Field Evidence for Asian Outflow and Fast Depletion of Total Gaseous Mercury in the Polluted Coastal Atmosphere

Atmospheric mercury (Hg) cycling in polluted coastal atmosphere is complicated and not fully understood. Here, we present measurements of total gaseous mercury (TGM) monitored at a coastal mountaintop in Hong Kong downwind of mainland China. Sharp TGM peaks during cold front passages were frequently observed due to Asian pollution outflow with typical TGM/CO slopes of 6.8 ± 2.2 pg m^-3 ppbv^-1. Contrary to the daytime maximums of other air pollutants, TGM exhibited a distinct diurnal variation with a midday minimum. Moreover, we observed four cases of extremely fast TGM depletion after sunrise, during which TGM concentrations rapidly dipped to 0.3-0.6 ng m^-3 accompanied by other pollutants on the rise. Simulated meteorological fields revealed that morning upslope flow transporting anthropogenically polluted but TGM-depleted air masses from the mixed layer caused morning TGM depletion at the mountaintop location. The TGM-depleted air masses were hypothesized to result mainly from fast photooxidation of Hg after sunrise with minor contributions from dry deposition (5.0%) and nocturnal oxidation (0.6%). A bromine-induced two-step oxidation mechanism involving abundant pollutants (NO₂, O₃, etc.) was estimated to play a dominant role, contributing 55%-60% of depleted TGM and requiring 0.20-0.26 pptv Br, an amount potentially available through sea salt aerosol debromination. Our findings suggest significant effects of the interaction between anthropogenic pollution and marine halogen chemistry on atmospheric Hg cycling in the coastal areas.

The influence of a single water molecule on the reaction of BrO + HONO

Quantum chemical computations and transition state theory are employed to systematically research the influence of a single molecule water on the BrO + HONO reaction. Two distinct reactions, namely BrO + trans-HONO and BrO + cis-HONO are explored for the reaction in the absence of water, which is mainly decided by the configuration of HONO. With introduction a single water molecule to the reaction, the rate coefficient of the channel starting from BrO + cis-HONO and BrO + trans-HONO are 2.43 × 10^-19 and 5.22 × 10^-22 cm³ molecule^-1 s^-1, which is larger than the reaction in the absence of water. For further comprehend the impact of water on the BrO + HONO reaction, it is necessary to compute the effective rate coefficient by taking into account the concentration of water. The water-assisted effective rate coefficients for the BrO + HONO reaction are smaller than that the reaction in the absence of water. The reaction of BrO with cis-HONO is feasible both in absence and existence of water.

Cellular and genetic mechanism of bacterial mercury resistance and their role in biogeochemistry and bioremediation

Mercury (Hg) is a highly toxic element that occurs at low concentrations in nature. However, various anthropogenic and natural sources contribute around 5000 to 8000 metric tons of Hg per year, rapidly deteriorating the environmental conditions. Mercury-resistant bacteria that possess the mer operon system have the potential for Hg bioremediation through volatilization from the contaminated milieus. Thus, bacterial mer operon plays a crucial role in Hg biogeochemistry and bioremediation by converting both reactive inorganic and organic forms of Hg to relatively inert, volatile, and monoatomic forms. Both the broad-spectrum and narrow-spectrum bacteria harbor many genes of mer operon with their unique definitive functions. The presence of mer genes or proteins can regulate the fate of Hg in the biogeochemical cycle in the environment. The efficiency of Hg transformation depends upon the nature and diversity of mer genes present in mercury-resistant bacteria. Additionally, the bacterial cellular mechanism of Hg resistance involves reduced Hg uptake, extracellular sequestration, and bioaccumulation. The presence of unique physiological properties in a specific group of mercury-resistant bacteria enhances their bioremediation capabilities. Many advanced biotechnological tools also can improve the bioremediation efficiency of mercury-resistant bacteria to achieve Hg bioremediation.

A single water molecule accelerating the atmospheric reaction of HONO with ClO

The role of a single water molecule on the atmospheric reaction of HONO + ClO is systematically investigated employing quantum chemical calculation combined with harmonic transition state theory. Two reaction pathways, cis-HONO + ClO and trans-HONO + ClO, are identified for the naked reaction, which depends on the configurations of HONO. When adding a single water molecule to this reaction, the rate constants of cis-HONO + ClO and trans-HONO + ClO pathways are 7.97 × 10^-21 and 2.29 × 10^-17 cm³ molecule^-1 s^-1, respectively, larger than the corresponding naked reaction. To further understand the role of water on the HONO + ClO reaction, it is necessary to calculate the effective rate constant by considering the concentration of water. It shows that the effective rate constants of water-assisted cis-HONO + ClO pathway are much smaller than those of the naked reaction, whereas the presence of water accelerates the trans-HONO + ClO at room temperature. This study demonstrates that water has a positive role in the pathway of trans-HONO + ClO by modifying the stabilities of reactant complexes and transition states through the hydrogen bond formation, which contributes to the sink of atmospheric HONO. In addition, the kinetic branching ratio indicates that the favorable reaction is the trans-HONO + ClO instead of the cis-HONO + ClO pathway, in contrast to the naked reaction. These results reveal the importance of water in the evaluation of the fate of active species in the atmosphere. Graphical Abstract.

