A51C-0576
A Multiphase Study of the Chemical Composition of Air, Aerosol Particles, Snow, and Ice Forms Collected Near Barrow, Alaska Provides Information on Bromine Activation
Unique chemistry that releases halogens from sea salts into the atmosphere depletes ozone and deposits mercury in the springtime in the Arctic. It is believed that Br- is present in the snow pack and other ice reservoirs over the ice-covered ocean during winter, and is liberated by poorly understood chemistry in the springtime. We undertook a study of halogens in different ice reservoirs like snow, aerosol particles, and in the gas phase in the vicinity of Barrow, Alaska in the springtime of 2007. For the first time, we observed a time series of most of the relevant bromine reservoirs simultaneously. The size and time-resolved aerosol data help us to complete a picture of what ice reservoirs are involved in releasing Br- into the gas phase. The smallest particles sampled with a 3-stage DRUM impactor contain a larger fraction of sulfate than the larger particles, and probably are the result of Arctic Haze sulfate pollution overlaid on a background of mostly larger sea-salt particles. The smallest particles are mostly enhanced in Br- (as compared to sea salt reference) while larger-sized particles could show near sea-salt Br- enrichment factors or slight Br- depletions. Enhancement in Br- can come from termination of halogen activation chemistry, which produces HBr. These HBr molecules would then be scavenged efficiently by the smaller particles due to their high surface area, which is consistent with our data. Depletion of Br- indicates halogen activation to the gas phase. During halogen activation events, surface snow shows higher depletions than the aerosol particles. Because the surface snow received its salts from the sea-salt aerosol that is mostly enhanced in Br- or is comparable to sea salt, and we observe depletions in snow, this indicates that snowpack is releasing halogens to the atmosphere. Knowing what ice surfaces contribute more to active Br- species can help us to predict ozone depletion and mercury deposition events. This mechanistic understanding is needed to make meaningful predictions of how Arctic pollution and Arctic atmospheric chemistry will respond to the drastic changes in sea ice.
A51C-0577
Chloracetone in California: A Tracer of Cl-atom Chemistry in Urban Air
Chloroacetone is a relatively long lived photoproduct from the Cl-atom initiated oxidation of propene. We report measurements of this compound from Malibu California as part of a collaborative multi-investigator project to study the sources and impact of halogen atom chemistry on ozone in coastal California. As part of this study we performed online measurements of trace organics gases using GC-MS to measure chloroacetone. This species was present in all samples with highest mixing ratios of ~ 15 pptv in air masses that were impacted by the LA urban plume. Additional canister sampling was also performed in Los Angeles. Canisters samples showed the presence of chloroacetone, often at much higher mixing ratios than the coastal samples. This paper will describe the analysis methodology, our results, and potential use of chloracetone as a tracer of Cl-atom oxidation history in polluted air masses.
A51C-0578
Destruction of Iodocarbons in Surface Seawater - Implications for Sea-Air Flux Calculations and the Atmospheric Iodine Budget
Although it is now widely accepted that CH3I is the major volatile organic source of iodine from the surface ocean to the atmosphere, CH3I emissions alone cannot balance the global iodine budget, which implies that there must be some additional source(s). Seawater and air measurements made during two cruises in the Atlantic Ocean during summer 2006 and spring 2007 suggest that other iodocarbons, in particular the dihaloalkanes CH2I2, CH2IBr and CH2ICl, may provide a combined global iodine atom source which is comparable to that of CH3I. However, deriving sea-to-air fluxes of these volatile gases is not straightforward. Established flux parameterizations are based on air and surface water concentrations, but seawater samples from ship campaigns are typically taken from at least 2-6 m depth (since there are technical difficulties associated with sampling closer to the surface). Given its relatively long lifetime in the oceans, sea-air fluxes of CH3I may be adequately approximated directly from concentrations measured a few metres below the surface, however for the dihalomethanes this is likely to give rise to considerable inaccuracies due to their short lifetimes with respect to photolysis in surface waters. Consequently, we have used a one-dimensional oceanic mixed layer model to constrain the extent of dihalomethane photodecay within the top few metres of the water column such that we can extrapolate surface seawater concentrations from sub-surface measurements, which in turn allows more accurate sea-air fluxes to be determined for these gases. A number of mono-iodinated alkanes were also detected in Atlantic seawater, including C2H5I and 1- C3H7I. Laboratory based studies have been carried out in order to parameterize the temperature- dependent chemical destruction of a number of mono-iodinated alkanes in saltwater, such that the oceanic lifetimes of these species may be predicted as a simple function of the surface seawater temperature. In light of these results we also consider the potential impact of rising global seawater temperatures on emissions of these iodocarbons to the marine boundary layer.
