Ozone uptake to the polar snowpack at Summit,Greenland

The uptake of atmospheric ozone to the polar, year-round snowpack on glacial ice was studied at Summit, Greenland during three experiments in 2003, 2004, and 2005. Ozone was measured at up to three depths in the snowpack, on the surface, and above the surface at three heights on a tower along with supporting meteorological parameters. Ozone in interstitial air decreased with depth, albeit ozone gradients showed a high variation depending on environmental conditions of solar radiation and wind speed. Under low irradiance levels, up to 90% of ozone was preserved up to 1 m depth in the snowpack. Ozone depletion rates increased significantly with the seasonal and diurnal cycle of solar irradiance, resulting in only 10% of ozone remaining in the snowpack following solar noon during summertime. Faster snowpack air exchange from wind pumping resulted in smaller above-surface-to-within snowpack ozone gradients. These data indicate that the uptake of ozone to polar snowpack is strongly dependent on solar irradiance and wind pumping. Ozone deposition fluxes to the polar snowpack are consequently expected to follow incoming solar radiation levels and to exhibit diurnal and seasonal cycles. The Summit observations are in stark contrast to recent findings in the seasonal, midlatitude snowpack [Bocquet, F., Helmig, D., Oltmans, S.J., 2007. Ozone in the mid-latitude snowpack at Niwot Ridge, Colorado. Arctic, Antarctic and Alpine Research, in press], where mostly light-independent ozone behavior was observed. These contrasting results imply different ozone chemistry and snowpack–atmosphere gas exchange in the snow-covered polar, glacial conditions compared to the temperate, mid-latitude environment.     

Hydroxyl concentration estimates in the sunlit snowpack at Summit,Greenland

Experiments were performed at Summit, Greenland (72°34′ N, 38°29′ W) to investigate hydroxyl mixing ratios in the sunlit surface snowpack (or firn). We added a carefully selected mixture of hydrocarbon gases (with a wide range of hydroxyl reactivities) to a UV and visible light transparent flow chamber containing undisturbed natural firn. The relative decrease in mixing ratios of these gases allowed estimation of the lower limit mixing ratio of hydroxyl radicals in the near-surface firn pore spaces. Hydroxyl mixing ratios in the firn air followed a diurnal cycle in summer 2003 (10–12 July), with peak values of more than 3.2×10⁶ molecules cm⁻³ between 13:00 and 16:00 local time. The minimum value estimated was 1.1×10⁶ molecules cm⁻³ at 20:00 local time. Results during spring of 2004 showed lower, but rapidly increasing, peak hydroxyl mixing ratios of 1.1×10⁶ molecules cm⁻³ in the early afternoon on 15 April and 1.5×10⁶ molecules cm⁻³ on 1 May. Our firn hydroxyl estimates were similar to directly measured above-snow ambient levels during the spring field season, but were only about 30% of ambient levels during summer.     

Boundary-layer dynamics and its influence on atmospheric chemistry at Summit,Greenland

Sonic anemometer turbulence measurements were made at Summit, Greenland during summer 2004 and spring 2005. These measurements allow for the characterization of the variability of the atmospheric boundary layer at this site by describing seasonal and diurnal changes in sensible heat flux and boundary layer stability as well as providing estimates of mixing layer height. Diurnal sensible heat fluxes at Summit ranged from −18 to −2 W m⁻² in the spring and from −7 to +10 W m⁻² in the summer. Sustained stable surface layer conditions and low wind speeds occured during the spring but not during the summer months. Unstable conditions were not observed at Summit until late April. Diurnal cycles of convective conditions during the daytime (0700–1700 h local time) were observed throughout July and August. Boundary layer heights, which were estimated for neutral to stable conditions, averaged 156 m for the spring 2005 observations. Comparisons of the boundary layer characteristics of Summit with those from South Pole, Antarctica, provide possible explanations for the significant differences in snowpack and surface-layer chemistry between the two sites.     

