A11B-0035 0800h
Fluxes of Nitrous Acid from Snow Surfaces in Antarctica
Mixing ratios of nitrous acid (HONO), 3-D micrometeorology, physical and chemical snow characteristics and UV insolation in and above the snow surface are being measured at Browning Pass ($163\deg$52' East, $74\deg$38' South), near the Italian Antarctic station "Mario Zucchelli" (formerly "Terra Nova Bay") between mid October and mid December 2004. The field site is ca. 10 km inland, but effectively removed from coastal/marine influences by the Northern Foothills (with peaks up to 800 m altitude), and the prevailing katabatic winds. Previous measurements in coastal Antarctica during springtime of 2003 showed minute HONO fluxes. This was ascribed to the sea salt deposition onto the snow surface and resulting alkaline snow, which reduced the mobility of nitrate anions in the snow, and such the availability of HONO precursors in the photochemically active surface layer. Here we will present preliminary results from the ongoing field campaign, and discuss the relationship between snow properties and HONO emissions.
A11B-0036 0800h
Episodes of High Surface Ozone Amounts During the Summer at South Pole, Antarctica
During the months of November, December and January episodes of enhanced ozone amounts greater than 35 ppb are seen in the surface ozone record at South Pole, Antarctica. Using the long-term surface and vertical profile measurements of ozone the character and frequency of these high ozone events are compared with the more quiescent periods. In order to better characterize the vertical structure of ozone in the lower layers, a procedure has been developed for correcting for the time response of the ozonesondes. This correction is based on a campaign in December 2003 in which slowly ascending ozonesondes on a tethered balloon were compared with the more rapidly ascending balloon release ozonesondes. At times of high ozone mixing ratio at the surface there is a strong gradient in the lowest 0.5 km layer with the largest ozone amounts near the surface. On the other hand when ozone near the surface is less than about 20-25 ppb, ozone is well mixed without a strong gradient. For intermediate conditions (ozone in the range 25-35 ppb) there is usually a gradient similar to that seen for the high ozone cases but with somewhat less vertical depth. Conditions of high or moderate surface ozone amounts dominate overall from mid November to mid January indicating that a surface source is active through much of this time.
A11B-0037 0800h
Composition of Aerosol Particles From the South Pole During ANTCI
As part of the Antarctic Tropospheric Chemistry Investigation (ANTCI), daily, high-volume, aerosol samples were collected from 22 November to 23 December 2003 at the South Pole Clean Air Facility. The samples were shipped frozen to the Carlsbad Environmental Monitoring & Research Center where they were analyzed for several major ions (nitrate, sulfate, methanesulfonate), up to thirty-three trace elements, and three radionuclides ($^{210}$Pb, $^{210}$Po, and $^{7}$Be). These samples were collected at the same site and with the same bulk collectors used during two field experiments conducted in 1998 and 2000 for a related program, the Investigation of Sulfur Chemistry in the Antarctic Troposphere (ISCAT). The $^{7}$Be activities were consistent with the long-term record produced by the Environmental Measurements Laboratory (EML); in particular, a slight decrease observed in the ANTCI $^{7}$Be samples compared with the ISCAT samples is in keeping with decade-scale trends in sunspot activity. The $^{210}$Pb activities were towards the high end of long-term EML record, but they were similar to the earlier ISCAT data. The arithmetic mean sulfate (124 ng m$^{-3}$), and methanesulfonate (MSA, 9.1 ng m$^{-3}$) concentrations were comparable to those observed during the ISCAT experiments, and the MSA/sulfate ratios for all experiments were generally similar, but there were several days during ANTCI (29 November to 3 December) on which high MSA concentrations and high MSA/sulfate concentration ratios occurred. Sodium concentrations, an indicator of atmospheric sea-salt, also were high in that set of samples, suggesting impacts from relatively fresh maritime air. Most notably, the arithmetic mean nitrate concentration (250 ng m$^{-3}$) during ANTCI was higher than that for either of the ISCAT data sets (39 ng m$^{-3}$ for ISCAT 1998 and 150 ng m$^{-3}$ for ISCAT 2000). Furthermore, while several peaks in nitrate were synchronous with MSA and sulfate, the last two samples collected (21 to 23 December 2003) had high nitrate but not high MSA or sulfate. In addition, the concentrations of elements typically associated with mineral aerosol, such as Al, Fe, K, etc. were roughly 5-times higher in the ANTCI samples compared with those collected during ISCAT. Several elements, including Pb, Sb, and Zn, that have strong anthropogenic sources also were higher during ANTCI than in the previous studies. These results indicate that there were relatively strong continental influences during ANTCI as well as the aforementioned maritime effect.