Springtime Nitrogen Oxide-Influenced Chlorine Chemistry in the Coastal Arctic

Atomic chlorine (Cl) is a strong atmospheric oxidant that shortens the lifetimes of pollutants and methane in the springtime Arctic, where the molecular halogens Cl₂ and BrCl are known Cl precursors. Here, we quantify the contributions of reactive chlorine trace gases and present the first observations, to our knowledge, of ClNO₂ (another Cl precursor), N₂O₅, and HO₂NO₂ in the Arctic. During March - May 2016 near Utqiaġvik, Alaska, up to 21 ppt of ClNO₂, 154 ppt of Cl₂, 27 ppt of ClO, 71 ppt of N₂O₅, 21 ppt of BrCl, and 153 ppt of HO₂NO₂ were measured using chemical ionization mass spectrometry. The main Cl precursor was calculated to be Cl₂ (up to 73%) in March, while BrCl was a greater contributor (63%) in May, when total Cl production was lower. Elevated levels of ClNO₂, N₂O₅, Cl₂, and HO₂NO₂ coincided with pollution influence from the nearby town of Utqiaġvik and the North Slope of Alaska (Prudhoe Bay) Oilfields. We propose a coupled mechanism linking NO_x with Arctic chlorine chemistry. Enhanced Cl₂ was likely the result of the multiphase reaction of Cl^-_(aq) with ClONO₂, formed from the reaction of ClO and NO₂. In addition to this NO_x-enhanced chlorine chemistry, Cl₂ and BrCl were observed under clean Arctic conditions from snowpack photochemical production. These connections between NO_x and chlorine chemistry, and the role of snowpack recycling, are important given increasing shipping and fossil fuel extraction predicted to accompany Arctic sea ice loss.

Direct detection of atmospheric atomic bromine leading to mercury and ozone depletion

Bromine atoms play a central role in atmospheric reactive halogen chemistry, depleting ozone and elemental mercury, thereby enhancing deposition of toxic mercury, particularly in the Arctic near-surface troposphere. However, direct bromine atom measurements have been missing to date, due to the lack of analytical capability with sufficient sensitivity for ambient measurements. Here we present direct atmospheric bromine atom measurements, conducted in the springtime Arctic. Measured bromine atom levels reached 14 parts per trillion (ppt, pmol mol^-1; 4.2 × 10⁸ atoms per cm^-3) and were up to 3-10 times higher than estimates using previous indirect measurements not considering the critical role of molecular bromine. Observed ozone and elemental mercury depletion rates are quantitatively explained by the measured bromine atoms, providing field validation of highly uncertain mercury chemistry. Following complete ozone depletion, elevated bromine concentrations are sustained by photochemical snowpack emissions of molecular bromine and nitrogen oxides, resulting in continued atmospheric mercury depletion. This study provides a breakthrough in quantitatively constraining bromine chemistry in the polar atmosphere, where this chemistry connects the rapidly changing surface to pollutant fate.

Organic bromine compounds produced in sea ice in Antarctic winter

During polar springtime, active bromine drives ozone, a greenhouse gas, to near-zero levels. Bromine production and emission in the polar regions have so far been assumed to require sunlight. Here, we report measurements of bromocarbons in sea ice, snow, and air during the Antarctic winter that reveal an unexpected new source of organic bromine to the atmosphere during periods of no sunlight. The results show that Antarctic winter sea ice provides 10 times more bromocarbons to the atmosphere than Southern Ocean waters, and substantially more than summer sea ice. The inclusion of these measurements in a global climate model indicates that the emitted bromocarbons will disperse throughout the troposphere in the southern hemisphere and through photochemical degradation to bromine atoms, contribute ~ 10% to the tropospheric reactive bromine budget. Combined together, our results suggest that winter sea ice could potentially be an important source of atmospheric bromine with implications for atmospheric chemistry and climate at a hemispheric scale.