A51C-0579
Development and Deployment of a Field Instrument for the Detection of Iodine Atoms Using Resonance Fluorescence
Recently there have been several measurements of I2, alkyl iodides (CH3I, CH2I2), iodine monoxide (IO) and OIO at sites including coastal, polar and open ocean environments. I2 and the alkyl iodides are rapidly photolysed in the atmosphere to liberate iodine atoms which may then react with ozone to form iodine monoxide (IO) radicals. Subsequent reactions of IO lead to the catalytic depletion of tropospheric ozone, affect OH:HO2 and NO:NO2 ratios, oxidise dimethyl sulphide and result in the formation of new particles. While the IO radical and its stable precursors have previously been detected and measured in the marine boundary layer, little is known about the levels of reactive inorganic halogen species, or the local distribution of iodine activity at coastal sites such as Mace Head (Ireland). We have developed an instrument which employs vacuum UV resonance-fluorescence using a microwave discharge lamp to detect atomic iodine via the (5p5) \ 2P3/2 ← \ 2P3/2 \ (6s1) transition near 178 nm. The instrument is calibrated by generating a known concentration of iodine atoms from the photolysis of I2 at 185 nm. The system can be operated in two modes; either to measure ambient iodine atoms (in photochemical steady state with their precursors and IO), or to measure the total photolabile iodine loading, through broadband visible photolysis of ambient air, with detection of the iodine atoms formed. This technique allows for the in-situ measurement of the species detected, which is advantageous for gathering information about their local distribution and source. The instrument was deployed for the first time in August 2007 at Mace Head on the west coast of Ireland. We present initial results from this field trial, which represent the first observation of ambient iodine atoms in the marine atmosphere. The results will be compared with the measurements of IO obtained from the University of Leeds laser-induced fluorescence (LIF) instrument.
A51C-0580
Development of a new method for determination of atmospheric halogen atom and halogen oxide concentrations
A flowtube reactor has been developed for the quantitative determination of atmospheric halogen atom concentrations. The technique operates by drawing atmospheric halogen atoms (Cl, Br, I) into a 20-mm diameter quartz flowtube, in which they quickly react with an introduced alkene (trifluoropropene) and NO to produce a halogenated ketone that is detected by gas chromatography with electron capture detection (GC-ECD). This method allows for the "direct" determination of atmospheric halogen atom concentrations, which have so far only been determined by indirect methods, e.g. as inferred from hydrocarbon decay rates. The fluorinated alkene reagent is utilized to increase the final product's ECD sensitivity as well as provide a compound not found in the environment (i.e. very low blanks), leading to greater overall instrument sensitivity. Halogen monoxide radicals (XO), typically only measured spectroscopically, can also be quantitatively determined by conversion to X via reaction with NO at the inlet. The resulting halogen atom and halogen oxide concentration measurements allow for the greater understanding of halogen photochemistry in the lower troposphere. Here we show results from laboratory tests and system calibrations in preparation for upcoming field work aboard the icebreaker CCGS Amundsen in the Canadian Arctic as part of IPY 2007-2008.