A11B-0038 0800h
Measurements of methyl chloride in air trapped in an Antarctic ice core
Methyl chloride (CH$_{3}$Cl), the most abundant atmospheric halocarbon, is emitted into the atmosphere from various natural sources, and is considered to contribute to the stratospheric ozone depletion. In order to deduce past variations of the atmospheric CH$_{3}$Cl mixing ratio, air samples extracted from an ice core were analyzed for CH$_{3}$Cl. The ice core was collected at Dome Fuji (77\deg 19' S, 39\deg 42' E, 3810 m a.s.l.) in the Antarctica. Air was extracted from the ice core samples of about 300 g by milling the samples by a cutter into fine powder under vacuum at -20 \deg C. The extracted air was dried by passing through a glass trap held at -100 \deg C and then collected into a stainless steel tube cooled at about -263 \deg C. The CH$_{3}$Cl mixing ratios in the air samples were determined using a gas chromatography/mass spectrometer system with prior cryogenic pre-concentration. The measured mixing ratios of CH$_{3}$Cl in the Holocene were found to be similar to the present day atmospheric mixing ratios. However, the CH$_{3}$Cl levels were much higher in last glacial period, presumably responding to changes in the natural sources and sinks due to low air temperature. These results were compared to other climatic and environmental signals reconstructed from the Dome Fuji ice core and the cause of the glatial-interglatial variation of the atmospheric CH$_{3}$Cl will be discussed.
A11B-0039 0800h
Observed Low Ozone Events in Coastal Antarctica - The Critical Role of Meteorology
Episodic loss of tropospheric ozone has been observed in both polar regions. The destruction of ozone appears to be associated with halogen chemistry, generally accepted as being driven by bromine released from sea ice surfaces. Since March 2003, measurements of surface ozone have been made at the British Antarctic Survey Clean Air Sector Laboratory (CASLab) at Halley station in coastal Antarctica. Detailed measurements of boundary layer meteorology as well as standard meteorology are also measured at the CASLab. Combining the data allows us to probe the role of meteorology in these "low ozone events". Low ozone events are observed at Halley on numerous occasions during Antarctic spring; on occasions the development of the event and its recovery are strongly associated with the build-up and decline of a stable boundary layer; on occasions, extremely rapid loss of ozone is observed (loss of 20ppbv in 3 minutes on one occasion) which are associated with larger scale transport. We report here on the events recorded during spring 2003, and show the critical influence of meteorology. The association suggests that the role of meteorology must be considered when striving to understand the mechanisms controlling observed low ozone events, and hence extremely good meteorology will need to be included in any modeling calculations trying to reproduce observed events.
A11B-0040 0800h
Observations of HONO at the South Pole during ANTCI
Observations of nitrous acid (HONO) at the South Pole taken during the Antarctic Troposphere Chemistry Investigation (ANTCI) are presented. ANTCI took place in November and December of 2003. The photofragmentation/laser-induced fluorescence (PF-LIF) technique was used to measure atmospheric HONO 10 meters above the surface. HONO is photo fragmented at 355 nm and the generated OH radical is probed at 282nm to excited state; the excited state OH fluorescence at 309 nm is detected by photomultiplier tube. This technique is both highly sensitive and specific for the detection of HONO. The system is calibrated by standard addition every 3 hours and verified by UV/VIS differential optical absorption spectroscopy. Calculated HONO concentrations using a simple steady state photochemical model are compared with the measurements and the difference is clarified.