Role of water on the H-abstraction from methanol by ClO

The influence of a single water molecule on the reaction mechanism and kinetics of hydrogen abstraction from methanol (CH₃OH) by the ClO radical has been investigated using ab initio calculations. The reaction proceeds through two channels: abstraction of the hydroxyl H-atom and methyl H-atom of CH₃OH by ClO, leading to the formation of CH₃O+HOCl (+H₂O) and CH₂OH+HOCl (+H₂O), respectively. In both cases, pre- and post-reactive complexes were located at the entrance and exit channel on the potential energy surfaces. Results indicate that the formation of CH₂OH+HOCl (+H₂O) is predominant over the formation of CH₃O+HOCl (+H₂O), with ambient rate constants of 3.07×10^-19 and 3.01×10^-23cm³/(molecule·sec), respectively, for the reaction without water. Over the temperature range 216.7-298.2K, the presence of water is seen to effectively lower the rate constants for the most favorable pathways by 4-6 orders of magnitude in both cases. It is therefore concluded that water plays an inhibitive role on the CH₃OH+ClO reaction under tropospheric conditions.

Constraints on Arctic Atmospheric Chlorine Production through Measurements and Simulations of Cl2 and ClO

During springtime, unique halogen chemistry involving chlorine and bromine atoms controls the prevalence of volatile organic compounds, ozone, and mercury in the Arctic lower troposphere. In situ measurements of the chlorine monoxide radical, ClO, and its precursor, Cl₂, along with BrO and Br₂, were conducted using chemical ionization mass spectrometry (CIMS) during the Bromine, Ozone, and Mercury Experiment (BROMEX) near Barrow, Alaska, in March 2012. To our knowledge, these data represent the first ClO measurements made using CIMS. A maximum daytime ClO concentration of 28 ppt was observed following an early morning peak of 75 ppt of Cl₂. A zero-dimensional photochemistry model was constrained to Cl₂ observations and used to simulate ClO during a 7-day period of the field campaign. The model simulates ClO within the measurement uncertainty, and the model results highlight the importance of chlorine chemistry participation in bromine radical cycling, as well as the dependence of halogen chemistry on NO_x levels. The ClO measurements and simulations are consistent with Cl₂ being the dominant Cl atom source in the Arctic boundary layer. Simulated Cl atom concentrations, up to ∼1 × 10⁶ molecules cm^-3, highlight the importance of chlorine chemistry in the degradation of volatile organic compounds, including the greenhouse gas methane.

Mass-Dependent and -Independent Fractionation of Mercury Isotope during Gas-Phase Oxidation of Elemental Mercury Vapor by Atomic Cl and Br

This study presents the first measurement of Hg stable isotope fractionation during gas-phase oxidation of Hg(0) vapor by halogen atoms (Cl(•), Br(•)) in the laboratory at 750 ± 1 Torr and 298 ± 3 K. Using a relative rate technique, the rate coefficients for Hg(0)+Cl(•) and Hg(0)+Br(•) reactions are determined to be (1.8 ± 0.5) × 10(-11) and (1.6 ± 0.8) × 10(-12) cm(3) molecule(-1) s(-1), respectively. Results show that heavier isotopes are preferentially enriched in the remaining Hg(0) during Cl(•) initiated oxidation, whereas being enriched in the product during oxidation by Br(•). The fractionation factors for (202)Hg/(198)Hg during the Cl(•) and Br(•) initiated oxidations are α(202/198) = 0.99941 ± 0.00006 (2σ) and 1.00074 ± 0.00014 (2σ), respectively. A Δ(199)Hg/Δ(201)Hg ratio of 1.64 ± 0.30 (2σ) during oxidation of Hg(0) by Br atoms suggests that Hg-MIF is introduced by the nuclear volume effect (NVE). In contrast, the Hg(0) + Cl(•) reaction produces a Δ(199)Hg/Δ(201)Hg-slope of 1.89 ± 0.18 (2σ), which in addition to a high degree of odd-mass-number isotope MIF suggests impacts from MIF effects other than NVE. This reaction also exhibits significant MIF of (200)Hg (Δ(200)Hg, up to -0.17‰ in the reactant) and is the first physicochemical process identified to trigger (200)Hg anomalies that are frequently detected in atmospheric samples.

Ab initio study of the reaction of ozone with bromide ion

Surface level ozone destruction in polar environments may be initiated by oxidation of bromide ions by ozone, ultimately leading to Br2 production. Ab initio calculations are used to support the development of atmospheric chemistry models, but errors can occur in study of the bromide-ozone reaction due to inappropriate treatment of the many-electron species and the ionic nature of the reaction. In this work, a high level ab initio study is used to take into account the electronic correlation and the polarization effects. Our results show three possible pathways for the reaction. In particular, we find that this process, though endothermic on the singlet spin state surface, can be energetically feasible on the triplet surface. The triplet surface can be reached through photoexcitation of ozone or by the spin crossing of the potential energy surface. Because this process is known to occur in the dark, it may be that it occurs after intersystem crossing to a triplet surface. This paper also provides a starting point calibration for any future ab initio calculation studies of the bromide-ozone reaction, from the gas to the condensed phase.