Observations of hydroxyl and the sum of peroxy radicals at Summit,Greenland during summer 2003

The first measurements of peroxy (HO₂+RO₂) and hydroxyl (OH) radicals above the arctic snowpack were collected during the summer 2003 campaign at Summit, Greenland. The median measured number densities for peroxy and hydroxyl radicals were 2.2×10⁸ mol cm⁻³ and 6.4×10⁶ mol cm⁻³, respectively. The observed peroxy radical values are in excellent agreement ($R^2=0.83$, $\mathrm{M}/\mathrm{O}=1.06$) with highly constrained model predictions. However, calculated hydroxyl number densities are consistently more than a factor of 2 lower than the observed values. These results indicate that our current understanding of radical sources and sinks is in accord with our observations in this environment but that there may be a mechanism that is perturbing the (HO₂+RO₂)/OH ratio. This observed ratio was also found to depend on meteorological conditions especially during periods of high winds accompanied by blowing snow. Backward transport model simulations indicate that these periods of high winds were characterized by rapid transport (1–2 days) of marine boundary layer air to Summit. These data suggest that the boundary layer photochemistry at Summit may be periodically impacted by halogens.     

Concentrations and sources of carbonaceous aerosol in the atmosphere of Summit,Greenland

High-volume PM_2.5 samples were collected at Summit, Greenland for approximately six months from late May through December of 2006. Filters were composited and analyzed for source tracer compounds. The individual organic compounds measured at Summit are orders of magnitude smaller than concentrations measured at other sites, including locations representative of remote oceanic, and remote and urban continental aerosol. The measured tracers were used to quantify the contribution of biomass burning (0.6–0.9 ng C m^?3), vegetative detritus (0.3–0.9 ng C m^?3), and fossil fuel combustion (0.1–0.8 ng C m^?3) sources, 4% of OC total, to atmospheric organic carbon concentrations at the remote location of Summit, Greenland. The unapportioned organic carbon (96%) during the early summer period correlates well with the fraction of water soluble organic carbon, indicating secondary organic aerosol as a large source of organic carbon, supported by the active photochemistry occurring at Summit. To the author's knowledge, this paper represents the first source apportionment results for the polar free troposphere.     

Release of gaseous bromine from the photolysis of nitrate and hydrogen peroxide in simulated sea-salt solutions

We have carried out a series of laboratory experiments to investigate the oxidation of bromide (Br⁻) by hydroxyl radical (OH) in solutions used to mimic sea-salt particles. Aqueous halide solutions with nitrate or hydrogen peroxide (HOOH) as a photochemical source of OH were illuminated with 313 nm light and the resulting gaseous bromine (Br*(g)) was collected. While illumination of these solutions nearly always formed gaseous bromine (predominantly Br₂ based on modeling results), there was no evidence for the release of gaseous chlorine. The rate of Br*(g) release increased (up to a plateau value) with increasing concentrations of bromide and was enhanced at lower pH values for both nitrate and HOOH solutions. Increased ionic strength in nitrate solutions inhibited Br*(g) release and the extent of inhibition was dependent upon the salt used. In HOOH solutions, however, no ionic strength effects were observed and the presence of Cl⁻ strongly enhanced Br*(g) release.Overall, for conditions typical of aged, deliquesced, sea-salt particles, the efficiencies of gaseous bromine release, expressed as mole of Br*(g) released per mole of OH photochemically formed, were typically 20–30%. Using these reaction efficiencies, we calculated the Br₂(g) release rate from aged, ambient sea-salt particles due to OH oxidation to be approximately 0.07 pptv h⁻¹ with the main contributions from nitrate photolysis and partitioning of gas-phase OH into the particle. While our solution conditions are simplified compared to ambient particles, this estimated rate of Br₂ release is high enough to suggest that OH-mediated reactions in sea-salt particles could be a significant source of reactive bromine to the marine boundary layer.     

Hydrogen peroxide photolysis, fenton reagent and photo-fenton for the degradation of nitrophenols: a comparative study

The kinetics of the degradation of seven nitrophenols (2-nitrophenol, 4-nitrophenol, 2,5-dinitrophenol, 2,6-dinitrophenol, 2,4-dinitrophenol, 2-methyl-4,6-dinitrophenol, 4-methyl-2,6-dinitrophenol) with the Fenton reagent, photo-Fenton, and hydrogen peroxide photolysis was investigated. The efficiency and operating costs for the studied treatments were evaluated and compared. The Fenton reagent was found to be the most efficient and the cheapest way for the nitrophenols (NPs) degradation. The formation of nitrate as a result of mineralisation of organically bounded nitrogen was observed during the treatment of NPs with studied treatment processes. The degrees of organically bounded nitrogen conversion to nitrate after 90% degradation of NPs with the Fenton, photo-Fenton and hydrogen peroxide photolysis were 51-67%, 85-90%, and 50-60%, respectively. According to Daphnia magna acute toxicity test, the Fenton treatment led to complete detoxification of NPs.     