A11B-0041 0800h
Comparison of Ozone Measurements From a Tethered Balloon Sampling Platform at South Pole Station in December, 2003
Ozone was measured from a tethered balloon platform during the 2003/2004 Antarctic Tropospheric Chemistry Investigation (ANTCI) at South Pole, Antarctica. Electrochemical concentration cell (ECC) ozonesondes were used during the 52 ascent/descent profile measurements from December 13-30, 2003. Five of the measurements used dual ECC ozonesondes. Quality data checks were done before and after each tether run by comparing to a surface ozone ultra-violet absorption (UV) monitor located next to the tether balloon site at 2 meters above ground level. During the 18 day period, ozonesonde spot measurement checks at the surface agreed to within 3 percent of the continuous ozone measurements under ambient concentrations ranging from 19 to 51 ppbv. This agreement was also consistent when compared to the nearby NOAA Climate Monitoring and Diagnostics Lab (CMDL) UV instrument sampling at 17 meters above ground level. However, during a 5-day period of stable conditions with relatively high surface ozone (near 45 ppbv), the 17 meter location measured 2 to 3 ppbv lower ozone than the 2 meter site due to the steep vertical gradient in ambient ozone. The ECC sonde and UV monitor agreement was also consistent during several vertical profiles up to 100 meters when the surface ozone monitor was connected to a long Teflon sampling tube attached to the tether platform.
A11B-0042 0800h
Measurement of Pernitric Acid, Nitric Acid and Sulfur Dioxide at the South Pole during ANTCI 2003
Sulfur dioxide, pernitric and nitric acid were measured with a chemical ionization mass spectrometer (CIMS) from the Atmospheric Research Observatory during the ANTCI 2003 austral summer field campaign. Preliminary mean mixing ratios of HNO4 (28 pptv) are comparable to values observed during ISCAT 2000, while the HNO3 mixing ratios (89 pptv) are elevated by a factor of 4. The mean SO2 mixing ratio (10 pptv ) displayed little variation throughout the mission. While the mean values for temperature, dewpoint and ozone between the two field missions are comparable, the NOx mixing ratios were ~2.5 higher during the 2003 ANTCI campaign. The NOx increase is mirrored in the HNO3 mixing ratio but is not as apparent in the HNO4 mixing ratio. Two probable causes for the increase of the HNO3/HNO4 ratio have been identified: 1) Advection of enhanced HNO3 mixing ratios to the South Pole 2) An increase in local production of HNO3 at the South Pole due to elevated NOx concentrations. The relative importance of each will be examined with a simple 0-D photochemical box model.
A11B-0043 0800h
A coupled physical, optical, and photochemical model of snow: relating measurements of specific surface area to snow optical properties.
Recent experiments and modeling studies have shown that chemical processes in the snow pack have significant impacts on the chemistry of the atmosphere. Solar ultraviolet radiation penetrating the snow pack is the driving force for some of these chemical processes. Therefore, factors controlling photochemical processes in the snowpack need to be understood. Here, we present field investigations of the relationship between physical and optical properties of the snowpack and laboratory studies validating radiation models that predict photochemical reaction rates within the snowpack. A critical parameter in modeling snow photochemistry is the scattering coefficient for the snow. Steve Warren (University of Washington) proposed that the best physical measurement that correlates with scattering is the specific surface area (SSA); however, this correlation has not been tested to our knowledge. Therefore, we performed field experiments comparing optical measurements of scattering and the SSA. The measurement of the snow SSA was achieved by using the Brunauer-Emmett-Teller method to analyse CH$_4$ adsorption onto snow samples. The optical determination of scattering in the snowpack used a variation of the method of Beaglehole. In this method, the light penetration of a snow sample was measured using decreasing thicknesses of the sample on a black base. In the thin layers, the scattering dominates the light attenuation within the snow. In the thicker layers both the absorption and scattering determine the light attenuation. Kubelka-Munk two-flux theory was used to model the data and calculate the scattering and absorption within the samples. This determination of the scattering was found to be proportional to the measured SSA. This linkage between SSA and optical properties confirms Warren`s hypothesis and allows the literatures of optical and physical properties of snow to be coupled. A laboratory study of the performance of snow radiation models was also carried out. A delta-Eddington model and the TUV-Snow model of Lee-Taylor and Madronich were used to calculate the photolysis rate coefficient of an actinometer molecule in an artificial snow analog. The total depth integrated photolysis rates calculated by the models were compared with photolysis rates determined from the conversion of the actinometer. The models were found to be in good agreement with the experiments. The validated models will made available to the wider research community via the www. Through the combination of these field and laboratory experiments, we are now able to quantify snow photochemistry either by optical measurements or by simply knowing snow physical properties.