A51C-0581
DOAS Measurements of Reactive Halogens at Greenland Summit in May and June, 2007
Reactive halogens in the arctic atmosphere have been a subject of measurement and analysis since the mid- 1980s, when sudden arctic springtime ozone depletion events at arctic coastal stations were correlated with increases in reactive bromine compounds. Previous evidence of reactive halogens at Greenland Summit, where the coast is over 400 km away and the elevation is 3200 m, provided some of the motivation for a large field campaign at Greenland Summit during the late spring of 2007. This campaign was designed to quantify reactive halogens and other species and factors influencing the arctic atmosphere at higher elevations and far away from the coast. UCLA participated in this campaign with two DOAS instruments. The UCLA long path instrument, LP- DOAS, was operated on light paths of 2 or 5 km (one way) 2-3 m above the snow. A dedicated multi-axis (MAX)- DOAS, designed for the Summit environment and installed in 2006, was also used and collected scattered sunlight at elevation angles of 1, 3 and 10 degrees, as well as from the zenith. LP-DOAS measurements were made in 4 regions from 290 to 580 nm and the MAX-DOAS was set to record spectra from 318 to 385 nm, so that the BrO absorption in the 300 – 350 nm region was accessible to both. This poster presents results from the DOAS instruments at Summit, including observation of BrO in the lower boundary layer during the 2007 campaign. A number of periods of elevated BrO were observed during the month-long observation period of May 15 – June 15, with mixing ratios as high as 3 ppt.
A51C-0582
Halogen Activation by N2O5 Reaction on Sea Ice Surfaces
The reaction of N2O5 on sea spray aerosol particles is known to release nitryl halides. Recent air mass trajectory analyses suggest ozone depletion events are correlated with air mass passage over young sea ice. To our knowledge, the reaction of N2O5 on sea ice surfaces has not yet been studied. We have examined the rate and product yields from the uptake of N2O5 on natural and artificial sea ice surfaces using a coated wall flow tube technique coupled to a chemical ionization mass spectrometer (CIMS). A wide range of halogenated species, including ClNO2, Cl, BrNO2, and Br2 and Br are promptly produced resulting in a near unity yield of gaseous halogen release. Cl containing species dominate the halogenated products, but only by a factor of 4 over Br containing products, suggesting preferential activation of Br. The product yield and reaction rates are interpreted in the context of expected sea ice and atmospheric composition for a range of polar conditions. Our findings imply that for certain conditions this chemistry may contribute substantially to polar halogen chemistry, for example, by affecting the gaseous Br/Cl radical ratio.
A51C-0583
Measurements of Cl2, Br2, I2, and BrCl in Polluted Coastal Air
Halogen atoms have the potential to produce and destroy ozone in polluted air, but there are few field studies of reactive halogens and their precursors in coastal urban environments. Here we present API/MS/MS measurements of dihalogen mixing ratios in Malibu, CA during October, 2006, as part of a field campaign to determine the impact of halogens on urban ozone photochemistry and air quality. During this study, Cl2 mixing ratios ranged from <1.2-11 ppt with a geometric mean of 1.1±1.5. Cl2 was observed both day and night with no dependence on wind direction. Br2 levels ranged from <0.3-11 ppt with a geometric mean of 0.3±0.6. Br2 was frequently observed at night and the highest levels were detected in off-shore winds, suggesting a possible continental or anthropogenic source. BrCl was observed above detection limit for the first time in marine air, at levels ranging from 0.5-2.2 ppt, with a geometric mean of 0.2±0.8. I2 ranged from <0.3-1.4 ppt with a mean of 0.2±0.4. The highest I2 mixing ratios were observed in westerly winds that recently passed over local kelp beds. I2 exhibited a consistent diurnal cycle with typical daytime levels 0.2-0.4 ppt higher than nighttime levels. A photochemical box model was used to infer the impact of dihalogen chemistry on ozone under the moderately polluted conditions encountered during this study. The results suggest that chlorine oxidation of hydrocarbons may be responsible for up to 5% of the observed O3 levels. At the observed Br2 levels, destruction of ozone by bromine radicals is minor, lowering O3 levels by only 1-2%. Iodine radicals generated from photolysis of I2 contribute to O3 destruction, partially off-setting the O3 produced from chlorine radicals. The efficiency of ozone destruction by Br and I atoms in polluted air is reduced in polluted air compared to clean air, due to halogen-NOx interactions that short-circuit catalytic O3 destruction cycles.