Distinguishing the relative contribution of fossil fuel and biomass combustion aerosols deposited at Summit,Greenland through isotopic and molecular characterization of insoluble carbon

Quantifying combustion aerosols transported to Summit, Greenland has typically involved the measurement of water-soluble inorganic and organic ions in air, snow, and ice. However, the ubiquitous nature of atmospheric soluble ions makes it difficult to separate the combustion component from the natural component. More specific combustion indicators are therefore needed to accurately quantify inputs from biomass and fossil-fuel burning. This work reports on radiocarbon (¹⁴C) analysis of elemental carbon (EC) and quantification of polycyclic aromatic hydrocarbons (PAHs) of water-insoluble particles from a snowpit excavated at Summit, Greenland in 1996. The ¹⁴C measurements allowed us to quantify the relative contribution of EC from biomass burning and fossil-fuel combustion transported to and deposited at Summit during periods of 1994 and 1995. Specific PAHs associated with conifer combustion helped to identify snowpit layers impacted by forest fires. Our results show that fossil EC was the major component during spring and fall 1994, while biomass EC and fossil EC were present in roughly equal amounts during summer 1994. PAH ratios in spring layers of the snowpit indicate substantial inputs from anthropogenic sources and the ΣPAH depth profile displays springtime maxima that coincided with non-sea-salt sulfate ion maximum concentrations. In other layers, ammonium ion concentrations were independent of the isotopic and molecular carbon measurements. This work demonstrates the utility of radiocarbon techniques to quantify the two different sources of combustion-generated particles at Summit; however, portions of the ¹⁴C results were indeterminate due to large uncertainties that were the result of chemical impurities introduced in the EC isolation technique. Additionally, PAH measurements were successfully performed on as little as 100 ml of snowmelt water, demonstrating the potential for future finer sample resolution.     

Vertical mixing above Summit,Greenland: Insights into seasonal and high frequency variability from the radionuclide tracers 7Be and 210Pb

The activity of the natural radionuclide tracers ⁷Be and ²¹⁰Pb has been determined in bulk aerosol samples collected over 2-day intervals for nearly five full years at Summit, Greenland. Year-round sampling was conducted in three campaigns; summer 1997 to summer 1998, summer 2000 to summer 2002, and summer 2003 to present. As in previous summer campaigns at Summit, and a year-round investigation at Dye 3, variations in the activities of the tracers on short time scales were strongly correlated despite the upper troposphere/lower stratosphere source of ⁷Be and the continental surface source of ²²²Rn (precursor of ²¹⁰Pb). This behavior is attributed to boundary layer dynamics exerting the dominant control on activities in air just above the ice sheet. Aerosols and associated species are depleted from the boundary layer above the snow when a strong inversion limits exchange with the free troposphere. Episodic weakening of the inversion allows ventilation of the boundary layer. This cycle drives simultaneous decreases and increases in the radionuclide tracers. The correlation between ⁷Be and ²¹⁰Pb on seasonal and annual bases was found to be stronger than at Dye 3, and the average activity of ⁷Be was lower at Summit despite the higher elevation (3.0 versus 2.5 km). These observations indicate that the boundary layer at Summit is more effectively isolated than at Dye 3. The activity of ⁷Be at Summit peaked in June or July all 5 years, closely following the seasonality of stratospheric injection of ⁷Be into the Arctic troposphere (based on seasonality of the ¹⁰Be/⁷Be ratio previously measured at Alert, NWT). This suggests that when the boundary layer at Summit is replenished by ventilation, it receives air reflecting the composition of the mid and upper troposphere.     