A11B-0044 0800h
Observations of Halogen Concentrations in Polar Snow near Barrow, Alaska Indicate that Bromide is Highly Affected by Atmospheric Chemistry
The polar atmosphere shows unique and important atmospheric chemistry in the related phenomena of ozone depletion and mercury deposition. During these ozone depletion episodes (ODEs), ozone is depleted from background levels to nearly zero. At the same time, gas-phase mercury is oxidized to reactive gaseous mercury that subsequently deposits potentially providing a source of toxic mercury to the Arctic ecosystem. These ODEs are clearly related to reactive halogen chemistry and particularly gaseous bromine species (Br and BrO). The origin of these reactive halogen species is most likely sea salts that are liberated by poorly understood chemistry, possibly assisted by frost flower formation. These reactive halogen species eventually react to form the stable halogen anions (e.g. Bromide, Br-), that is subsequently deposited to the snowpack. Therefore, we undertook a study of halogens in the snow in the vicinity of Barrow, Alaska. Snow samples were collected in three phases along a 100 km transect from shore to inland in 2004. Phase I (29 Feb - 5 Mar), phase II (31 Mar - 13 Apr), and phase III (7 May - 11 May) snow samples were analyzed for ions to investigate snow-air interactions. Anions (F-, Cl-, Br-, NO3-, SO42-) were analyzed with ion chromatography, and cations (Ca2+, Mg2+ and Na+) with flame atomic absorption spectroscopy. Frost flower samples were approximately 4 times more concentrated in ions than sea water, but most ions (except sulfate and to a smaller extent sodium) were not fractionated with respect to sea water. The bromide to chloride ratio in the frost flowers was identical to that of sea water, indicating that possible fractionation of bromide during frost flower formation does not occur. The sulfate to chloride ratio in frost flowers was about half of that in sea water, indicating fractionation probably due to mirabalite (Na2SO4 hydrate) precipitation. In snow samples, the bromide to chloride ratio showed bromide depletion in salty samples (higher chloride) (generally from offshore and near-shore locations) and enhancement in less salty snow (generally from inland). The bromide and chloride falloff from the coast are presented. The falloff in bromide is very different from that of chloride, so neither chloride nor specific conductance are good tracers for bromide in these snow samples. The observed patterns are consistent with the hypothesis that Br- in offshore and near-shore samples is activated into the gas phase and then it can be transported farther inland.
A11B-0045 0800h
Multiple Oxygen Isotope Photochemistry of Nitrate in Ice
Nitrate (NO$_{3}$ $^{-}$) is a major anion in polar ice [de Angelis and Legrand, 1995;Dibb et al., 1998;Silvente and Legrand, 1995]. As the primary sink for atmospheric NOx, nitrate's chemical history is a link to understanding the nitrogen budget and oxidation capacity of the polar atmosphere over time. Our understanding of variations in ice core nitrate concentrations has been limited by depositional and post-depositional loss processes, associated with temperature, accumulation rate, diffusion, photochemistry and volatilization [Wolff, 1995]. The first report of multiple oxygen isotope composition of nitrate in ice cores (Alexander et al., 2004) demonstrates the ability to observe changes in the oxidation capacity of past atmospheres. To accurately interpret this and future isotopic data, we need to ascertain the oxygen isotope fractionation ($\delta$$^{17}$O, $\delta $^{18}$O) associated with the photolysis of nitrate in ice. Post-depositional processes, such as nitrate photolysis and volatilization may alter the initial isotopic signal of nitrate in the snowpack. A series of nitrate photolysis experiments were conducted on 10 mM solutions of Fisher KNO$_{3}$ and USGS-35 NaNO$_{3}$ at 313 \pm$ 20 nm over 12 to 48 hours and between -30 and 25 $\deg$C. With initial mass-dependent nitrate, a strict mass-dependent fractionation was observed in the residual irradiated nitrate. However, after 12 and 24 hours of irradiation, mass-independent USGS-35 NaNO$_{3}$ ($\Delta$$^{17}$O = 21.0 \pm$ 0.4 \permil) displayed a decrease of 1.6 \pm$ 0.4 and 2.0 \pm$ 0.4 % at 25 $\deg$C, 1.2 \pm$ 0.4 and 1.3 \pm$ 0.4 \permil at $-5\deg$C, 0.2 \pm$ 0.4 \permil and 1.1 \pm$ 0.4 \permil at $-30\deg$C, respectively. The greater isotope effect at higher temperatures may be due to a thicker quasi-liquid layer (QLL) allowing faster rates for secondary nitrate producing reactions between H$_{2}$O and photoproducts NO, NO$_{2}$, NO$_{2}$$^{-}$. In the aqueous phase this effect is even greater. Hence, we infer that the production of nitrate via these 'secondary processes' has positive temperature dependence, causing a noticeable but minor decrease in $\Delta$$^{17}$O. The potential application of utilizing $\Delta$$^{17}$O -NO$_{3}$$^{-}$ as a new technique to interpret the nitrate ice record to increase our understanding of the polar paleoatmosphere is evaluated in the context of the present measurements.
