Che-Jen Lin

The chemistry of atmospheric mercury: a review

The atmosphere is an important transient reservoir of mercury. In addition to its great capacity, the chemical processes transforming mercury between the elemental and divalent states strongly influence the transport characteristics and deposition rate of this toxic metal back to the ground. Modeling efforts to assess global cycling of mercury require an in-depth knowledge of atmospheric mercury chemistry. This review article provides selected physical and chemical properties of atmospheric mercury, and discusses the identified mercury transformation pathways mediated by ozone, S(IV), hydroperoxyl radical, hydroxyl radical, chlorine, nitrate radical and photolysis of Hg(II) complexes. Special attention is paid to the kinetics and mechanisms of the reactions interconverting mercury between elemental and divalent states. The significance and implications of each transformation pathway under atmospheric conditions are addressed. Future research areas that must be pursued to better understand the fate and transformation of mercury in the atmosphere are also projected.

Aqueous free radical chemistry of mercury in the presence of iron oxides and ambient aerosol

Abstract The effect of goethite (α-FeOOH), hematite (α-Fe203) and maghemite (γ-Fe203) on the aqueous photoreduction of divalent mercury with organic acids (oxalate, formate and acetate) is investigated. Laboratory photochemistry experiments with synthetic iron oxides and simulated sunlight were performed to assess the role of the oxides on the photoreduction. Ambient aerosol was also collected and introduced as the solid phase to compare its effect with that of synthetic oxides. It is observed that the presence of various iron oxides or aerosol particles enhances the photoreduction. It is also found that the hydroxyl radicals produced in the hematite-oxalate systems can re-oxidize the reduced mercury back to Hg(II). Based on the experimental observations, mechanisms responsible for the Hg(II) reduction are proposed. The kinetics of Hg0 oxidation by hydroxyl radicals was also studied by a steady-state kinetic technique using nitrate photolysis as the * OH radical source. The second-order rate constant is determined to be 2.0 × 109 M− s−1. The implications of the studied reactions on the atmospheric chemistry of mercury are discussed.

Aqueous Photochemistry of Mercury with Organic Acids

To understand the dynamic chemical changes of mercury taking place in atmospheric water, a more detailed knowledge of the reactions of mercury with organic acids must be obtained. The rate of the above chemical reactions with different Hg(II) species is especially essential for understanding the dominant pathway of mercury transformation in the atmosphere. The objective of this research is to study the rate of photochemical reactions of dissolved divalent mercury with organic acids such as oxalate, acetate, and formate, which are abundant in atmospheric water. Laboratory photochemical experiments with simulated sunlight were conducted to assess the role of homogeneous photochemistry in changing the redox states of mercury in atmospheric water. It is observed that Hg(II) is readily reduced by hydroperoxyl radicals produced by the photolysis of oxalate. The second-order rate constant for the Hg(II)-hydroperoxyl radical reaction was found to be 1.7 x 104 M-1 s-1 when no chloride is present and 1.1 x 104 M-1 s-1 when chloride is present in the system. Modeling of mercury speciation has been performed using MICROQL equilibrium software.

ARM Southern Great Plains Site Observations of the Smoke Pall Associated with the 1998 Central American Fires

Drought-stricken areas of Central America and Mexico were victimized in 1998 by forest and brush fires that burned out of control during much of the first half of the year. Wind currents at various times during the episode helped transport smoke from these fires over the Gulf of Mexico and into portions of the United States. Visibilities were greatly reduced during favorable flow periods from New Mexico to south Florida and northward to Wisconsin as a result of this smoke and haze. In response to the reduced visibilities and increased pollutants, public health advisories and information statements were issued by various agencies in Gulf Coast states and in Oklahoma. This event was also detected by a unique array of instrumentation deployed at the U.S. Department of Energys Atmospheric Radiation Measurement (ARM) program Southern Great Plains Cloud and Radiation Testbed and by sensors of the Oklahoma Department of Environmental Quality/Air Quality Division. Observations from these measurement devices sugg...