A51C-0584
Measurements of Molecular Halogens on the California Coast
Although the hydroxyl radical is generally considered to be the dominant daytime oxidizing species in the troposphere, there is increasing evidence that halogen atoms are also significant oxidants in coastal areas. The importance of reactive halogen species (RHS), such as chlorine atoms and bromine and iodine oxides, is not well characterized, partly due to the few observations of RHS in the atmosphere. A collaborative field measurement campaign was conducted in October 2006 in Malibu, CA to measure a suite of trace gases, including halogen molecules, halogen oxides, Cl + VOC reaction products, aerosol composition, ozone, nitrogen oxides, carbon monoxide, and others. An atmospheric pressure ionization mass spectrometer was used to detect molecular bromine and iodine in the marine boundary layer during that field experiment. The results of those measurements are presented here.
A51C-0585
Modelling bromine and ozone over the Dead Sea
Measurements of Ozone and BrO concentrations over the Dead Sea indicate that Ozone Depletion Events widely known to happen in polar regions are also likely to occur over the Dead Sea due to the very high bromine content of Dead Sea water. However, BrO and ozone levels as they are detected can not solely be explained by high Br- levels in the Dead Sea water and the release of gas phase halogen species out of sea borne aerosol particles and their conversion to reactive halogen species. It is likely that other sources for reactive halogen compounds are needed to explain the observed concentrations for BrO and ozone. To explain the chemical mechanism taking place over the Dead Sea leading to BrO levels of several ppt we used the single column model MISTRA which calculates microphysics, meteorology, gas and aerosol phase chemistry. We performed pseudo Lagrangian studies by letting the model column first move over the desert which surrounds the Dead Sea region and then let it move over the Dead Sea itself. To include an additional source for gas phase halogen compounds gas exchange between the Dead Sea water and the atmosphere is treated explicitly. Model calculations indicate that this process has to be included to explain the measurements.
A51C-0586
Modeling the halogen chemistry in the Antarctic boundary layer
Since the mid-1980s, events of drastic losses of ozone have been reported in the Arctic boundary layer in spring. Subsequently, similar ozone depletions have been observed in the Antarctic boundary layer. It is now recognized that reactive halogens play a major role in these ozone depletion events. Interestingly, significant levels of iodine oxides (IO) have been reported from stations near the Antarctic coast, while no such levels have yet been measured in the Arctic. The importance of these levels of IO in the Antarctic remains unclear as regard to the chemical mechanisms associated with ozone depletions and the bromine chemistry. We investigated the potential sources of iodine for the Antarctic boundary layer using the model MISTRA in the one-dimensional mode. The relative contributions of sea salt aerosols, frost flowers, and organoiodine species for the release of reactive iodine to the gas phase were assessed. Typical, as well as increased sea salt aerosol number concentrations, do not contain enough iodine ions to account for a significant source of iodine. Modeling the presence of frost flowers containing increased concentrations of iodide compared to seawater also showed no relevant release of reactive iodine species. Model runs investigating typical levels of organoiodine (2 ppt CH3I, 1 ppt C3H7I) showed mean mixing ratios of IO and OIO nearly 103 times higher than modeled sea salt or frost flower aerosols, but reaching levels of only ~10-1 ppt. The rapid photodissociation of CH2I2 was found important for the release of reactive iodine. A prescribed flux of CH2I2 from the surface (set to maintain observed levels of ~0.5 ppt in the Antarctic) induced IO and OIO mixing ratios approximating 1 ppt. In order to release observed mean levels of reactive iodine to the boundary layer (5-10 ppt IO), a prescribed flux of molecular iodine from the surface had to be set to nearly 1.0×109 molec~cm-2~s-1. Modeled vertical distribution of IO within the boundary layer is not consistent with observations of well-mixed IO. Interactions between the bromine and iodine chemistry are investigated and possible model improvements are presented.
