• Chemical Composition Of Ice Surfaces: Implications For Springtime Bromine Chemistry

      Alvarez-Aviles, Laura; Simpson, William R. (2008)
      Reactive bromine chemistry is responsible for events of almost total tropospheric O3 destruction and the deposition of mercury during the Arctic spring. The source of the majority of the atmospheric bromine loading is salts from seawater, but many questions remain on the mechanism by which salts are transported and chemically activated to reactive species. Specifically, the role of snow and ice surfaces in exchanging bromine with the atmosphere needed investigation. Therefore, we undertook a detailed study of the ionic composition of selected ice surfaces near Barrow, Alaska and tracked modifications with respect to Cl- and Na+ (sea-salt tracers) in approximately 1,400 samples. We developed data analysis tools to observe modifications and related these methods to the traditional enrichment factor and the non-sea-salt abundance methods. Surface snow was highly modified in Br- composition by atmospheric exchanges that both add and remove bromine, providing evidence for snow's involvement in reactive bromine chemistry. Calcium was enriched by dust input. Sulfate in surface snow was fractionated at the source by mirabilite (Na2SO 4 • 6H2O) precipitation and enriched by Arctic haze inputs. Frost flowers are vapor-grown ice crystals that wick brine and may be involved in sea-salt aerosol production and production of reactive halogen species. Detailed examination of frost flower growth and chemical composition shows that they are sites of mirabilite precipitation and separation, which can lead to sulfate-depleted aerosol particles, but show no sign of direct reactive bromine production. By simultaneously studying snow, ice, aerosol particles, and gas-phase bromine species, we made a mass balance of bromine in various reservoirs. This mass balance points away from frost flowers and towards snow as the dominant source of reactive bromine. This work develops a mechanistic picture of how reactive bromine chemistry depends upon snow and sea ice that is needed to make meaningful predictions of how the recent changes to the Arctic sea ice cover will affect air pollution chemistry.
    • Controls On Antimony And Arsenic Speciation Via Sorption And Redox Chemistry At The Clay Mineral - Water Interface In Natural And Laboratory Settings

      Ilgen, Anastasia Gennadyevna; Trainor, Thomas P. (2010)
      Adsorption and redox transformations of contaminants in soil and aqueous environments are often controlled by the available mineral substrates. Aluminosilicates and aluminum oxides are ubiquitous and can influence the speciation and, therefore, the transport and bio-availability of toxic elements such as arsenic (As) and antimony (Sb). It is important to understand the partitioning and redox reactions promoted by these substrates in order to understand and model the transport of As and Sb in soils and surface waters. This study provides a detailed investigation of the sorption and redox behavior of As and Sb in clay-rich natural and laboratory systems. Since Fe 3+ is often found substituting for Al3+ in clay mineral structures, we also investigated the role of structural Fe in redox transformations of As and Sb adsorbed at the clay surface in controlled laboratory experiments. In a natural system affected by the release of spent geothermal fluids (Mutnovsky geothermal fields, Kamchatka, Russia), As concentrations are elevated above background levels in the Falshivaia River water and sediments (< 65 microm size fraction). Arsenic from the geothermal source fluids is in the reduced As3+ form, and is oxidized to As5+ after mixing with river water. Both As3+ and As5+ are found in aqueous and adsorbed forms. Analysis of the extended x-ray absorption fine structure (EXAFS) spectra shows that sediment-phase arsenic is associated with both Al- and Fe-rich phases with a bi-dentate corner-sharing local geometry. A series of laboratory experiments were performed in order to investigate Sb adsorption by Al-rich mineral substrates at a macroscopic and molecular level. The EXAFS analysis of the experimental samples concluded that both Sb3+ and Sb5+ form inner-sphere sorption complexes on the surfaces of hydrous aluminum oxide (HAO), and the clay minerals kaolinite (KGa-1b) and nontronite (NAu-1). Primarily, bi-dentate corner-sharing with a minor amount of mono-dentate corner-sharing complexes were formed. The oxidation state of the clay structural Fe affects the adsorption capacity of nontronite; if the clay is partially reduced, the Sb5+ uptake is increased significantly. The long term dynamics of the As aqueous speciation in clay suspensions where reduced arsenic (As3+) was added initially is complex. A fast disappearance of As3+ due to oxidation to As 5+ was followed by a slow increase of aqueous As3+. This behavior is explained by two simultaneous reactions: fast oxidation of As3+ by structural Fe3+ (anaerobic) or Fe 3+ and dissolved O2 (aerobic) and the slow reduction of As5+ by dissolved Fe2+. The ability of the structural Fe in nontronite clay to promote oxidation of As3+/Sb 3+ was greatly affected by its oxidation state: if all structural Fe was in an oxidized Fe3+ form, no oxidation was observed; however, when ~ 20 % of structural Fe was reduced to Fe2+, the clay promoted the most extensive oxidation under both aerobic and anaerobic conditions. The structural Fe2+ is not able to reduce As 5+/Sb5+, but reduction was seen when aqueous Fe 2+ was present in the systems. These research findings indicate that As and Sb can be effectively immobilized by Al-rich phases, while the substrate nature and oxidation state of structural Fe, along with the presence of dissolved Fe2+, can greatly affect the fate and transport of As and Sb. The increase in Sb5+ uptake in response to reducing structural Fe, possible increase or decrease in uptake of As due to As5+ reduction by aqueous Fe 2+, or oxidation of As3+ by clay structural Fe 3+, is likely to take place in a natural clay-rich soil or aquifer environment in moderate to slightly reducing conditions.
    • New instrumentation for the detection of sulfur dioxide in the remote atmosphere