Atmospheric mercury in the Canadian Arctic. Part I: a review of recent field measurements

Long-range atmospheric transport and deposition are important sources of mercury (Hg) to Arctic aquatic and terrestrial ecosystems. We review here recent progress made in the study of the transport, transformation, deposition and reemission of atmospheric Hg in the Canadian Arctic, focusing on field measurements (see Dastoor et al., this issue for a review of modeling studies on the same topics). Redox processes control the speciation of atmospheric Hg, and thus impart an important influence on Hg deposition, particularly during atmospheric mercury depletion events (AMDEs). Bromine radicals were identified as the primary oxidant of atmospheric Hg during AMDEs. Since the start of monitoring at Alert (NU) in 1995, the timing of peak AMDE occurrence has shifted to earlier times in the spring (from May to April) in recent years, and while AMDE frequency and GEM concentrations are correlated with local meteorological conditions, the reasons for this timing-shift are not understood. Mercury is subject to various post-depositional processes in snowpacks and a large portion of deposited oxidized Hg can be reemitted following photoreduction; how much Hg is deposited and reemitted depends on geographical location, meteorological, vegetative and sea-ice conditions, as well as snow chemistry. Halide anions in the snow can stabilize Hg, therefore it is expected that a smaller fraction of deposited Hg will be reemitted from coastal snowpacks. Atmospheric gaseous Hg concentrations have decreased in some parts of the Arctic (e.g., Alert) from 2000 to 2009 but at a rate that was less than that at lower latitudes. Despite numerous recent advances, a number of knowledge gaps remain, including uncertainties in the identification of oxidized Hg species in the air (and how this relates to dry vs. wet deposition), physical-chemical processes in air, snow and water-especially over sea ice-and the relationship between these processes and climate change.

Development, testing, and deployment of an air sampling manifold for spiking elemental and oxidized mercury during the Reno Atmospheric Mercury Intercomparison Experiment (RAMIX)

The Reno Atmospheric Mercury Intercomparison Experiment (RAMIX) was in Reno, NV from August 22, 2011 to September 16, 2011. The goals of the experiment were to compare existing and new methods for measurements of ambient elemental and oxidized Hg, and to test these with quantitative spikes of Hg(0), HgBr2, O3 and water vapor. In this paper we describe the design, testing, and deployment of a high flow manifold system designed to deliver ambient air and spiked compounds to multiple instruments simultaneously. The manifold was constructed of 1" OD PFA tubing and heated to 115 °C for the entire active zone. Manifold flow was controlled at ∼200 LPM using a blower and a velocity sensor in a feedback control system. Permeation tubes in controlled ovens were used to deliver Hg(0) and HgBr2. Ozone was generated from a small UV lamp in a flow of high purity O2. Water vapor was generated by pumping a flow of purified N2 through heated, high purity water. The spiking delivery for Hg(0), HgBr2, O3, and water vapor after dilution in the manifold ranged up to 20 ng m(-3), 0.64 ng m(-3), 100 ppbv, and 20 g kg(-1), respectively. During laboratory tests the average transmission efficiencies for Hg(0), HgBr2, and O3 were found to be 92%, 76%, and 93%, respectively.

Fast time resolution oxidized mercury measurements during the Reno Atmospheric Mercury Intercomparison Experiment (RAMIX)

The Reno Atmospheric Mercury Intercomparison Experiment (RAMIX) was carried out from 22 August to 16 September, 2011 in Reno, NV to evaluate the performance of new and existing methods to measure atmospheric mercury (Hg). Measurements were made using a common sampling manifold to which controlled concentrations of Hg species, including gaseous elemental mercury (GEM) and HgBr2 (a surrogate gaseous oxidized mercury (GOM) compound), and potential interferents were added. We present an analysis of Hg measurements made using the University of Washington's Detector for Oxidized Hg Species (DOHGS), focusing on tests of GEM and HgBr2 spike recovery, the potential for interference from ozone (O3) and water vapor (WV), and temporal variability of ambient reactive mercury (RM). The mean GEM and HgBr2 spike recoveries measured with the DOHGS were 95% and 66%, respectively. The DOHGS responded linearly to HgBr2. We found no evidence that elevated O3 interfered in the DOHGS RM measurements. A reduction in RM collection and retention efficiencies at very high ambient WV mixing ratios is possible. Comparisons between the DOHGS and participating Hg instruments demonstrate good agreement for GEM and large discrepancies for RM. The results suggest that existing GOM measurements are biased low.