Modeling chemistry in and above snow at Summit, Greenland – Part 2: Impact of snowpack chemistry on the oxidation capacity of the boundary layer (original) (raw)
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EGU General Assembly Conference Abstracts, 2012
Sunlit snow is increasingly recognized as a chemical reactor that plays an active role in uptake, transformation, and release of atmospheric trace gases. Snow is known to influence boundary layer air on a local scale, and given the large global surface coverage of snow may also be significant on regional and global scales. We present a new detailed one-dimensional snow chemistry module that has been coupled to the 1-D atmospheric boundary layer model MISTRA. The new 1-D snow module, which is dynamically coupled to the overlaying atmospheric model, includes heat transport in the snowpack, molecular diffusion, and wind pumping of gases in the interstitial air. The model includes gas phase chemical reactions both in the interstitial air and the atmosphere. Heterogeneous and multiphase chemistry on atmospheric aerosol is considered explicitly. The chemical interaction of interstitial air with snow grains is simulated assuming chemistry in a liquid-like layer (LLL) on the grain surface. The coupled model, referred to as MISTRA-SNOW, was used to investigate snow as the source of nitrogen oxides (NO x) and gas phase reactive bromine in the atmospheric boundary layer in the remote snow covered Arctic (over the Greenland ice sheet) as well as to investigate the link between halogen cycling and ozone depletion that has been observed in interstitial air. The model is validated using data taken 10 June
Atmos. Chem. Phys., 2014
To provide a theoretical framework towards a better understanding of ozone depletion events (ODEs) and atmospheric mercury depletion events (AMDEs) in the polar boundary layer, we have developed a one-dimensional model that simulates multiphase chemistry and transport of trace constituents from porous snowpack and through the atmospheric boundary layer (ABL) as a unified system. This paper constitutes Part 1 of the study, describing a general configuration of the model and the results of simulations related to reactive bromine release from the snowpack and ODEs during the Arctic spring. A common set of aqueous-phase reactions describes chemistry both within the liquid-like layer (LLL) on the grain surface of the snowpack and within deliquesced "haze" aerosols mainly composed of sulfate in the atmosphere. Gas-phase reactions are also represented by the same mechanism in the atmosphere and in the snowpack interstitial air (SIA). Consequently, the model attains the capacity of simulating interactions between chemistry and mass transfer that become particularly intricate near the interface between the atmosphere and the snowpack. In the SIA, reactive uptake on LLL-coated snow grains and vertical mass transfer act simultaneously on gaseous HOBr, a fraction of which enters from the atmosphere while another fraction is formed via gas-phase chemistry in the SIA itself. A "bromine explosion", by which HOBr formed in the ambient air is deposited and then converted heterogeneously to Br 2 , is found to be a dominant process of reactive bromine formation in the Published by Copernicus Publications on behalf of the European Geosciences Union. 4102 K. Toyota et al.: Air-snowpack exchange of bromine and ozone calm weather conditions may undergo persistent ODEs without substantial contributions from blowing/drifting snow and wind-pumping mechanisms, whereas the column densities of BrO in the ABL will likely remain too low in the course of such events to be detected unambiguously by satellite nadir measurements. www.atmos-chem-phys.net/14/4101/2014/ Atmos. Chem. Phys., 14, 4101-4133, 2014
Geophysical Research Letters, 1995
A simple model is presented to estimate atmospheric concentrations of chemical species that exist primarily as aerosols based on snow core/ice core chemistry at Summit, Greenland. The model considers the processes of snow, fog, and dry deposition. The deposition parameters for each of the processes are estimated for SO42' and Ca 2+ and are based on e•ents conducted during the 1993 and 1994 stmuner field seasons. The seasonal mean atmospheric concentrations are estimated based on the deposition parameters and snow cores obtained during the field seasons. The ratios of the estimated seasonal mean airborne concentration divided by the measured mean concentration (Ua,est/Ua,me•) for 8042' over the 1993 and 1994 field seasons are 0.85 and 0.95, respectively. The U•,est /U•,me• ratios for Ca 2+ are 0.45 and 0.90 for the 1993 and 1994 field seasons. The uncertainties in the estimated atmospheric concentrations range from 30% to 40% and are due to variability in the input parameters. The model estimates the seasonal mean atmospheric 8042' and Ca 2' concentrations to within 15% and 55%, respectively. Although the model is not directly applied to ice cores, the application of the model to ice core chemical signals is briefly discussed.