      Nicks, Dennis Keith, Jr.; Benner, Richard (1999)
      Sulfur gases are an important chemical component of the atmosphere. Gaseous sulfur compounds effect the acidity of rainwater and are important precursors to aerosol particles which affect public health, climate and visibility of scenic vistas such as the Grand Canyon. Sulfate aerosols are also known to participate in ozone catalysis in the stratosphere. A vast majority of the gaseous sulfur cycling through the atmosphere will exist as sulfur dioxide (SO2) at some time during its atmospheric lifetime. Since SO 2 is a primary component of the atmospheric sulfur cycle, quality measurements of this gas are important to understanding the cycling of sulfur through the atmosphere. The mixing ratio of SO2 in the atmosphere can be as low as a few 10's of parts-per-trillion by volume (pptv) in unpolluted areas and as high as 100's of parts-per-billion by volume (ppbv) near industrial centers. Obtaining SO2 measurements with mixing ratios that can differ by 105 in magnitude is a difficult task, especially for mixing ratios less than a few hundred pptv. The Diffusion Denuder/Sulfur Chemiluminescence Detector (DD/SCD) was developed further and tested in a rigorously blind comparison under controlled laboratory conditions. The DD/SCD exhibited excellent sensitivity and little-to-no interference from other trace gases. The DD/SCD performance was comparable to that of other state-of-the-art instruments developed for measuring SO 2 in the remote atmosphere. The Continuous SO2 Detector was developed to overcome the limitation of long sampling times (4 to 90 minutes) inherent in the DD/SCD and other state-of-the-art techniques. The Continuous SO2 Detector (CSD) was developed based on the design of the DD/SCD, but has been optimized for sensitive, high-time resolved measurements of SO2 in air. Sensitive, high-time resolved measurements would be beneficial for studying atmospheric SO2 over large geographical areas from a moving sampling platform such as an aircraft. The current prototype of the CSD is capable of measuring SO2 at mixing ratios of less than 100 pptv on the order of seconds. The DD/SCD, CSD and an automated, computer controlled dynamic dilution system described in this thesis represent a suite of instruments for the measurement of SO2 in the remote atmosphere.
    • Nitrogen oxide photochemistry in high northern latitudes during spring

      Beine, Harald Jurgen; Jaffe, Daniel A.; Benner, Richard; Shaw, Glenn; Stolzberg, Richard; Wendler, Gerd (1996)
      The transport of NOy reservoir species from midlatitudes into the Arctic and the thermal and photochemical breakup of these species has been proposed to be the most important NOx source during spring, and may have an important influence on the ozone budget. This has not yet been shown to be correct. The objective of this research is to understand the sources of NOx and ozone in high latitudes during spring. To measure NOx, a high sensitivity chemiluminescence NO detector and a photolytic converter for NO$\sb2$ were constructed. The detection limits for NO and NO$\sb2$ were 1.70 and 5.67 part per trillion (pptv) in a one-hour average, respectively. Springtime NOx measurements were carried out concurrently with measurements of ozone, PAN, J(NO$\sb2$), and other species during 1994 at the Zeppelin station on Svalbard, and during 1993 and 1995 at Poker Flat, Alaska. The median mixing ratios of NOx, PAN and ozone at Svalbard were 23.7, 237.0 pptv, and 39.0 parts per billion (ppbv), respectively. During a few ozone depletion events in the Arctic marine boundary layer ozone and NOx mixing ratios were as low as 4 ppbv and 0.9 pptv, respectively. Halogen chemistry is probably responsible for both effects. The median NOx, PAN and ozone mixing ratios at Poker Flat were 79.5 pptv, 85.9 pptv, and 40.6 ppbv, respectively. During April and May diurnal cycles of PAN, ozone and temperature were observed and anticorrelated with the water mixing ratio. We interpret this to be the result of mixing with higher layers of the troposphere during the day. At both locations thermal PAN decomposition was an important NOx source. At Svalbard PAN decomposition was small, and the in-situ ozone production rates are an insignificant contribution to the ozone budget. Because of the higher temperatures, PAN decomposition rates, NOx mixing ratios, and in-situ ozone production rates are higher at Poker Flat. A contribution from this production to the overall ozone budget was visible during some periods. These results indicate that stable ozone precursors which are transported into the Arctic from anthropogenic sources can influence the ozone budget in high latitudes.
    • Nitrogen oxides in the Arctic troposphere

