Parisa A. Ariya

The Arctic: a sink for mercury

Mercury is a persistent, toxic and bio-accumulative pollutant of global interest. Its main mass in the troposphere is in the form of elemental gas-phase mercury. Rapid, near-complete depletion of mercury has been observed during spring in the atmospheric boundary layer of frozen marine areas in Arctic, sub-Arctic and Antarctic locations. It is strongly correlated with ozone depletion. To date, evidence has indicated strongly that chemistry involving halogen gases from surface sea-salt is the mechanism of this destruction. Precisely which halogen gases are the main players has remained unresolved. Our novel kinetic data and multiscale modelling show that Br atoms and BrO radicals are the most effective halogens driving mercury oxidation. The reduction of oxidized mercury deposited in the snow pack back to Hg0 and subsequent diffusion to the atmosphere is observed. However, it cannot compensate for the total deposition, and a net accumulation occurs. We use a unique global atmospheric mercury model to estimate that halogen-driven mercury depletion events result in a 44% increase in the net deposition of mercury to the Arctic. Over a 1-yr cycle, we estimate an accumulation of 325 tons of mercury in the Arctic.

Studies of ozone initiated reactions of gaseous mercury: kinetics, product studies, and atmospheric implications

Kinetics of O3 initiated oxidation reaction of Hg0 was performed in N2 and over the temperature range of 283–323 K at pressure of 750 ± 1 Torr, in order to provide further understanding of geochemical mercury cycling. Kinetic studies were carried out using relative and absolute techniques by gas chromatography with mass spectroscopy (GC-MS). The room temperature reaction rate constant was determined to be (7.5 ± 0.9) × 10−19 cm3 molecule−1 s−1. The calculated activation energy (EA) and pre-exponential factor (A) were 11.7 ± 0.27 kJ mol−1 and 8.43 × 10−17 cm3 molecule−1 s−1, respectively. For the first time, products of O3-initiated oxidation of elemental mercury have been studied in the gas-phase, from the suspended aerosols, and from the wall of the reactor, using chemical ionization mass spectrometry (CI-MS), GC-MS, and inductively coupled plasma mass spectrometry (ICP-MS). Under our experimental conditions, the dominant identified product was HgO, which was mainly adsorbed on the reaction walls, and there was less than 1% of the reaction product collected on the micron filters (0.5 μ) from the suspended aerosols. The implications of our kinetics and product studies to the chemistry of the atmosphere are herein discussed.

Mercury Physicochemical and Biogeochemical Transformation in the Atmosphere and at Atmospheric Interfaces: A Review and Future Directions

Atmosphere and at Atmospheric Interfaces: A Review and Future Directions Parisa A. Ariya,*,†,‡ Marc Amyot, Ashu Dastoor, Daniel Deeds,‡ Aryeh Feinberg,† Gregor Kos,‡ Alexandre Poulain, Andrei Ryjkov, Kirill Semeniuk, M. Subir, and Kenjiro Toyota †Department of Chemistry and ‡Department of Atmospheric and Oceanic Sciences, McGill University, 801 Sherbrooke Street West, Montreal, Quebec, Canada, H3A 2K6 Department of Biological Sciences, Universite ́ de Montreál, 90 avenue Vincent-d’Indy, Montreal, Quebec, Canada, H3C 3J7 Air Quality Research Division, Environment Canada, 2121 TransCanada Highway, Dorval, Quebec, Canada, H9P 1J3 Department of Biology, University of Ottawa, 30 Marie Curie, Ottawa, Ontario, Canada, K1N 6N5 Department of Chemistry, Ball State University, 2000 West University Avenue, Muncie, Indiana 47306, United States Air Quality Research Division, Environment Canada, 4905 Dufferin Street, Toronto, Ontario, Canada, M3H 5T4

Potential for Mercury Reduction by Microbes in the High Arctic

ABSTRACT The contamination of polar regions due to the global distribution of anthropogenic pollutants is of great concern because it leads to the bioaccumulation of toxic substances, methylmercury among them, in Arctic food chains. Here we present the first evidence that microbes in the high Arctic possess and express diverse merA genes, which specify the reduction of ionic mercury [Hg(II)] to the volatile elemental form [Hg(0)]. The sampled microbial biomass, collected from microbial mats in a coastal lagoon and from the surface of marine macroalgae, was comprised of bacteria that were most closely related to psychrophiles that had previously been described in polar environments. We used a kinetic redox model, taking into consideration photoredox reactions as well as mer-mediated reduction, to assess if the potential for Hg(II) reduction by Arctic microbes can affect the toxicity and environmental mobility of mercury in the high Arctic. Results suggested that mer-mediated Hg(II) reduction could account for most of the Hg(0) that is produced in high Arctic waters. At the surface, with only 5% metabolically active cells, up to 68% of the mercury pool was resolved by the model as biogenic Hg(0). At a greater depth, because of incident light attenuation, the significance of photoredox transformations declined and merA-mediated activity could account for up to 90% of Hg(0) production. These findings highlight the importance of microbial redox transformations in the biogeochemical cycling, and thus the toxicity and mobility, of mercury in polar regions.