Reactive trace gases measured in the interstitial air of surface snow at Summit, Greenland
Atmospheric Environment, 2004
Concentration measurements of nitric oxide (NO), nitrogen dioxide (NO 2 ), nitrous acid (HONO), nitric acid (HNO 3 ), formaldehyde (HCHO), hydrogen peroxide (H 2 O 2 ), formic acid (HCOOH) and acetic acid (CH 3 COOH) were performed in air filtered through the pore spaces of the surface snowpack (firn air) at Summit, Greenland, in summer 2000. In general, firn air concentrations of NO, NO 2 , HONO, HCHO, HCOOH, and CH 3 COOH were enhanced compared to concentrations in the atmospheric boundary layer above the snow. Only HNO 3 and H 2 O 2 normally exhibited lower concentrations in the firn air. In most cases differences were highest during the day and lowest during nighttime hours. Shading experiments showed a good agreement with a photochemical NO x source in the surface snow. Patterns of H 2 O 2 , CH 3 COOH, and HNO 3 observed within the surface snow-firn air system imply that the number of molecules in the snow greatly exceeded that in the firn air. Deduced partitioning indicates that the largest fractions of the acids were present at the ice grain-air interface. In all cases, the number of molecules located at the interface was significantly higher than the amount in the firn air. Therefore, snow surface area and surface coverage are important parameters, which must be considered for the interpretation of firn air concentrations. r
Journal of Geophysical Research, 2001
Hydrogen peroxide (H202) contributes to the atmosphere's oxidizing capacity, which determines the lifetime of atmospheric trace species. Measured bidirectional summertime H202 fluxes from the snowpack at Summit, Greenland, in June 1996 reveal a daytime H202 release from the surface snow reservoir and a partial redeposition at night. The observations also provide the first direct evidence of a strong net summertime H202 release from the snowpack, enhancing average boundary layer H20•. concentrations approximately sevenfold and the OH and HO2 concentrations by 70% and 50%, respectively, relative to that estimated from photochemical modeling in the absence of the snowpack source. The total H202 release over a 12-day period was of the order of 5x10 •3 molecules m -2 s -• and compares well with observed concentration changes in the top snow layer. Photochemical and air-snow interaction modeling indicate that the net snowpack release is driven by temperature-induced uptake and release of H202 as deposited snow, which is supersaturated with respect to ice-.air partitioning, approaches equilibrium. The results show that the physical cycling of H202 and possibly other volatile species is a key to understanding snowpacks as complex physicalphotochelnical reactors and has far reaching implications for the interpretation of ice core records as well as for the photochemistry in polar regions •nd in the vicinity of snowpacks in general.
Relationships between aerosol and snow chemistry at King Col, Mt. Logan Massif, Yukon, Canada
Atmospheric Environment, 2006
Simultaneous samples of aerosol (n=48) and recent snow (n=193) chemistry were collected at King Col (4135m) in the St. Elias Mountains, Yukon, between 17 May and 11 June 2001. Major ion concentrations in aerosol samples were low with the total ionic burden averaging 5.52neqm−3 at standard temperature and pressure (STP). Interspecies aerosol relationships indicate the presence of (NH4)2SO4 aerosol at
Snow-sourced bromine and its implications for polar tropospheric ozone
Atmospheric Chemistry and Physics, 2010
In the last two decades, significant depletion of boundary layer ozone (ozone depletion events, ODEs) has been observed in both Arctic and Antarctic spring. ODEs are attributed to catalytic destruction by bromine radicals (Br plus BrO), especially during bromine explosion events (BEs), when high concentrations of BrO periodically occur. However, neither the exact source of bromine nor the mechanism for sustaining the observed high BrO concentrations is completely understood. Here, by considering the production of sea salt aerosol from snow lying on sea ice during blowing snow events and the subsequent release of bromine, we successfully simulate the BEs using a global chemistry transport model. We find that heterogeneous reactions play an important role in sustaining a high fraction of the total inorganic bromine as BrO. We also find that emissions of bromine associated with blowing snow contribute significantly to BrO at mid-latitudes. Modeled tropospheric BrO columns generally compare well with the tropospheric BrO columns retrieved from the GOME satellite instrument (Global Ozone Monitoring Experiment). The additional blowing snow bromine source, identified here, reduces modeled high latitude lower tropospheric ozone amounts by up to an average 8% in polar spring.