A11B-0046 0800h
Quantum Yields for OH Formation from the Photolysis of Nitrite on Ice: Comparison to Nitrate and Hydrogen Peroxide
The photodecomposition of nitrate, nitrite, and hydrogen peroxide generates hydroxyl radicals (OH) in ice and snow. These photoreactive species can affect snow chemistry as well as ice-core records and the atmospheric boundary layer. For example, it has been hypothesized that photoformed OH can oxidize organic carbon in the snowpack and lead to the release of volatile organic compounds such as formaldehyde. While the quantum yields of OH radicals from the photolysis of nitrate and hydrogen peroxide in ice have been measured,[1-3] the photochemical behavior of nitrite in ice and snow remains an open issue. In this study, we are measuring the quantum yields of the OH radical ($\Phi$(NO$_{2}^{-} \longrightarrow$ OH)) from the photolysis of frozen aqueous NO$_{2}^{-}$ solutions using benzoate as a chemical probe. For this presentation we will discuss: (1) NO$_{2}^{-}$ molar absorptivities; (2) The temperature dependence of ($\Phi$(NO$_{2}^{-} \longrightarrow$ OH)) for ice pellets at temperatures between 243 K and 268 K, and (3) The pH dependence of ($\Phi$(NO$_{2}^{-} \longrightarrow$ OH)) and the relative reactivities of NO$_{2}^{-}$ and HNO$_{2}$ at 263 K. We will also compare the nitrite results with those previously determined for nitrate and hydrogen peroxide, including the relative contributions for OH formation from nitrate, nitrite, and hydrogen peroxide in the snow, and the atmospheric implications of these reactions. References: 1. Dubowski, Y.; Colussi, A. J.; Hoffmann, M. R. J.Phys.Chem.A 2001, 105, 4928. 2. Chu, L.; Anastasio, C. J.Phys.Chem.A 2003, 107, 9594. 3. Chu, L.; Anastasio, C. Quantum Yields of OH from the Photolysis of Hydrogen Peroxide in Ice, in preparation.
A11B-0047 0800h
Comparison of Spring and Summer Hydroxyl Concentrations in the Snowpack at Summit, Greenland
The concentration of hydroxyl radical in near-surface snowpack at Summit, Greenland (72\deg34' N, 38\deg28' W) was estimated during two field campaigns. The first took place in the summer (2003) when hydroxyl radical should be at its peak due to 24 hours of sunlight. The second occurred in the spring (mid-March through April) of 2004, a period when Summit goes from 12 hours of sunlight to near complete sunlight and experiences rapidly changing photochemistry. The experiment consisted of adding a carefully selected mixture of hydrocarbon gases, with a wide variety of radical reactivities, to a UV and visible transparent flow chamber containing undisturbed natural firn. The relative decrease in mixing ratios of these gases allowed estimation of the mixing ratio of hydroxyl radicals in the near- surface snowpack. Hydrocarbon samples were collected in 2L stainless steel canisters and analyzed in Irvine, CA. The residence time of gases in the chamber was characterized by injection of SF$_{6}$ and monitored by an on-site GC. Graphing the decay of 1-butene, i-butene, cis-2-butene, and trans-2-butene versus their respective rate constants with hydroxyl yields a straight line with a slope equal to -[OH]$\times$$\tau$ where [OH] is the gaseous hydroxyl concentration in the firn pore spaces and $\tau$ is the residence time of the gases in the firn. During the summer of 2003, the calculated OH mixing ratios followed a diurnal cycle. The peak hourly average was 5.0$\times$10$^{6}$ molecules/cm$^{3}$ between 1PM and 2PM local time. The minimum hourly average was 1.6$\times$10$^{6}$ molecules/cm$^{3}$ between 8PM and 9PM. Initial results from spring 2004 will be presented, and are expected to show hydroxyl radical concentrations that are significantly lower.
