Kinetics and products study of the reaction of BrO radicals with gaseous mercury

Bro reactions of elemental mercury was as a major candidate for near complete depletion of elemental mercury in polar region. The kinetics of the reaction between BrO radicals with gaseous mercury was identified using relative rate method by Gas Chromatography with Mass spectroscopic Detection (GC-MS) at room temperature ($298\pm1$ K) and at atmospheric pressure $760 \pm 1$ Torr in the $N_2$ diluent. Propane, DMS and butane were used as reference molecules. The upper and lower limits rate constant for reaction of $\rm Hg^0$ with BrO was estimated to be $1.0 \times 10^{-13}$ and $1.0 \times 10^{15}$ cm$^3$ molecules$^{-1}$ s$^{-1}$, respectively. BrO radicals were produced from the photolysis of bromine and dibromomethane in presence of ozone and detected by MS at $\rm m/e = 95$ or 97. Ozone was produced in a Silent discharged generator. Reaction products were identified using direct mass spectrometer with chemical ionization ion source

Trichloroethene and tetrachloroethene: tropospheric probes for Cl- and Br-atom reactions during the polar sunrise

We report the results of laboratory and modeling investigations of the atmospheric fate of chlorinated ethenes and their rôle as indicators of halogen reactions in the springtime Arctic troposphere. The kinetics and mechanism of the gas-phase reactions of Cl- and Br-atoms with tetrachloroethene were studied using a Fourier transform infrared spectrometer (FTIR) in 93.3 kPa air and T = 296 ± 2 K. Along with our previous study on Cl and Br atom reactions of trichloroethene, using the known rate of the Cl + ethane reaction as reference, the values of 7.2 ± 0.2 × 10−11 and 3.8 ± 0.2 × 10−11 cm3 molecule−1 s−1 were obtained for the Cl-atom reaction rate constants of tri- and tetrachloroethene, respectively. For the Br-atom reactions, using ethene and propane as the reference molecules, we report the absolute values of 1.1 ± 0.1 × 10−13 and 9.0 ± 0.1 × 10−17 cm3 molecule−1 s−1 for the rates of Br attack on tri- and tetrachloroethene. The major products were XCl2C-C(O)Cl (X = H in trichloroethene and X = Cl in tetrachloroethene) and XBrClC-C(O)Cl in Cl-atom and Br-atom initiated reactions, respectively. We also observed phosgene and formyl chloride in the reactions of trichloroethene and phosgene in the tetrachloroethene reactions and report the branching ratios for these channels. Our observations show that the addition of Cl-atoms to the less substituted carbon is the major reaction pathway and that a Cl-atom is subsequently released after either Cl- or Br-atoms add to trichloroethene or tetrachloroethene, leading to the recycling of active halogen. We carried out a box modelling exercise to apply the kinetic and mechanistic information obtained in this work to the interpretation of measurements of C2HCl3, C2Cl4 and other hydrocarbons in the Arctic troposphere. Our results demonstrate that the presence of both Cl- and Br-atoms is required to explain the decreases in the concentrations of these species during ozone depletion events in the Arctic, that the amount of Cl- and Br-atoms required to account for the observed decreases of chlorinated ethenes is also sufficient to completely destroy ozone. However, at the low concentrations of chlorinated species observed, the cycling of Br to Cl through the studied reactions is not a significant effect in the atmosphere

Atmospheric transoformation of elemental mercury upon reactions with halogens

Rapid depletions of atmospheric mercury (Hg$^0$) and ozone have recently been observed in several regions of high-and sub-Arctic, as well as Antarctica. There are a large body of ground based and satellite observations implicating the reactions of halogens, during these depletion events. In our laboratory, we have performed extensive kinetic and product studies on the oxidation reactions of elemental mercury with atmospheric oxidants, along with complementary thermochemical studies based on our kinetics and product studies of reactions of gaseous Hg$^0$ with molecular and atomic halogens. In this study, we incorporated our laboratory data in a chemical box model (MOCCA). Our results on the potential importance of some key molecular and atomic halogen reactions in depletion of mercury within atmospheric boundary layer, will be discussed

Polar sunrise experiment 1995: Hydrocarbon measurements and tropospheric Cl and Br-atoms chemistry

As part of the Polar Sunrise Experiments (PSE) 1995, we report our results on measurement of non-methane hydrocarbons (NMHC) collected at Alert, Northwest Territories, Canada (82.5° N, 62.3° W) from Julian days 57 to 113, 1995 along with our data on continuous methane and ozone measurements during the same period. The concentration of NMHCs such as alkanes correlated well with that of methane during the dark period before polar sunrise. However, no correlation was observed after the sunrise. Several ozone depletion events and concurrent decreases in hydrocarbon concentrations relative to their background levels were observed. In all ozone depletion periods, concentration changes of alkanes and toluene were consistent with the occurrence of Cl-atom reactions. The characteristics of alkane isomer concentration changes suggest the dominance of HO chemistry prior to, and a switch to Cl chemistry after, the polar sunrise (during the ozone depletion events). The changes in ethyne concentration from their background level were in excess of those expected from Cl-atom kinetics alone and are attributed to additional Br-atom reactions. The time integral for Br mixing ratios is a few orders of magnitudes higher than that for Cl-atoms, suggesting much higher Br-atom concentrations compared to Cl-atom concentrations, if they are present simultaneously

C2-C7 hydrocarbon concentrations in Arctic snowpack interstitial air: potential presence of active Br within the snowpack