Investigation of the role of the snowpack on atmospheric formaldehyde chemistry at Summit, Greenland
Journal of Geophysical Research, 2002
1] Ambient gas-phase and snow-phase measurements of formaldehyde (HCHO) were conducted at Summit, Greenland, during several summers, in order to understand the role of air-snow exchange on remote tropospheric HCHO and factors that determine snowpack HCHO. To investigate the impact of the known snowpack emission of HCHO, a gas-phase model was developed that includes known chemistry relevant to Summit and that is constrained by data from the 1999 and 2000 field campaigns. This gas-phase-only model does not account for the high ambient levels of HCHO observed at Summit for several previous measurement campaigns, predicting approximately 150 ppt from predominantly CH 4 chemistry, which is $25-50% of the observed concentrations for several years. Simulations were conducted that included a snowpack flux of HCHO based on HCHO flux measurements from 2000 and 1996. Using the fluxes obtained for 2000, the snowpack does not appear to be a substantial source of gas-phase HCHO in summer. The 1996 flux estimates predict much higher HCHO concentrations, but with a strong diel cycle that does not match the observations. Thus, we conclude that, although the flux of HCHO from the surface likely has a significant impact on atmospheric HCHO above the snowpack, the time-dependent fluxes need to be better understood and quantified. It is also necessary to identify the HCHO precursors so we can better understand the nature and importance of snowpack photochemistry. INDEX TERMS: 0322 Atmospheric Composition and Structure: Constituent sources and sinks; 0365 Atmospheric Composition and Structure: Tropospherecomposition and chemistry; 1863 Hydrology: Snow and ice (1827); 3367 Meteorolgy and Atmospheric Dynamics: Theoretical modeling Citation: Dassau, T. M., et al., Investigation of the role of the snowpack on atmospheric formaldehyde chemistry at Summit, Greenland,
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On the origin of the occasional spring nitrate peak in Greenland snow
Atmospheric Chemistry and Physics, 2014
Ice core nitrate concentrations peak in the summer in both Greenland and Antarctica. Two nitrate concentration peaks in one annual layer have been observed some years in ice cores in Greenland from samples dating post-1900, with the additional nitrate peak occurring in the spring. The origin of the spring nitrate peak was hypothesized to be pollution transport from the mid-latitudes in the industrial era. We performed a case study on the origin of a spring nitrate peak in 2005 measured from a snowpit at Summit, Greenland, covering 3 years of snow accumulation. The effect of long-range transport of nitrate on this spring peak was excluded by using sulfate as a pollution tracer. The isotopic composition of nitrate (δ 15 N, δ 18 O and 17 O) combined with photochemical calculations suggest that the occurrence of this spring peak is linked to a significantly weakened stratospheric ozone (O 3 ) layer. The weakened O 3 layer resulted in elevated UVB (ultraviolet-B) radiation on the snow surface, where the production of OH and NO x from the photolysis of their precursors was enhanced. Elevated NO x and OH concentrations resulted in enhanced nitrate production mainly through the NO 2 + OH formation pathway, as indicated by decreases in δ 18 O and 17 O of nitrate associated with the spring peak. We further examined the nitrate concentration record from a shallow ice core covering the period from 1772 to 2006 and found 19 years with double nitrate peaks after the 1950s. Out of these 19 years, 14 of the secondary nitrate peaks were accompanied by sulfate peaks, suggesting long-range transport of nitrate as their source. In the other 5 years, low springtime O 3 column density was observed, suggesting enhanced local production of nitrate as their source. The results suggest that, in addition to direct transport of nitrate from polluted regions, enhanced local photochemistry can also lead to a spring nitrate peak. The enhanced local photochemistry is probably associated with the interannual variability of O 3 column density in the Arctic, which leads to elevated surface UV radiation in some years. In this scenario, enhanced photochemistry caused increased local nitrate production under the condition of elevated local NO x abundance in the industrial era.