A51C-0587
Cl2, Br2, and I2 Measurements in the Tropical Marine Boundary Layer in Cape Verde, Africa
The dihalogens Cl2, Br2, and I2 were measured in marine boundary layer air at a windward tropical coastal site in Cape Verde, off West Africa, as part of the RHaMBLe halogen campaign (May-June 2007). These halogen species are thought to be released via heterogeneous processes on aerosols and photolyze quickly during the daytime to generate highly reactive halogen atoms. These radicals can catalytically destroy ozone and oxidize organic molecules. The measurements were made with an atmospheric pressure ionization triple- quadrupole mass spectrometer by negative atom detection following collisional dissociation of the parent molecular ion (e.g. Cl2- to Cl-). Each species exhibited a consistent and unique diel cycle. Cl2 had nighttime maxima of ~3-20 ppt, based on preliminary calibrations, and was below the detection limit during the day. I2 showed a semi-diurnal cycle, peaking most strongly at night (up to ~1 ppt) and also around solar noon (up to ~0.5 ppt). Br2 also peaked near solar noon but was still detectable at other times of day for almost the entire campaign. The midday Br2 peaks, based on our current calibrations, were usually a few ppt but reached levels upwards of 20 ppt. Such levels in the daytime are incompatible with observed ozone concentrations, which did not show dramatic depletion. These observations suggest a particularly strong local source or some positive analytical artifact.
A51C-0588
Prototype of a Laser-Induced Fluorescence Ground-Based Instrument for Measurements of Atmospheric Iodine Monoxide (IO)
High abundances of iodine monoxide (IO) are known to exist and to participate in local photochemistry of the marine boundary layer: (1) IO participates in depletion episodes of O3 and in the removal of mercury in the Arctic polar spring by enhancing atomic Br mixing ratios. Recent observations and computer simulations suggest that mercury sequestration is closely tied to halogen photochemistry and that gaseous atomic Hg depletion can be enhanced significantly by the presence of small amounts of iodine-containing compounds. (2) IO and higher- order iodine oxides are involved in the formation of new particles in coastal marine environments. Studies using smog chamber experiments simulating coastal atmospheric conditions have demonstrated that new particles can form from condensable iodine-containing vapors and that their concentrations over the open ocean are sufficient to influence marine particle formation. (3) IO has also been shown to affect the oxidizing capacity of the troposphere by altering the partitioning of NO2/NO and HO2/HO and by activating chlorine and bromine in sea salt aerosols. In the stratosphere, these same processes can lead to enhanced ozone loss rates. Detailed photochemical models that include iodine photochemistry, however, are hampered by the lack of observational data. The distribution of IO in vertical, horizontal, and temporal coordinates is unknown, so the impact of IO on global photochemistry cannot be predicted. The resolution of these important scientific issues requires an in situ IO instrument. A fully functional nanosecond Nd:YAG-pumped Ti:Sapphire laser system and a prototype IO ground-based instrument have been built in our lab. With the current setup, the laser system was situated 10 m from the field station, and the laser light was coupled via an optical fiber. With the use of highly efficient fluorescence detection optics and photon counting techniques, sensitivities of better than 0.1 ppt in 1 s for IO was achieved in the laboratory and 1-2 ppt in the field. The design of the instrument and data acquisition system will be described. The prototype was deployed to the Northeastern University Marine Science Center in Nahant, MA in August 2007. Laminaria macroalgae is known to be responsible for coastal I2 emissions, and Nahant has extensive populations of kelp within the shallow subtidal zone. Results from the field mission will be reported, and future developments of the instrument will be discussed. It is expected that a modified version of this new system will, in time, be capable of being integrated with the existing instrumentation used for the detection of halogen (ClO, BrO), nitrogen (NO2), and hydrogen (OH and HO2) free radicals.