A11B-0048 0800h
Global Modelling of NOx Emissions From Snow
Recent measurements from Antarctica and other regions have demonstrated that NOx is produced photochemically within snowpacks and then released to the overlying atmosphere. Such a release is likely to be an important source of NOx in snow covered regions, particularly those remote from anthropogenic sources. Given the extent of snow fields globally, NOx emissions from snow may also provide a significant additional global source of NOx. We report here on numerical simulations run in a 3D chemical transport model (p-TOMCAT) aimed at assessing the significance of snowpack emissions of NOx on the global troposphere and snow covered regions. The model has a detailed tropospheric chemistry into which NOx emissions based on observed snow cover have been included. We present results from a number of multi-annual integrations designed to assess the impact and sensitivity, both globally and regionally, to snowpack NOx emissions.
A11B-0049 0800h
Diurnal and Seasonal Variation of Alkenes, Halocarbons, and Alkyl Nitrates in the Surface Snowpack at Summit, Greenland
We present measurements of alkenes, halocarbons, and alkyl nitrates made at Summit, Greenland ($72\deg$34' N, $38\deg$28' W) as part of a larger project to elucidate the processes that produce and consume HOx radicals within and above sunlit surface snow (or firn) over a wide range of environmental conditions. During the summer of 2003 we observed firn mixing ratios of these gases that are typically a factor of 2 to 10 higher in concentration within the firn air than in the ambient air 1 m above the snow. We compare these findings to repeat measurements made at Summit from March to May 2004 as the site emerges from the dark winter months.
A11B-0050 0800h
In Situ Measurement of OH and (HO2 +RO2) during the 2003 and 2004 Summit, Greenland Campaigns
During the summer of 2003 and early spring of 2004 a Chemical Ionization Mass Spectrometer (CIMS) measured gas phase OH and (HO2 +RO2) in the boundary layer at Summit, Greenland. To measure HO2 +RO2, NO is added 3 minutes out of every 20. The addition of NO allows the HO2 +RO2 to be converted to the OH radical which then reacts with SO2 to form H2SO4. The H2SO4 is measured with the CIMS and the OH and HO2 +RO2 concentrations are inferred from the resultant [H2SO4]. The calculated sensitivity of HO2 +RO2 to the added NO of the sample is compared to observations. In addition to the response of the CIMS to the NO addition, the system field calibrations for both OH and HO2 are shown. The calibrations are performed in situ by photolyzing ambient H2O to form OH and HO2. Various aspects of the system performance such as detection limits and time response will be presented.
A11B-0051 0800h
Selective Mass Spectrometer Characterization of Halogen Gases in Air at Atmospheric Pressure
We have developed a new interface for use with a commercial ion-trap mass spectrometer equipped with atmospheric pressure chemical ionization (APCI-MS). The new interface uses a mechanical pump to draw gaseous analyte through a glass manifold and into the corona discharge area of the APCI-MS. This new method of delivering a gaseous sample at atmospheric pressure directly to the MS has been used to obtain real-time measurements of Br$_{2}$ and Cl$_{2}$ over synthetic seawater ice. The ion intensity of a halogen gas measured by the MS is affected by the pumping rate and the position of the glass manifold. The MS signals for Br$_{2}$ are linear in the 0.1 to 10.6 ppbv range, and the estimated 3 sigma detection limit is 20.7 pptv. The MS signals for Cl$_{2}$ are linear in the 0.2 to 25 ppbv range, and the estimated 3 sigma detection limit is 1.081 ppbv. This lab-based technique is suitable to be the basis for a portable field-based design. Such a design, a miniaturized instrument, will help elucidate the role of seawater snow and ice surfaces on the photochemical production of Br$_{2}$ and Cl$_{2}$ in the high Arctic.