Nitrate postdeposition processes in Svalbard surface snow
Journal of Geophysical Research: Atmospheres, 2014
The snowpack acts as a sink for atmospheric reactive nitrogen, but several postdepositional pathways have been reported to alter the concentration and isotopic composition of snow nitrate with implications for atmospheric boundary layer chemistry, ice core records and terrestrial ecology following snow-melt. Careful daily sampling of surface snow during winter (11 -15 February, 2010) and spring-time (April 9 -May 5, 2010) near Ny-Ålesund, Svalbard reveals a complex pattern of processes within the snowpack. Dry deposition was found to dominate over post-depositional losses, with a net nitrate deposition rate of (0.6±0.2) µmol m -2 d -1 to homogeneous surface snow. At Ny-Ålesund, such surface dry deposition can either solely result from long-range atmospheric transport of NO x,y or include the redeposition of photolytic/bacterial emission originating from deeper snow layers. Our data further confirm that polar basin air masses bring 15 N-depleted nitrate to Svalbard, while high nitrate δ( 18 O) values only occur in connection with ozone-depleted air, and show that these signatures are reflected in the deposited nitrate. Such ozone-depleted air is attributed to active halogen chemistry in the air-masses advected to the site. However, here the Ny-Ålesund surface snow was shown to have an active role in the halogen dynamics for this region, as indicated by declining bromide concentrations and increasing nitrate δ( 18 O), during high BrO (low ozone) events. The data also indicates that the snow-pack BrO-NO x cycling continued in post-event periods, when ambient ozone and BrO levels recovered. Key points -NO 3 deposition and snowpack processes quantified by surface snow sampling -NO 3 dry deposition dominates over post-dep. loss in Ny-Ålesund surface snow. -Brdecreases whilst NO 3 d( 18 O) increases, suggesting tied BrO-NO x chemistry
Atmospheric Chemistry and Physics, 2013
The first measurements of atmospheric nitric oxide (NO) along with observations of ozone (O 3), hydroperoxides (H 2 O 2 and MHP) and snow nitrate (NO − 3) on the West Antarctic Ice Sheet (WAIS) were carried out at the WAIS Divide deep ice-coring site between 10 December 2008 and 11 January 2009. Average ±1σ mixing ratios of NO were 19 ± 31 pptv and confirmed prior model estimates for the summer boundary layer above WAIS. Mean ±1σ mixing ratios of O 3 of 14 ± 4 ppbv were in the range of previous measurements from overland traverses across WAIS during summer, while average ±1σ concentrations of H 2 O 2 and MHP revealed higher levels with mixing ratios of 743 ± 362 and 519 ± 238 pptv, respectively. An upper limit for daily average NO 2 and NO emission fluxes from snow of 8.6×10 8 and 33.9×10 8 molecule cm −2 s −1 , respectively, were estimated based on photolysis of measured NO − 3 and nitrite (NO − 2) in the surface snowpack. The resulting high NO x emission flux may explain the little preservation of NO − 3 in snow (∼ 30 %) when compared to Summit, Greenland (75-93 %). Assuming rapid and complete mixing into the overlying atmosphere, and steady state of NO x , these snow emissions are equivalent to an average (range) production of atmospheric NO x of 30 (21-566) pptv h −1 for a typical atmospheric boundary-layer depth of 250 (354-13) m. These upper bounds indicate that local emissions from the snowpack are a significant source of short-lived nitrogen oxides above the inner WAIS. The net O 3 production of 0.8 ppbv day −1 triggered with NO higher than 2 pptv is too small to explain the observed O 3 variability. Thus, the origins of the air masses reaching WAIS Divide during this campaign were investigated with a 4-day back-trajectory analysis every 4 h. The resulting 168 back trajectories revealed that in 75 % of all runs air originated from the Antarctic coastal slopes (58 %) and the inner WAIS (17 %). For these air sources O 3 levels were on average 13 ± 3 ppbv. The remaining 25 % are katabatic outflows from the East Antarctic Plateau above 2500 m. When nearsurface air from the East Antarctic Plateau reaches WAIS Divide through a rapid transport of less than 3 days, O 3 levels are on average 19 ± 4 ppbv with maximum mixing ratios of 30 ppbv. Episodes of elevated ozone at WAIS Divide are therefore linked to air mass export off the East Antarctic Plateau, demonstrating that outflows from the highly oxidizing summer atmospheric boundary layer in the interior of the continent can episodically raise the mixing ratios of longlived atmospheric chemical species such as O 3 and enhance the oxidative capacity of the atmosphere above WAIS.