Physical and chemical characterization of bioaerosols – Implications for nucleation processes

The importance of organic compounds in the oxidative capacity of the atmosphere, and as cloud condensation and ice-forming nuclei, has been recognized for several decades. Organic compounds comprise a significant fraction of the suspended matter mass, leading to local (e.g. toxicity, health hazards) and global (e.g. climate change) impacts. The state of knowledge of the physical chemistry of organic aerosols has increased during the last few decades. However, due to their complex chemistry and the multifaceted processes in which they are involved, the importance of organic aerosols, particularly bioaerosols, in driving physical and chemical atmospheric processes is still very uncertain and poorly understood. Factors such as solubility, surface tension, chemical impurities, volatility, morphology, contact angle, deliquescence, wettability, and the oxidation process are pivotal in the understanding of the activation processes of cloud droplets, and their chemical structures, solubilities and even the molecular configuration of the microbial outer membrane, all impact ice and cloud nucleation processes in the atmosphere. The aim of this review paper is to assess the current state of knowledge regarding chemical and physical characterization of bioaerosols with a focus on those properties important in nucleation processes. We herein discuss the potential importance (or lack thereof) of physical and chemical properties of bioaerosols and illustrate how the knowledge of these properties can be employed to study nucleation processes using a modeling exercise. We also outline a list of major uncertainties due to a lack of understanding of the processes involved or lack of available data. We will also discuss key issues of atmospheric significance deserving future physical chemistry research in the fields of bioaerosol characterization and microphysics, as well as bioaerosol modeling. These fundamental questions are to be addressed prior to any definite conclusions on the potential significance of the role of bioaerosols on physico-chemical atmospheric processes and that of climate.

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.

A theoretical study of the reactions of parent and substituted Criegee intermediates with water and the water dimer

A theoretical study was performed involving the reactions of a series of atmospherically significant Criegee intermediates (CIs), including parent, mono- and dimethyl - substituted CIs, with water and the water dimer at the CCSD(T)//B3LYP/6-311+G(2d,2p) level. We investigated two reaction routes leading to α-hydroxyalkyl hydroperoxide (HAHP). According to our calculations, the most favorable route is the formation of HAHP as the result of reactions of CIs with the water dimer. The rate constants for both pathways were calculated and the dependence of overall rate constant on temperature and relative humidity was also evaluated. The implication of our results to the chemistry of the troposphere is herein discussed.

Significance of HO x and peroxides production due to alkene ozonolysis during fall and winter: A modeling study

In an attempt to identify new mechanisms for the generation of oxidants during fall and winter, we carried out a modeling investigation in which ozonolysis reactions of alkenes that were primarily anthropogenic in origin were considered. Our results indicate that the ozonolysis reactions of these molecules can be the major sources of HOx, H2O2, and organic peroxides during the night and therefore especially during dark seasons. These O3-initiated oxidation reactions produce more peroxy radicals than those initiated by HO or NO3. This increase in RO2 also results in an increase in HO, HO2, and H2O2. The direct HO formation pathways by ozonolysis of alkenes can form more HO radicals than that from the reaction of O(1D) + H2O during the dark seasons. This additional source of HO can augment significantly atmospheric oxidation. H2O2 formation by ozonolysis also appears to be the most important dark season tropospheric sources of this oxidant. Our modeling results suggest that the existence of pollutant hydrocarbons and trace amount of biogenically produced terpenes can also lead to important production of HOx, H2O2, and organic peroxides. Substantially enhanced gas-phase production of H2O2 and organic peroxides due to ozonolysis reactions can cause significant liquid-phase oxidation of S(IV) to S(VI), and hence the role of ozonolysis reactions can be important for the sulfur conversion studies.

A theoretical study of the reactions of carbonyl oxide with water in atmosphere: the role of water dimer

Carbonyl oxide is a well-known intermediate formed in gas-phase reactions of ozone with alkenes. Secondary reactions of carbonyl oxide are suggested to lead to the formation of HO, H2O2 and organic peroxides in the atmosphere. We performed a theoretical study of reactions of carbonyl oxide with water and a water dimer. Using CCSD(T)/6-311+G(2d,2p)//B3LYP/6-311+G(2d,2p) calculations we found that the most energetically favourable channel is the formation of hydroxymethyl hydroperoxide (HMHP) as the result of reactions of carbonyl oxide with the water dimer. The potential importance of water dimer reactions in the chemistry of the troposphere is discussed herein.

Kinetics of the gas-phase reactions of Cl atom with selected C2–C5 unsaturated hydrocarbons at 283 < T < 323 K

The reaction of Cl atoms with a series of C2–C5 unsaturated hydrocarbons has been investigated at atmospheric pressure of 760 Torr over the temperature range 283–323 K in air and N2 diluents. The decay of the hydrocarbons was followed using a gas chromatograph with a flame ionization detector (GC-FID), and the kinetic constants were determined using a relative rate technique with n-hexane as a reference compound. The Cl atoms were generated by UV photolysis (λ ≥ 300 nm) of Cl2 molecules. The following absolute rate constants (in units of 10−11 cm3 molecule−1 s−1, with errors representing ±2σ) for the reaction at 295 ± 2 K have been derived from the relative rate constants combined to the value 34.5 × 10−11 cm3 molecule−1 s−1 for the Cl + n-hexane reaction: ethene (9.3 ± 0.6), propyne (22.1 ± 0.3), propene (27.6 ± 0.6), 1-butene (35.2 ± 0.7), and 1-pentene (48.3 ± 0.8). The temperature dependence of the reactions can be expressed as simple Arrhenius expressions (in units of 10−11 cm3 molecule−1 s−1): kethene = (0.39 ± 0.22) × 10−11 exp{(226 ± 42)/T}, kpropyne = (4.1 ± 2.5) × 10−11 exp{(118 ± 45)/T}, kpropene = (1.6 ± 1.8) × 10−11 exp{(203 ± 79)/T}, k1-butene = (1.1 ± 1.3) × 10−11 exp{(245 ± 90)/T}, and k1-pentene = (4.0 ± 2.2) × 10−11 exp{(423 ± 68)/T}. The applicability of our results to tropospheric chemistry is discussed.