Kinetics of the oxidation of hydrogen sulfite by hydrogen peroxide in aqueous solution:: ionic strength effects and temperature dependence

Conductometry was used to study the kinetics of the oxidation of hydrogen sulfite, HSO⁻₃, by hydrogen peroxide in aqueous non-buffered solution at the low concentration level of 10⁻⁵–10⁻⁶ M, typically found in cloud water. The kinetic data confirm that the rate law reported for the pH range 3–6 at higher concentration levels, rate=k_H·[H⁺]·[HSO⁻₃]·[H₂O₂], is valid at the low concentration level and at low ionic strength I_c. At 298 K and I_c=1.5×10⁻⁴ M, third-order rate constant k_H was found to be k_H=(9.1±0.5)×10⁷ M⁻² s⁻¹. The temperature dependence of k_H led to an activation energy of E_a=29.7±0.9 kJ mol⁻¹. The effect of the ionic strength (adjusted with NaCl) on rate constant k_H was studied in the range I_c=2×10⁻⁴–5.0 M at pH=4.5–5.2 by conductometry and stopped-flow spectrophotometry. The dependence of k_H on I_c can be described with a semi-empirical relationship, which is useful for the purpose of comparison and extrapolation. The kinetic data obtained are critically compared with those reported earlier.     

Oxidation of chlorophenols in soil at natural pH by catalyzed hydrogen peroxide: the effect of soil organic matter.

This investigation reports on the effects of soil organic matter (SOM) during the oxidation of chlorophenols with Fe2+-catalyzed H2O2 (Fenton oxidation) system. The soil pH was 7.1 and was not altered. Sorption experiments of soil pre-treated under various oxidation conditions were performed. Concentrations of organic matter in the liquid phase and soil before and after oxidation were analyzed. The results were correlated to the observation in batch Fenton oxidation tests. They showed that the oxidation of chlorophenols at natural soil pH depended on the dose of H2O2 and Fe2+. The soil organic content did not vary significantly after various Fenton treatments, while the sorption of chlorophenols was 10-25% less by the oxidation. The concentration of chlorophenols in the liquid phase exhibited a "decrease and rebound" phenomenon in the batch Fenton oxidation tests. It appeared that the oxidation of SOM resulted in the release of sorbed chlorophenols which were then oxidized by the excess H2O2. An "oxidation-desorption-oxidation" scheme was proposed to describe one of the interaction mechanisms among the oxidant, SOM, and chlorophenols during oxidation.     

Hydroxyl (OH) radical production rates in snowpacks from photolysis of hydrogen peroxide (H2O2) and nitrate (NO3−)

Atmospheric chemistry directly above snowpacks is strongly influenced by ultraviolet (UV) radiation initiated emissions of chemicals from the snowpack. The emission of gases from the snowpack to the atmosphere is in part due to chemical reactions between hydroxyl radical, OH (produced from photolysis of hydrogen peroxide (H₂O₂) or nitrate (NO₃⁻)) and impurities in the snowpack. The work presented here is a radiative-transfer modelling study to calculate the depth-integrated production rates of hydroxyl radical from the photolysis of hydrogen peroxide and nitrate anion in snow for four different snowpacks and for solar zenith angles 30°–90°. This work also demonstrates the importance of hydrogen peroxide photolysis to produce hydroxyl radical relative to nitrate photolysis with (a) different snowpacks, (b) different ozone column depths, and (c) snowpack depths. The importance of hydrogen peroxide photolysis over nitrate photolysis for hydroxyl radical production increases with increasing depth in snowpack, column ozone depth, and solar zenith angle. With a solar zenith angle of 60° the production of hydroxyl radical from hydrogen peroxide photolysis accounts for 91–99% of all hydroxyl radical production from hydrogen peroxide and nitrate photolysis.     

Regional surveys of the chemistry of the snowpack in Norway,late winter 1973, 1974, 1975 and 1976

In late winter 1973, 1974, 1975 and 1976 samples of the snowpack were collected from 23 to 165 sites in Norway (and adjacent areas of Sweden and Finland in 1976) and the concentrations of major ions and several trace metals determined. In southeastern and northernmost Norway the snowpack was characterized by high concentrations of H⁺, SO₄, NO₃, and NH₄, components that come principally from the long-range transport of air pollutants. Along the west and northwest coast the snowpack was dominated by the seawater salts, Cl, Na, and Mg.The patterns of concentration of these two groups of components in the snowpack were similar to those in fresh precipitation collected at 16 stations in Norway. The concentrations in the snowpack, however, were generally less than those measured in winter precipitation, probably due to mid-winter melting that preferentially depletes the snowpack in chemical impurities.Snowpack and precipitation chemistry in Norway apparently results from the admixture of continental long-range transported air pollutants with marine aerosols. Deposition is enhanced by orographie effects.     