Analysis of nitrate in the snow and atmosphere at Summit, Greenland: Chemistry and transport
Journal of Geophysical Research: Atmospheres, 2016
As a major sink of atmospheric nitrogen oxides (NO x = NO + NO 2), nitrate (NO 3 À) in polar snow can reflect the long-range transport of NO x and related species (e.g., peroxyacetyl nitrate). On the other hand, because NO 3 À in snow can be photolyzed, potentially producing gas phase NO x locally, NO 3 À in snow (and thus, ice) may reflect local processes. Here we investigate the relationship between local atmospheric composition at Summit, Greenland (72°35′N, 38°25′W) and the isotopic composition of NO 3 À to determine the degree to which local processes influence atmospheric and snow NO 3 À. Based on snow and atmospheric observations during May-June 2010 and 2011, we find no connection between the local atmospheric concentrations of a suite of gases (BrO, NO, NO y , HNO 3 , and nitrite (NO 2 À)) and the NO 3 À isotopic composition or concentration in snow. This suggests that (1) the snow NO 3 À at Summit is primarily derived from long-range transport and (2) this NO 3 À is largely preserved in the snow. Additionally, three isotopically distinct NO 3 À sources were found to be contributing to the NO 3 À in the snow at Summit during both 2010 and 2011. Through the complete isotopic composition of NO 3 À , we suggest that these sources are local anthropogenic particulate NO 3 À from station activities (δ 15 N = 16‰, Δ 17 O = 4‰, and δ 18 O = 23‰), NO 3 À formed from midlatitude NO x (δ 15 N = À10‰, Δ 17 O = 29‰, δ 18 O = 78‰) and a NO 3 À source that is possibly influenced by or derived from stratospheric ozone NO 3 À (δ 15 N = 5‰, Δ 17 O = 39‰, δ 18 O = 100‰).
Atmospheric Chemistry and Physics
Column ozone variability has important implications for surface photochemistry and the climate. Ice-core nitrate isotopes are suspected to be influenced by column ozone variability and δ 15 N(NO − 3) has been sought to serve as a proxy of column ozone variability. In this study, we examined the ability of ice-core nitrate isotopes to reflect column ozone variability by measuring δ 15 N(NO − 3) and 17 O(NO − 3) in a shallow ice core drilled at the South Pole. The ice core covers the period 1944-2005, and during this period δ 15 N(NO − 3) showed large annual variability ((59.2 ± 29.3) ‰), but with no apparent response to the Antarctic ozone hole. Utilizing a snow photochemical model, we estimated 6.9 ‰ additional enrichments in δ 15 N(NO − 3) could be caused by the development of the ozone hole. Nevertheless, this enrichment is small and masked by the effects of the snow accumulation rate at the South Pole over the same period of the ozone hole. The 17 O(NO − 3) record has displayed a decreasing trend by ∼ 3.4 ‰ since 1976. This magnitude of change cannot be caused by enhanced post-depositional processing related to the ozone hole. Instead, the 17 O(NO − 3) decrease was more likely due to the proposed decreases in the O 3 / HO x ratio in the extratropical Southern Hemisphere. Our results suggest icecore δ 15 N(NO − 3) is more sensitive to snow accumulation rate than to column ozone, but at sites with a relatively constant snow accumulation rate, information of column ozone variability embedded in δ 15 N(NO − 3) should be retrievable.