A11B-0052 0800h
Influence of Snowpack Metamorphism on Tropospheric Chemistry
In recent years, many studies have highlighted the impact of photochemical reactions in snow on tropospheric chemistry. Snow metamorphism is another factor that affects snow-air interactions. Metamorphism consists of sublimation/condensation cycles driven by temperature gradients. It results in dramatic modifications of the physical properties of the snowpack that lead to exchanges of trace gases with the atmosphere. Even in the absence of light, the chemical compositions of both the snowpack and the troposphere are affected, influencing the chemical signal in ice cores. To better understand exchanges caused by physical processes, we undertook a winter-long study of a snowpack undergoing high-temperature gradient metamorphism, in Fairbanks, central Alaska. To monitor their relative influence on one another, we coupled our physical investigation of the snowpack (temperature, density, crystal morphology, specific surface area, permeability) to chemical measurements (ions, aldehydes, water isotopes). The effects of metamorphic intensity were studied by monitoring the evolution of a similar snowpack subjected to very low temperature gradients. This was achieved by letting the snowpack form on tables, under which air circulation allowed isothermal conditions. A much enhanced permeability and a faster decrease in specific surface area are observed under natural conditions compared to the table experiments. These characteristics considerably impact the concentration of chemical compounds within the snowpack and consequences on their remobilization depend on whether they are present as dissolved gases or particles. Gases such as formaldehyde are strongly released to the atmosphere under high grade metamorphism, while particulate species are less affected by the intensity of metamorphism. These data show that metamorphic intensity significantly influences snow composition, for a constant atmospheric composition. Climate change will decrease temperature gradients in the snow cover and thus modify chemical signals in ice cores. This paper attempts to elucidate the relationship between these two variables.
A11B-0053 0800h
Determination of Volatile Organic Compounds in Snow Using Solid Phase Micro Extraction (SPME)
Volatile organic compounds (VOC) in snow-samples from different environments were determined. Samples were collected in Resolute, Nunavut in the high Arctic ( latitude: 74.70$\deg$/longitude: - 94.91$\deg$), the Gasp, Peninsula in Quebec (at Mont Albert near Ste-Anne-des Monts: 49.12$\deg$/- 66.49$\deg$) and downtown Montreal, Quebec ( 45.54$\deg$/- 73.60$\deg$) in order to reflect different degrees of anthropogenic influence. In order to assess the ability of compounds contained in the sample to perform photochemistry, samples were irradiated with UV-light in the 300-400 nm range. Filtering through a 0.2 $\mu$m-filter provided information about compounds primarily associated with the liquid phase. A solid phase micro extraction (SPME) procedure was developed for sample preparation and VOC were identified using gas chromatography with mass spectrometric detection (GC-MS). We will present our results at several urban and remote sites, and the implication of the result to atmospheric chemistry will be discussed.
A11B-0054 0800h
Ozone Atmosphere-Snowpack Gas Exchange Processes in Polar and Midlatitude Snowpacks
Photochemical processes have been shown to drive the surface-atmosphere gas exchange of photochemically important tropospheric trace gases. This research investigates ozone-snowpack gas exchange processes and the chemical, physical and meteorological parameters that determine ozone fluxes. Research has been conducted over year-round snowpack at Summit, Greenland, and at South Pole, Antarctica. Furthermore, two seasonally snow-covered sites in the Rocky Mountains, Colorado have been investigated. Experiments include flux measurements by the tower gradient and by eddy correlation methods and measurements of ozone in interstitial air at several depths in the snowpack. These data illustrate that ozone deposition to snow depends on multiple parameters, the most important ones including snow chemical composition, snow depth and incoming solar radiation. These dependencies warrant consideration in atmospheric chemistry and transport models for accurate descriptions of the tropospheric ozone budget in the Polar Regions.
A11B-0055 0800h
Seasonal Trends in Snow Photochemistry Impacts on Soluble Acidic Gases at Summit, Greenland
Previous work at Summit has established that mixing ratios of HONO, HCOOH and CH$_3$COOH are greatly enhanced in firn air, and that snow to air fluxes support enhanced mixing ratios in the bottom of the atmosphere above the ice sheet. Experimental manipulations have shown that the processes producing the firn air enhancements of these acidic gases respond quickly to changes in light and temperature, but it has been difficult to separate photolytic production from thermal desorption. In March and April, 2004, an intensive sampling campaign was conducted during the spring season when solar elevation and temperatures were both naturally increasing dramatically. Comparison between the 2004 campaign and results from summer 2003 indicate that production of HONO is already active by the end of March and steadily increased through May, but had not reached peak summer intensity by early May. In contrast, the mixing ratios of the carboxylic acids were greatly suppressed through the third week of April, but then increased rapidly to levels characteristic of full summer. We are exploring relationships between temperature and actinic fluxes, in both the air and snow, and the mixing ratios of these gases. This natural experiment yielded non-synchronous changes in light and temperature that may yield greater insight into the relative importance of photochemical production and thermal desorption as sources of these acids in sunlit snow.