Homogeneous degradation of 1,2,9,10-tetrachlorodecane in aqueous solutions using hydrogen peroxide, iron and UV light

The homogeneous degradation of the polychlorinated n-alkane, 1,2,9,10-tetrachlorodecane (T4C10), was studied in aqueous solutions of hydrogen peroxide, including Fenton and photo-Fenton reaction conditions. All solutions were adjusted to a pH of 2.8 and an ionic strength of 0.1 M NaClO4 prior to photolysis. T4C10 (2 x 10(-6) M) was substantially degraded by the H2O2/UV system (1.0 x 10(-2) M H2O2), with 60% disappearance in 20 min of irradiation in a photoreactor equipped with 300 nm lamps of light intensity 3.6 x 10(-5) Ein L(-1) min(-1) (established by ferrioxalate actinometry). The reaction produced stoichiometric amounts of chloride ion indicating complete dechlorination of the chlorinated n-alkane. T4C10 degraded very slowly under Fenton (Fe2+/H2O2/dark) and Fenton-like (Fe3+/H2O2/dark) conditions. However, when the same solutions were irradiated, T4C10 degraded more rapidly than in the H2O2/UV system, with 61% disappearance in 10 min of exposure. The rapid degradation is related to the enhanced degradation of hydrogen peroxide to oxidizing *OH radicals under photo-Fenton conditions. Degradation was inhibited in both the H2O2/UV and photo-Fenton systems by the addition of KI and tert-butyl alcohol due to *OH scavenging.     

Measurements of the concentration in rainwater and of the Henry's law constant of hydrogen peroxide

A chemiluminescent method for measuring hydrogen peroxide was developed using hemin as a catalyst for luminol-based H₂O₂ oxidation, which gave a detection limit below 0.1 μg ∝⁻¹ in the solution. It was shown that most atmospheric species did not give serious interference, and that negative interference of SO₂; could be eliminated if pH of the collecting solution was above 10 whereas O₃, gave significant positive interference. The Henry's law constant of H₂O₂ was experimentally determined to be 1.42 × 10⁵ M atm⁻¹ at 20°C at ambient concentration levels of H₂O₂. This method was also applied to the measurements of H₂O₂ concentration in rainwater in Tokyo, Japan, which was in the range of 5–1065μg ∝⁻¹.     

Partitioning of hydrogen peroxide between the gas and liquid phases in the presence of surfactant

Gas-liquid phase partitioning is a key physical property that can predict the environmental fate of a compound between two phases. Several environmental factors have been known to affect the gas-liquid phase partitioning. We investigated the influence of surfactant on the gas-liquid phase partitioning of hydrogen peroxide (H(2)O(2)). The surfactant used was ammonium perfluorooctanoate (APFO). H(2)O(2) solution containing the surfactant was equilibrated in a closed system and gas phase H(2)O(2) concentration was measured by the peroxyoxalate chemiluminescence (PO-CL) method. Gas phase H(2)O(2) concentrations remained constant below the critical micelle concentration (CMC) and increased linearly with surfactant concentration above the CMC, which indicated that surfactant micelles influenced the gas-liquid phase partitioning of H(2)O(2). This result showed that H(2)O(2)-micelle interactions are less favorable than H(2)O(2)-H(2)O interactions. Surfactant monomers did not affect the gas-liquid phase partitioning of H(2)O(2) due to the absence of micelles. Solvent (methanol) effect was also investigated and showed that gas phase H(2)O(2) concentrations increased with the addition of solvent. This indicated the unfavorable interaction of H(2)O(2) with hydrophobic medium compared to hydrophilic one. It is consistent with the result that H(2)O(2)-micelles has a weaker interaction than H(2)O(2)-water because surfactant micelles are hydrocarbon-like organic phase rather than aqueous phase.