The reaction between HgBr and O3: kinetic study and atmospheric implications

The rate constants of many reactions currently considered to be important in the atmospheric chemistry of mercury remain to be measured in the laboratory. Here we report the first experimental determination of the rate constant of the gas-phase reaction between the HgBr radical and ozone, for which a value at room temperature of k(HgBr + O₃) = (7.5 ± 0.6) × 10^-11 cm³ molecule s^-1 (1σ) has been obtained. The rate constants of two reduction side reactions were concurrently determined: k(HgBr + O) = (5.3 ± 0.4) × 10^-11 cm³ molecule s^-1 and k(HgBrO + O) = (9.1 ± 0.6) × 10^-11 cm³ molecule s^-1. The value of k(HgBr + O₃) is slightly lower than the collision number, confirming the absence of a significant energy barrier. Considering the abundance of ozone in the troposphere, our experimental rate constant supports recent modelling results suggesting that the main atmospheric fate of HgBr is reaction with ozone to form BrHgO.

The Existence of Airborne Mercury Nanoparticles

Mercury is an important global toxic contaminant of concern that causes cognitive and neuromuscular damage in humans. It is ubiquitous in the environment and can travel in the air, in water, or adsorb to soils, snow, ice and sediment. Two significant factors that influence the fate of atmospheric mercury, its introduction to aquatic and terrestrial environments, and its bioaccumulation and biomagnification in biotic systems are the chemical species or forms that mercury exists as (elemental, oxidized or organic) and its physical phase (solid, liquid/aqueous, or gaseous). In this work, we show that previously unknown mercury-containing nanoparticles exist in the air using high-resolution scanning transmission electron microscopy imaging (HR-STEM). Deploying an urban-air field campaign near a mercury point source, we provide further evidence for mercury nanoparticles and determine the extent to which these particles contain two long suspected forms of oxidized mercury (mercuric bromide and mercuric chloride) using mercury mass spectrometry (Hg-MS). Using optical particle sizers, we also conclude that the conventional method of measuring gaseous oxidized mercury worldwide can trap up to 95% of nanoparticulate mercuric halides leading to erroneous measurements. Finally, we estimate airborne mercury aerosols may contribute to half of the oxidized mercury measured in wintertime Montréal urban air using Hg-MS. These emerging mercury-containing nanoparticle contaminants will influence mercury deposition, speciation and other atmospheric and aquatic biogeochemical mercury processes including the bioavailability of oxidized mercury to biota and its transformation to neurotoxic organic mercury.

Electrocrystallization and solubility of mercury in alkaline solution

The chemical thermodynamics of mercury in aqueous sodium hydroxide solution has been investigated through electrochemical polarisation and solubility experiments. A review of thermodynamic data allowed determination of the Hg/HgO electrode potential. Cyclic voltammetry revealed a complex anodic reaction beginning with aqueous dissolution of elemental mercury and subsequent electrocrystallization of mercuric oxide. Cathodic sweeps showed dual reduction reactions, attributed to the presence of aqueous mercury and mercuric oxide. The solubility and hence activity of elemental mercury in sodium hydroxide solution was determined, otherwise known as the Sechenov salt effect. Sodium hydroxide salted mercury out of solution.

Elemental mercury: Its unique properties affect its behavior and fate in the environment

Elemental mercury (Hg⁰) has different behavior in the environment compared to other pollutants due to its unique properties. It can remain in the atmosphere for long periods of time and so can travel long distances. Through air-surface (e.g., vegetation or ocean) exchange (dry deposition), Hg⁰ can enter terrestrial and aquatic systems where it can be converted into other Hg species. Despite being ubiquitous and playing a key role in Hg biogeochemical cycling, Hg⁰ behavior in the environment is not well understood. The objective of this review is to provide a better understanding of how the unique physicochemical properties of Hg⁰ affects its cycling and chemical transformations in different environmental compartments. The first part focuses on the fundamental chemistry of Hg⁰, addressing why Hg⁰ is liquid at room temperature and the formation of amalgam, Hg halide, and Hg chalcogenides. The following sections discuss the long-range transport of Hg⁰ as well as its redistribution in the atmosphere, aquatic and terrestrial systems, in particular, on the sorption/desorption processes that occur in each environmental compartment as well as the involvement of Hg⁰ in chemical transformation processes driven by photochemical, abiotic, and biotic reactions.

Testing the use of passive sampling systems for understanding air mercury concentrations and dry deposition across Florida, USA

This paper describes the use of passive sampling systems and surrogate surfaces for monitoring atmospheric mercury (Hg) concentrations and dry deposition, respectively, in Florida,USA. Although this area has been reported to have low air concentrations, wet deposition values, reported by the Mercury Deposition Network, are some of the highest in the United States, and little is known about the magnitude of dry deposition to the region. To address this uncertainty, dry deposition of gaseous oxidized mercury (GOM) was estimated based on data collected using surrogate surfaces and through the application of a dry deposition model that utilized Tekran® Mercury Analyzer data for three sites (Davie near Fort Lauderdale, Tampa and Pensacola) over a year (July 2009-July 2010). Passive sampler systems for monitoring GOM and total gaseous mercury (TGM) concentrations were also deployed. In general, higher surrogate surface deposition, and GOM and TGM passive sampler uptake were observed at the DVE location. Across all sites, empirically derived dry deposition was higher than that determined using modeled values. Tekran® Instrument derived GOM concentrations, as well as modeled deposition rates, followed the same seasonal and spatial patterns as that measured by the samplers, however there were some spatial and temporal trends captured by the samplers that were not seen in the Tekran® derived data. Results indicate that these samplers may be applied to identify spatial and temporal trends in air Hg concentrations and potential deposition at sites with low and fairly constant GOM concentrations as reported by the Tekran® system (2-8 pg m(-3)). When viewed collectively, trends in sampler and Tekran® derived data also suggest the potential for different forms of GOM in air. Using empirical and modeled values, dry deposition in Florida during the year of this study could account for 1.5 to 14% of total annual Hg deposition (wet+dry).