Oxidation of elemental mercury by aqueous chlorine (HOCl/OCl−): Implications for tropospheric mercury chemistry

The stoichiometry and kinetics of elemental mercury (Hg0) oxidation by aqueous chlorine (HOCl/OCl−) have been investigated. The stoichiometric ratio of Hg0 to HOCl/OCl− is found to be 1:1, the same as the electron transfer ratio. The rate constants of the oxidation are measured in a novel fashion by using chloramine (NH2Cl) as the free chlorine reservoir. The rate constants at room temperature (23°∼ 25°C) for the Hg0-HOCl and Hg0-OCl− are measured to be (2.09±0.06)×106 and (1.99±0.05)×106 M−1 s−1, respectively. Based on the solubility data of chlorine, the intrinsic Henrys law constant of chlorine is calculated to be 7.61×10−2 M atm−1 at 25°C. Model study using the kinetic data in this investigation shows that the oxidation of Hg0 by aqueous chlorine is an important pathway contributing dissolved divalent mercury (Hg(II)) in atmospheric water, especially at higher cloud water pH when solubility of chlorine is greatly increased and before sunrise when chlorine reaches its peak concentrations in the marine troposphere.

Degradation of monomethylmercury chloride by hydroxyl radicals in simulated natural waters

The degradation of methylmercury chloride by hydroxyl radicals (*OH) has been investigated using nitrate photolysis from 285 to 800 nm with a 450 W Xenon lamp as the (*OH source. The identified products are Hg(2+), Hg(0), CHCl(3) and formaldehyde. The second-order rate constant at pH of 5 at room temperature was determined to be 9.83(+-0.66)x10(9) M(-1) x s(-1)using benzoic acid as the *OH scavenger. The effects of chloride concentration and methylmercury speciation have also been investigated. A mechanism of the CH(3)HgCl-*OH reaction has been proposed. The calculated methylmercury degradation rates in natural waters using the above rate constant were comparable to the in situ photodegradation rates reported previously, indicating that degradation by (*)OH may be one of the important pathways of methylmercury degradation in sunlit surface waters.

Aqueous phase reactions of mercury with free radicals and chlorine : Implications for atmospheric mercury chemistry

The role of aqueous phase hydroperoxyl radical (HO 2 . ), hydroxyl radical ( . OH) and chlorine (HOCl/OCl . ) in the redox cycle of atmospheric mercury is investigated. The contributions of the above species to the aqueous phase Hg(II) concentration under various conditions are evaluated by a chemical kinetic model simulating the chemistry in atmospheric droplets. Based on the model results, . OH is an important daytime oxidant for Hg° and can account for up to 25 % of the total oxidation rate in the aqueous phase under the model conditions in this study. In the nighttime marine atmosphere, aqueous chlorine can be the most predominant oxidant for Hg°, contributing up to 90 % of the total oxidation rate in the aqueous phase. Increasing pH from 4.0 to 4.3 and decreasing [Cl - ] from 1.0 to 0.5 mM increase [Hg(II)] in the droplets by 50 % and 26 %, respectively. Compared to aqueous SO 2 , HO 2 . can reduce Hg(II) at a significant rate. In addition, HO 2 . is the only reductant balancing all the oxidation processes of Hg° after SO 2 is depleted.