Samples of interstitial air from within the snow pack on an ice floe on the Arctic Ocean were collected during the April 1994 Polar Sunrise Experiment. The concentrations of C2-C7 hydrocarbons are reported for the first time in the snow pack interstitial air. Alkane concentrations tended to be higher than concentrations in free air samples above the snow but very similar to winter measurements at various locations in the Arctic archipelago. However, ethyne concentrations in both interstitial and free air were highly correlated with ozone mixing ratios, consistent with previous demonstrations of the effects of Br atom chemistry. The analysis of total bromine within the snow pack indicate an enrichment in total Br at the interface layer between snow and free troposphere. The mixing ratios of some brominated compounds, such as CHBr3 and CHBr2Cl, are found to be higher in this top layer of snow relative to the boundary layer. Results were inconclusive due to the limited number of samples, but suggest the possible presence of active bromine in the snow pack and also that some differences exist between chemical reactions occurring in interstitial air compared to air in the boundary layer

Behavior of mercury in snow from different latitudes

Deposition of Hg via snow fall may represent an important Hg flux to terrestrial and aquatic ecosystem of temperate and polar regions. We have conducted a series of field and laboratory experiments to better understand the post-depositional behaviour of Hg in snow. We found that: 1) a significant portion of the snow-to-air Hg evasion results from photoreduction of Hg in snow; 2) this photoreduction is mainly driven by UV-B radiation; 3) this photoreduction can be observed even in the presence of halogens in the snow, although we further found that these halogens favour the reverse photooxidation reaction; 4) laboratory experiments show that this photoreduction is likely mediated by organic reducing agents found in the snow matrix. These data will be incorporated in a global/regional Hg model. We propose that Hg falling inland in temperate and Arctic areas could be more rapidly re-emitted than Hg falling in coastal Arctic areas (where mercury depletion events occur)

The gas-phase ozonolysis reaction of methylbutenol: A mechanistic study

The gas‐phase ozonolysis reaction of methylbutenol through the Criegee mechanism is investigated. The initial reaction leads to a primary ozonide (POZ) formation with barriers in the range of 10–28 kJ mol−1. The formation of 2‐hydroxy‐2‐methyl‐propanal (HMP) and formaldehyde‐oxide is more favorable, by 10 kJ mol−1, than the syn‐CI and formaldehyde. The unimolecular dissociation of the more stable syn‐CI via 1,5‐H transfer into an epoxide is more favored than the epoxide and 3O2 formation. The ester channel led to the formation of the acetone and formic acid favorably from the anti‐CI. The hydration of the anti‐CI with H2O and (H2O)2 is significantly barrierless with a higher plausibility to the latter, and thus they may lead to the formation of peroxides and ultimately OH radicals, as well as airborne particulate matter. Reaction of anti‐CI with water dimers enhances its atmospheric reactivity by a factor of 28 in reference to water monomers

Measurements of C2-C7 hydrocarbons during the polar sunrise experiment 1994: Further evidence for halogen chemistry in the troposphere

Air samples for nonmethane hydrocarbon (NMHC) analysis were collected at two ground‐based sites: Alert, Northwest Territories (82.5°N, 62.3°W) and Narwhal ice camp, an ice floe 140 km northwest of Alert, from Julian days 90 to 117, 1994, and on a 2‐day aerial survey conducted on Julian days 89 and 90, 1994 over the Arctic archipelago. Several ozone depletion events and concurrent decreases in hydrocarbon concentrations relative to their background levels were observed at Alert and Narwhal ice camp. At Narwhal, a long period (≥7 days) of ozone depletion was observed during which a clear decay of alkane concentration occurred. A kinetic analysis led to a calculated Cl atom concentration of 4.5 × 103 cm−3 during this period. Several low‐ozone periods concurrent with NMHC concentration decreases were observed over a widespread region of the Arctic region (82°–85°N, and 51°–65°W). Hydrocarbon measurements during the aerial survey indicated that the low concentrations of these species occurred only in the boundary layer. In all ozone depletion periods, concentration changes of alkanes and toluene were consistent with Cl atom reactions. The changes in ethyne concentration from its background level were in excess of those expected from Cl atom kinetics alone and are attributed to additional Br atom reactions. A box modeling exercise suggested that the Cl and particularly Br atom concentrations required to explain the hydrocarbon behavior are also sufficient to destroy ozone

Sources and cycling of mercury in the paleo Arctic Ocean from Hg stable isotope variations in Eocene and Quaternary sediments

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/155764/1/Gleason_et_al_2017_Sources.pd

Some like it cold: microbial transformations of mercury in polar regions

The contamination of polar regions with mercury that is transported from lower latitudes as inorganic mercury has resulted in the accumulation of methylmercury (MeHg) in food chains, risking the health of humans and wildlife. While production of MeHg has been documented in polar marine and terrestrial environments, little is known about the responsible transformations and transport pathways and the processes that control them. We posit that as in temperate environments, microbial transformations play a key role in mercury geochemical cycling in polar regions by: (1) methylating mercury by one of four proposed pathways, some not previously described; (2) degrading MeHg by activities of mercury resistant and other bacteria; and (3) carrying out redox transformations that control the supply of the mercuric ion, the substrate of methylation reactions. Recent analyses have identified a high potential for mercury-resistant microbes that express the enzyme mercuric reductase to affect the production of gaseous elemental mercury when and where daylight is limited. The integration of microbially mediated processes in the paradigms that describe mercury geochemical cycling is therefore of high priority especially in light of concerns regarding the effect of global warming and permafrost thawing on input of MeHg to polar regions