A51C-0589
Interfacial Chemistry of Aqueous Sulfur/Iodide Aerosol Microdroplets in Gaseous Ozone
The intermediates ISO3- (m/z = 207) and IS2O3- (m/z = 239) generated in aqueous (iodide - thiosulfate) microdroplets traversing dilute ozone gas plumes at atmospheric pressure are detected via online electrospray mass spectrometry within 1 ms, and their stabilities gauged by collision-induced dissociation. The simultaneous detection of anionic reactants and the S2O62-, HSO4-, IO3- and I3- products as a function of experimental conditions provides evidence of unique interfacial reaction kinetics. Although ozone reacts ~3-4 times faster with I- than S2O332- in bulk solution, only S2O32- is apparently oxidized in [I--]o/[S2O32- ]o = 10 microdroplets below [O3(g)] ~ 50 ppm. The sulfite to sulfate and iodide to triiodide and iodate oxidations in the interfacial layers of aqueous thiosulfate or mixed thiosulfate and iodide microdroplets briefly exposed to dilute O3(g) gas mixtures are also investigated. S(IV) oxidation kinetics in sodium thiosulfate solutions, where the rates are proportional to [S(IV)] [O3(g)] in the ranges investigated, correspond to a surface-specific reaction. I3-/IO3- yields based on interfacial I- losses exceed their stoichiometric limits in the presence of excess S(IV), revealing that interfacial I- is competitively replenished from the microdroplets inner layers. Present results provide unequivocal evidence of distinct interfacial chemistry in gas-aerosol reactions of atmospheric relevance.
A51C-0590
Iodide Catalyzes Bromide Oxidation by Ozone
Ozone depletion events (ODE) occurring in the lower troposphere during early spring involve unexpectedly high levels of gaseous bromine arising from the oxidation of sea salt bromide int he dark polar winter. It has been reported that sea salt is more reactive than pure bromide toward ozone. This fact implies that minor, still unidentified components of natural sea salt catalyze bromide oxidation. We investigate bromine production from aqueous solution microdroplets exposed to gaseous ozone by means of negative ion electrospray mass spectrometry (ESMS). Aqueous bromide and iodide solutions are nebulized in the presence of gaseous ozone, and gaseous products sampled and analyzed by online ESMS after being scavenged in situ by aqueous iodide microdroplets. Bromine formation is confirmed by the appearance and evolution of bromide, iododibromide ion, tribromide ion, and diiodobromide ion signals. We find that bromine gas production is significantly enhanced in mixed iodide and bromide aerosols over bromide solutions. We propose a mechanism for iodide catalysis of bromide by ozone and discuss its implications of ODE's in the Arctic.
A51C-0591
Investigation of the Influence of Microphysical Assumptions on Aerosol Chemistry Systems
We have developed an integrated chemical and microphysical box model, designed for examining the influence of aerosols on halogen activation in the marine boundary layer. The chemical scheme is based on that of Pechtl et al, pp 505-523, ACP (2006) and constructed using KPP (Sandu and Sander, pp 187-195, ACP, 2006). Non- ideal effects on component vapour pressures for the core HNO3/H2SO4/NH3/NaCl/HCl system are calculated using a parametrisation of the Pitzer, Simonson and Clegg mole fraction activity coefficient model. The aerosol size distribution is represented using the sectional Moving Centre method. Currently only condensational and evaporational particle growth is considered, coagulation is not included. We will present the results of a study of the differences between coupled and uncoupled chemical and microphysical aerosol representation and of the dependence of aerosol chemistry on the particle-size resolution of the model, as well as the influence of the mixing state assumptions for sulphate and sea-salt modes has. Such dependence critically affects the predictions of the impacts of reactive halogen cycling.