Assessment of modeled mercury dry deposition over the Great Lakes region

Three sets of model predicted values for speciated mercury concentrations and dry deposition fluxes over the Great Lakes region were assessed using field measurements and model intercomparisons. The model predicted values were produced by the Community Multiscale Air Quality Modeling System for the year 2002 (CMAQ2002) and for the year 2005 (CMAQ2005) and by the Global/Regional Atmospheric Heavy Metals Model for the year 2005 (GRAHM2005). Median values of the surface layer ambient concentration of gaseous elemental mercury (GEM) from all three models were generally within 30% of measurements. However, all three models overpredicted surface-layer concentrations of gaseous oxidized mercury (GOM) and particulate bound mercury (PBM) by a factor of 2-10 at the majority of the 15 monitoring locations. For dry deposition of GOM plus PBM, CMAQ2005 showed a clear gradient with the highest deposition in Pennsylvania and its surrounding areas while GRAHM2005 showed no such gradient in this region; however, GRAHM2005 had more hot spots than those of CMAQ2005. Predicted dry deposition of GOM plus PBM from these models should be treated as upper-end estimates over some land surfaces in this region based on the tendencies of all the models to overpredict GOM and PBM concentrations when compared to field measurements. Model predicted GEM dry deposition was found to be as important as GOM plus PBM dry deposition as a contributor to total dry deposition. Predicted total annual mercury dry deposition were mostly lower than 5 μg m(-2) to the surface of the Great lakes, between 5 and 15 μg m(-2) to the land surface north of the US/Canada border, and between 5 and 40 μg m(-2) to the land surface south of the US/Canada border. Predicted dry deposition from different models differed from each other by as much as a factor of 2 at regional scales and by a greater extent at local scales.

Two-phase model of mercury chemistry in the atmosphere

Abstract A hybrid kinetic and equilibrium model simulating the conditions in clouds is developed to assess mercury chemistry in the atmosphere. Simulation of an air parcel containing cloud droplets is used to examine the effects of a number of physical and chemical parameters on the evolution of dissolved divalent mercury [Hg(II)] concentration profiles. Sensitivity analysis on each parameter is performed and the resulting Hg(II) profiles are plotted. The modeled steady-state Hg(II) levels are then compared to the measured values in precipitation. It is found that gaseous-phase oxidation of elemental mercury by ozone contributes a significant fraction of dissolved Hg(II) in the droplets, and that aqueous phase radical (i.e. OH and HO 2 ) play an important role in mercury transformations. S(IV) is an important reductant for aqueous-phase Hg(II). However, the contribution of S(IV) in mercury chemistry is limited to the cases when aqueous phase S(IV) is present at a concentration to maximize HgSO 3 formation. After the system reaches a steady state, HO 2 is the only reductant to balance the Hg(II) production by various oxidation pathways in both phases. Based on the simulation results, it is suggested that more oxidation pathways should be identified to better describe mercury chemistry in the atmosphere.

Licklider transmission protocol (LTP)-based DTN for cislunar communications

Delay/disruption-tolerant networking (DTN) technology offers a new solution to highly stressed communications in space environments, especially those with long link delay and frequent link disruptions in deep-space missions. To date, little work has been done in evaluating the performance of the available “convergence layer” protocols of DTN, especially the Licklider Transmission Protocol (LTP), when they are applied to an interplanetary Internet (IPN). In this paper, we present an experimental evaluation of the Bundle Protocol (BP) running over various “convergence layer” protocols in a simulated cislunar communications environment characterized by varying degrees of signal propagation delay and data loss. We focus on the LTP convergence layer (LTPCL) adapter running on top of UDP/IP (i.e., BP/LTPCL/UDP/IP). The performance of BP/LTPCL/UDP/IP in realistic file transfers over a PC-based network test bed is compared to that of two other DTN protocol stack options, BP/TCPCL/TCP/IP and BP/UDPCL/UDP/IP. A statistical method of t-test is also used for analysis of the experimental results. The experiment results show that LTPCL has a significant performance advantage over Transmission Control Protocol convergence layer (TCPCL) for link delays longer than 4000 ms regardless of the bit error rate (BER). For a very lossy channel with a BER of around 10-5, LTPCL has a significant goodput advantage over TCPCL at all the link delay levels studied, with an advantage of around 3000 B/s for delays longer than 1500 ms. LTPCL has a consistently significant goodput advantage over UDPCL, around 2500-3000 B/s, at all levels of link delays and BERs.