      Honrath, Richard Edward, Jr.; Jaffe, Daniel A.; Gosnick, Joan P.; Stolzberg, Richard J.; Stamnes, Knut; Shaw, Glenn E. (1991)
      Nitrogen oxides play a critical role in tropospheric photochemistry. In order to characterize these compounds in the arctic troposphere, ground-level concentrations of total reactive nitrogen (NO$\sb{y}$) and NO were determined over an extended period at a site near Barrow, Alaska. A high-sensitivity instrument developed for this purpose was used in three measurement campaigns: summer 1988, spring 1989, and March-December 1990. During the 1990 campaign, the detection limit for NO was 3-10 pptv (depending on averaging period), and the NO$\sb{y}$ uncertainty was $\pm$26%. A screening algorithm was applied to the data to eliminate effects from local (Barrow) sources, and the remaining data were divided into "background periods" (unaffected by local or regional NO$\sb{x}$ sources), and "events" (periods when emissions from a regional NO$\sb{x}$ source--the Prudhoe Bay oil-producing region--apparently impacted Barrow). These measurements revealed a sharp seasonal cycle of background NO$\sb{y}$ concentrations, with high values in early spring (median 560-620 pptv) and $\sim$70 pptv (median) during summer. This cycle is similar to that of other compounds in arctic haze but is partially attributed to a reduction in NO$\sb{y}$ lifetime due to organic nitrate decomposition as temperatures and insolation increased. Evidence indicates that the springtime arctic NO$\sb{y}$ reservoir was primarily composed of stable removal-resistant species, including PAN and other organic nitrates. PAN decomposition as temperatures rose in late spring likely caused an observed pulse of NO to $\sim$35 pptv (maximum hourly average); hourly-average NO concentrations were otherwise generally $<$8 pptv. NO$\sb{x}$ production from PAN decomposition due to the onset of spring or southward advection may affect springtime O$\sb3$ levels both in the Arctic and in the northern mid-latitudes. NO$\sb{y}$ and O$\sb3$ concentrations were positively correlated during summer, possibly indicating long-range transport of both and/or the presence of a mid-tropospheric NO$\sb{y}$ reservoir combined with a stratospheric O$\sb3$ source. A number of events with substantially elevated NO$\sb{y}$ concentrations (to 16 ppbv) were observed in air not impacted by emissions from the town of Barrow. Substantial evidence indicates that these events were a result of NO$\sb{x}$ emissions from the Prudhoe Bay region ($\sim$300 km to the ESE), which is also expected to affect measurements of other compounds at the Barrow site.
    • Nocturnal Processing Of Nitrogen Oxide Pollution At High Latitudes: Off-Axis Cavity Ring-Down Spectroscopy Method Development And Field Measurement Results

      Apodaca, Randy Lee; Simpson, William (2008)
      Nitrogen oxides, or NOx, play a central role in ozone and nitric acid (HNO3) pollution in the troposphere. Reactions of nitrate radical (NO3) and dinitrogen pentoxide (N2O 5) result in the removal of NOx and ozone from the nighttime atmosphere. In this thesis, we describe the configuration, operation, and performance of an off-axis cavity ring-down spectroscopy (oa-CRDS) field instrument designed for measuring NO3 and N2O5. Furthermore, we report results of an N2O5 instrument intercomparison conducted using an atmospheric simulation chamber in Julich, Germany. The results of the intercomparison demonstrate that the oa-CRDS instrument is an excellent tool for measuring NO3 and N2O 5. Also reported in this thesis are the results of two field campaigns aimed at characterizing NOx removal from the nocturnal pollution plume arising from Fairbanks, AK. The results from the field campaigns suggest ice is responsible for catalyzing N2O5 heterogeneous hydrolysis in cold, high-latitude plumes. When air masses are sub-saturated with respect to ice, the data show longer lifetimes (&sim;20 minutes) and elevated N2O5 levels while ice-saturated air masses show shorter lifetimes (&sim;6 minutes) and suppressed N2O 5 levels. Lastly, we present vertical profiles of N2O 5 measured above the seasonal snow pack. The results of the profiling studies suggest that N2O5 can be removed by heterogeneous hydrolysis on ice in the snow pack. Our findings indicate that catalysis on ice surfaces is largely responsible for nocturnal processing of N2O 5 leading to nitric acid production and loss of NOx in high latitude plumes.
    • Surface Structure Of Hydrated And Iron(Ii) Reacted Hematite(11(-)02) And (0001)