A51C-0592
Diel Variability of Total and Speciated Water-Soluble Inorganic Iodine in PM2.5 Aerosol at a Southern California Coastal Site
PM2.5 aerosol was sampled over nominal 3-hour intervals at the head of Zuma Beach in Malibu, California (USA) from 6 to 24 October 2006 by filtration at 1.13 m3 min-1 (STP) through 20 x 25 cm cellulose fiber (Whatman 41) filters that had been rinsed with deionized water (DIW). Exposed filters were removed from support cartridges as soon as possible after retrieval (usually within 2 hours), immediately sealed in clean polyethylene bags, and stored frozen until further processing. Following the field campaign one quarter of each filter was pressed into a pellet (2.0 cm diameter x 0.5 cm thick) and analyzed by neutron activation for total concentrations of I and several other trace elements. Our preliminary analyses indicate that sodium and iodine show a clear diel variation characterized by higher concentrations from late morning to early evening. We hypothesize that this diel variability is related to a persistent land/sea breeze circulation associated with the nearby coastal region. Other elements are indicative of variability in other aerosol sources such as soil dust (Al, Mn) and fossil fuel combustion (V). Second quarters are currently being extracted in DIW and analyzed in two ways: 1) for iodide by ion chromatography, and 2) for inorganic iodine in higher oxidation states (i.e., V to 0) by chemical reduction with ascorbic acid followed by determination of iodide by ion chromatography. Results of the trace element and speciated iodine analyses will be presented.
A51C-0593
Development of Regional and Global Climatologies of Surface Ocean Bromoform Concentrations
Bromoform is produced in the oceans by phytoplankton and seaweeds (macroalgae). This production is thought to result in supersaturation of most of the surface oceans and therefore a net flux of bromoform to the marine boundary layer (MBL). In the MBL bromoform may photolyse resulting in bromine radicals, which catalytically destroy ozone. Bromoform may also persist long enough to perturb oxidative photochemistry in the free troposphere and stratosphere. An accurate assessment of the magnitude of the oceanic source of bromoform is therefore of great interest. Recent studies [Yang et al., 2005; Warwick et al., 2006] have developed approaches that model bromoform concentrations in the atmosphere. However these models are limited by poorly constrained estimations of the oceanic source; specifically very low skill in resolving spatial and temporal distribution. Here we present a climatological approach to parameterizing the distribution of bromoform in the surface ocean. Approximately 15,000 air and surface seawater concentration measurements from 52 previously published studies were collated and tagged with time, date and location information. For each one of these data points remote sensed data for a variety of relevant variables, including sea surface temperature, chlorophyll-a and wind speed were collated and assigned. These remotely sensed variables were used to parameterize the sources and sinks of bromoform in the surface ocean. This facilitated the development of a model formula for sea surface concentrations of bromoform. The results of regional parameterization experiments show that this method adequately captures the variability of sea surface bromoform concentrations; on a regional scale the R2 correlation co-efficient for a plot of the observed against modeled bromoform concentration ranged from 0.48 (Southern Ocean) - 0.52 (Tropics), with P values of less than 0.001. Sea-air fluxes were calculated from the climatology output, satellite wind speed and average bromoform air concentration, using the parameterization of Wanninkhof [1992]. These results show strongest fluxes in tropical regions and a global sea-air flux comparable to recent estimates. The temporal and spatial resolution that this approach provides may facilitate improved modeling of bromoform in the atmosphere. Furthermore this approach may allow tentative predictions of bromoform concentrations in a changing climate, or in regions where no data is available.