A11B-0056 0800h
Light Penetration in the Snowpack at Summit, Greenland: Measurements of Chemical Actinometry J-Values and Spectrally Resolved Actinic Flux
Rates of photochemical reactions within the snowpack, both on snow grains and in the firn air, depend on how actinic flux is attenuated as a function of depth. This snowpack photon flux can either be measured directly (e.g., with spectral radiometers in the snow) or indirectly (e.g., by chemical actinometry where the rate of a photochemical reaction is measured). The advantage of the former method is that it gives wavelength-resolved photon fluxes that can be used to calculate the rate constant for any chromophore whose quantum yield and molar absorptivity are known. In contrast, the advantage of chemical actinometry is that it is a more direct measure of the photolysis rate constants that is unaffected by uncertainties in quantum yields or molar absorptivities. Here we describe results from both chemical actinometry experiments and spectral radiometric measurements in the snowpack at Summit during spring and summer. For the chemical actinometers we used three chromophores that photolyze to form hydroxyl radical: hydrogen peroxide, nitrate, and nitrite. For the spectral radiometer measurements, we used a scanning double monochromator spectroradiometer with a bialkali photocathode that measured the photon flux from 280 to 560 nm every 30 seconds. Both techniques generally showed good exponential decays in j-values with depth. Chemical actinometry measurements of e-folding depths (the depth at which the actinic flux is 1/e of its surface value) show that nitrate consistently had the smallest values, with an average of 9 cm for a mid-day value during spring, and 12 cm for summer mid-day. Hydrogen peroxide and nitrite were much more sensitive actinometers than nitrate, with surface j values (spring mid-day) that were approximately 40 and 150 times greater, respectively. While e-folding depths for hydrogen peroxide were similar to those of nitrate, those for nitrite were significantly larger, a result of the fact that nitrite absorbs at longer wavelengths where the snow is less absorbing. Preliminary analyses indicate that j-values derived from the spectral radiometer measurements are often similar to the chemical actinometry values, but that there were sometimes differences. These comparisons, and the potential reasons for the observed differences, will be discussed.
A11B-0057 0800h
A New Technique for Sampling Firn Air
The discovery and subsequent interest in photochemical interactions between the polar snowpack and the atmosphere has spawned interest in reliable methods of measuring chemical concentrations in interstitial air. Consistent sampling of the interstitial air in the snowpack had been problematic due to great chemical differences possible from sampling different layers in the snow and the difficulties in acquiring a sample that could serve multiple investigators at the same time. This paper describes a new air sampling device that was developed to solve many of the sampling problems. This new system allows multiple simultaneous chemical analysis of air contained in the pore spaces of the arctic snowpack at unlimited increments from depths of 0 to 150 cm. The three major components are a 4 ft diameter highly UV transmittent acrylic "hood" with a 10 cm rim, a 10 cm diameter casing barrel, and an air probe head. These components operate along with a variety of sub-components that supplement the sampling process. The technique provides for a common sample collection, use for a variety of gases to be sampled, it eliminates short circuit air sampling, provides undisturbed snow for in-situ sampling at multiple sample depths in the same location. The design is discussed and possible extension as a platform for other sensors is described.
A11B-0058 0800h
Factors that influence UV actinic flux in and above Arctic snow
Photochemical rate constants (i.e., photolysis frequencies or j-values) are principle inputs to photochemical models used to illustrate atmospheric chemistry, both in the overlaying atmosphere and within the snowpack. We have measurements of the in situ actinic flux above and within the snowpack during most of the 2003 and 2004 Summit, Greenland measurement periods from which we calculated 39 different photolysis frequencies. Since the firn (or in-snow) actinic flux observations were made at a series of discrete depths while the firn air chemical measurements reflect variable subsurface volumes, it is necessary to test the various radiative transfer models that attempt to describe the actinic flux both in the atmosphere and snowpack environments. To do this we incorporate the available measurements and estimates of factors that impact the above-snow actinic flux, such as snow surface albedo, percent cloud cover, overhead ozone column and aerosol/blowing snow optical depth into an atmospheric radiative transfer model. We also investigate the impact of physical snow characteristics (e.g., density, grain size, etc.) on the penetration of UV radiation. We use our measurements of the UV actinic flux to test these radiative transfer models for a variety atmospheric and snow conditions to help identify which of these variables are the most important inputs to the models.