      Tanwar, Kunaljeet S.; Trainor, Thomas (2008)
      Reactions on naturally abundant hematite (alpha-Fe2O 3) surfaces significantly influence the transport and bio-availability of a number of important nutrients and contaminants. The surface reactivity of alpha-Fe2O3 is dependent on the surface structure, i.e. the identity and coordination of chemical moieties exposed at the surface. The surface structure is strongly influenced by the presence of water and common aqueous species such as Fe(II). Therefore, it is important to understand how the surface structure evolves in the presence of water and aqueous species (e.g. Fe(II)) in order to model the surface reactivity of hematite in natural aquatic systems. The current study provides a detailed experimental investigation of the surface structure of two predominant natural faces of alpha-Fe2O 3, the (1102) and (0001) surfaces under hydrated conditions in absence and presence of aqueous Fe(II). The surface structure of hydrated alpha-Fe2O3(1102) prepared via a room-temperature wet chemical and mechanical polishing (CMP) procedure is consistent with a surface termination where the top layer of iron atoms is absent compared to the stoichiometric bulk termination. The annealing of CMP prepared alpha-Fe2O3(1 102) in air at 773 K results in transformation of the surface to a structure consistent with the stoichiometric termination. For CMP prepared alpha-Fe2O3(0001), the experimental results show a co-existence of two distinct structural domains on the surface. The first domain corresponds to hydroxylation of surface Fe atoms, and the second domain is formed by complete removal of the surface Fe cation leading to an exposed oxygen layer on the surface. The exposure of CMP prepared alpha-Fe2O3(1 102) and (0001) to aqueous Fe(II) results in structural modification of both surfaces due to adsorption of Fe(II) at crystallographic lattice sites followed by oxidation to Fe(III). Preliminary research conducted to identify the effect of Fe(II) induced surface modification on reactivity using Pb(II) as a reactive probe indicates that the clean and Fe(II)-modified surfaces exhibit significantly different reactivity towards Pb(II). Overall, the systematic structural characterization of hydrated and Fe(II)-modified alpha-Fe 2O3 surfaces presented in the current study will provide a basis to elucidate surface structure-reactivity relationships for hematite and will aid in developing models of mineral-water interfacial reactivity.
    • The Role Of Ice Surfaces In Affecting Nighttime Removal Of Nitrogen Oxides In High Latitude Plumes

      Huff, Deanna M.; Simpson, William (2010)
      Nitrogen oxides play an important role in the atmosphere by affecting ozone-mediated oxidation pathways. Nitrogen oxide removal from the atmosphere occurs via nitric acid formation. This nitric acid deposits to Earth's surface, leading to acidification and nitrogen fertilization. Under dark and cold conditions that commonly exist in the winter at high latitudes, nighttime reactions oxidize NO2 to the nitrate radical, NO3, and these molecules react to form N2O5. The heterogeneous hydrolysis of N2O5, which is catalyzed by surfaces, forms nitric acid. Modeling studies indicate that a majority of the Nx removal at high latitudes results from nighttime N2O5 chemistry. The N2O5 intermediate molecules may react on snowpack surfaces or on atmospheric particles. Past field studies demonstrated that aerosol surfaces are not solely responsible for the removal of N2O 5 near Earth's surface at high latitudes. In this work, we have used aerodynamic gradient micrometeorological methods to measure the deposition velocity of N2O5 to snowpack. This measurement is the first time that snowpack deposition has been quantified directly. We have found that snowpack deposition near Earth's surface at high latitudes is a significant chemical loss process for N2O5. Further studies demonstrated higher mixing ratios and longer lifetimes of N2 O5 aloft. Increasing N2O5 abundance and longevity with altitude implicates different loss mechanisms contribute at various altitudes in the atmosphere. Near Earth's surface, N2O 5 is very reactive, while aloft it acts more as a reservoir species that can transport further. Understanding the controlling mechanisms for N x removal under high latitude conditions will lead to better characterization of the NOx transport in pollution plumes and nitric acid deposition patterns.