A51C-0594
Tallgrass Prairie as a Source and Sink of Methyl Halides
Temperate grasslands are believed to be a globally significant sink for methyl bromide (CH3Br) and perhaps methyl chloride (CH3Cl), compounds which lead to stratospheric ozone destruction. Fluxes of these compounds were measured at Konza Prairie, a tallgrass prairie in the Flint Hills of Kansas, during June 2006 and August 2007. A stable isotope tracer technique was used to distinguish between simultaneous production and oxidation processes, allowing the first gross flux measurements of CH3Cl and CH3Br from a tallgrass prairie. Observed gross uptake rates of CH3Cl and CH3Br were similar to what we previously observed from the shortgrass steppe in Colorado and annual grasslands in California, but much lower than reported fluxes from a grassland in northeastern North America. A water manipulation experiment was performed both under controlled laboratory conditions, as well as in the field, demonstrating that uptake rates of both CH3Cl and CH3Br were strongly affected by soil moisture. On the production side, new sources of methyl halides were identified in association with certain plant species. Fluxes of these halogenated trace gases were compared to environmental variables, such as air temperature and volumetric water content. Net fluxes of methyl iodide (CH3I), carbon tetrachloride (CCl4), and other halogenated volatile organic compounds (HVOCs), were also measured.
A51C-0595
A Study of Reactive Halogen Release From Frozen Sodium Halide Solutions: The Effect of Photons and Hydroxyl Radicals
It has been established that reactive halogen species are instrumental in ozone depletion events observed at high latitudes. Recent field campaigns also indicate that in other environments, such as the marine boundary layer, halogens influence the oxidizing potential, either by directly oxidizing reduced species or by cycling HO2 to OH. Gas phase chemistry has been unable to account for the amount of reactive halogens observed during field campaigns. Heterogeneous autocatalytic mechanisms, such as the bromine explosion, have been invoked to help balance the ledgers. Still the initial release of reactive halogens remains an open question. In our study we explore the role of photons and gas phase hydroxyl radicals as potential oxidants of halide species frozen in solution. Depending on the reactant required, either a xenon arc lamp (photons) or microwave plasma (OH) was employed to generate the oxidizing species. In the case of photons, the pathway for oxidant formation was via dissolved nitrate photolysis. A chemical ionization mass spectrometer was subsequently used to detect the gas phase halogen species liberated from the substrate. We see the effects of temperature, halide concentration, pH, and nitrate concentration within the frozen solution upon the halogen yield.
A51C-0596
Biogenic Production of Reactive Bromocarbons: New Field Data and sea-air Fluxes in the Atlantic Ocean
Biogenic bromine production by phytoplankton and macroalgae is thought to represent an important link between ocean biology, climate and atmospheric composition. Models of atmospheric bromine chemistry suggest that natural sources of bromocarbons such as CHBr3 and CH2Br2 may account for up to 30% of stratospheric and tropospheric O3 depletion. However, at present these models are limited by the accuracy to which the bromine source terms can be described. In particular, simultaneous measurements of ocean surface water and marine boundary layer bromocarbon concentrations are lacking, limiting the ability to estimate sea to air fluxes to a reasonable degree of accuracy. Furthermore, little is known regarding the factors that control biogenic bromine production, or the temporal and spatial variability of the bromine source term at the regional scale. We present new data from two research cruises during which we measured a range of bromocarbons, including CHBr3, CH2Br2 and CH2IBr, in both surface seawater and the marine boundary layer using two GC-MS systems. The first cruise was to the North Eastern Atlantic (latitudinal range 53-59°N) in summer 2006, while the second cruise was to the Tropical and Subtropical Atlantic and the Mauritanian Upwelling (latitudinal range 16-30°N) in spring 2007. Concentration data and resulting sea air fluxes generally decrease in the order coastal > shelf > upwelling ~ open ocean. Although a broad trend of elevated seawater concentrations in waters with high chlorophyll a (phytoplankton productivity proxy) is observed, the relationship is not simple. We explore this complex relationship between phytoplankton and bromocarbon production in more detail, examining changes in phytoplankton assemblage and health as indicated by cell counts and pigment distributions. We then use these relationships to present a revised regional estimate for the North Atlantic sea to air flux of biogenic bromine.