with previously known Hg gas and aqueous phase chemistry (Pleijel and Munthe, 1995, Lin and Pekhonen, 1999). The AMCOTS results can be compared directly to results from measurement campaigns, (Hedgecock et al., 2003, Hedgecock and Pirrone, 2004). The high measured concentrations of RGM coupled with the modelling support have meant that the cycle of Hg in the MBL, its emission, transport, chemistry and deposition have had to be reconsidered, and the role of the ocean-atmosphere in the global Hg cycle and the global Hg budget reassessed (Mason and Sheu, 2002). The MBL chemistry of Hg therefore cannot be removed from the transport and physical processes which ultimately have an influence on the concentrations of Hg species in the MBL. Where Hg in the MBL comes from, and which chemical species are emitted or transported with it, can directly or indirectly affect Hg chemistry. The scavenging of Hg compounds by deliquesced aerosol particles or rain droplets, and the dry deposition of oxidised Hg compounds not only exert a major influence over the concentrations of Hg species in the MBL but also on the marine emission/deposition budget. The chemistry of Hg is however not only the central theme of this chapter, but also the central process which links emissions to deposition, governs the lifetime of Hg(g) in the MBL, and therefore the influence that marine emissions may have on the global Hg budget.