Related papers
Atmospheric Environment, 2002
Tower-based measurements of hydrogen peroxide (H 2 O 2 ) and formaldehyde (HCHO) exchange were performed above the snowpack of the Greenland ice sheet. H 2 O 2 and HCHO fluxes were measured continuously between 16 June and 7 July 2000, at the Summit Environmental Observatory. The fluxes were determined using coil scrubber-aqueous phase fluorometry systems together with micrometeorological techniques. Both compounds exhibit strong diel cycles in the observed concentrations as well as in the fluxes with emission from the snow during the day and the evening and deposition during the night. The averaged diel variations of the observed fluxes were in the range of +1.3 Â 10 13 molecules m À2 s À1 (deposition) and À1.6 Â 10 13 molecules m À2 s À1 (emission) for H 2 O 2 and +1.1 Â 10 12 and À4.2 Â 10 12 molecules m À2 s À1 for HCHO, while the net exchange per day for both compounds were much smaller. During the study period of 22 days on average ð0:8 þ4:6 À4:3 Þ Â 10 17 molecules m À2 of H 2 O 2 were deposited and ð7:0 þ12:6 À12:2 Þ Â 10 16 molecules m À2 of HCHO were emitted from the snow per day. A comparison with the inventory in the gas phase demonstrates that the exchange influences the diel variations in the boundary layer above snow covered areas. Flux measurements during and after the precipitation of new snow shows that o16% of the H 2 O 2 and more than 25% of the HCHO originally present in the new snow were available for fast release to the atmospheric boundary layer within hours after precipitation. This release can effectively disturb the normally observed diel variations of the exchange between the surface snow and the atmosphere, thus perturbing also the diel variations of corresponding gas-phase concentrations. r
Concentrations and snow-atmosphere fluxes of reactive nitrogen at Summit, Greenland
Journal of Geophysical Research, 1999
Concentrations and fluxes of NO y (total reactive nitrogen), ozone concentrations and fluxes of sensible heat, water vapor and momentum were measured from May 1 to July 20, 1995 at Summit, Greenland. Median NO y concentrations declined from 947 ppt in May to 444 ppt by July. NO y fluxes were observed into and out of the snow, but the magnitudes were usually below 1µmol m -2 hr -1 because of the low HNO 3 concentration and weak turbulence over the snow surface. Some of the highest observed fluxes may be due to temporary storage by equilibrium sorption of PAN or other organic nitrogen species on ice surfaces in the upper snowpack. Sublimation of snow at the surface or during blowing snow events is associated with efflux of NO y from the snowpack. Because the NO y fluxes during summer at Summit are bi-directional and small in magnitude, the net result of turbulent NO y exchange is insignificant compared to the 2 µmol m -2 d -1 mean input from fresh snow during the summer months. If the arctic NO y reservoir is predominantly PAN (or compounds with similar properties) thermal dissociation of this NO y is sufficient to support the observed flux of nitrate in fresh snow. Very low HNO 3 concentrations in the surface layer (1% of total NO y ) reflect the poor ventilation of the surface layer over the snowpack combined with the relatively rapid uptake of HNO 3 by fog, falling snow, and direct deposition to the snowpack.
Snow and firn properties and air–snow transport processes at Summit, Greenland
Atmospheric Environment, 2002
Snow-air exchange processes affect the chemistry of the atmosphere as well as the chemistry of underlying snow and firn. An understanding of the transport processes is important for quantifying and predicting changes in atmospheric chemistry and also for improving ice core interpretation. This paper focuses on the nature of diffusive and advective (ventilation) interstitial transport processes at Summit, Greenland. Field measurements of snow and firn density, permeability and microstructure are presented and compared with measurements from previous years. Density and permeability profiles follow similar general patterns from year to year; however, the specifics of the profiles show interannual variation. Field measurements of the diffusion of an inert tracer gas, SF6, through the surface wind pack yields an SF6 diffusion coefficient for the June 2000 surface wind pack at Summit of B0.06 cm 2 /s; the tortuosity of the surface wind pack was 0.5. The first direct measurements of interstitial air flow in snow due to natural ventilation in undisturbed snow are presented, for light (3 m/s) winds and moderately strong (9 m/s) winds in a hoar layer 15 cm beneath the surface. The measurements during light winds showed results characteristic of diffusion profiles, while the measurements under strong winds showed evidence of ventilation. The interstitial air flow velocities are consistent with previous modeling results. Published by Elsevier Science Ltd.
Atmospheric Environment, 2002
Measurements at Summit, Greenland, performed from June-August 1999, showed significant enhancement in concentrations of several trace gases in the snowpack (firn) pore air relative to the atmosphere. We report here measurements of alkenes, halocarbons, and alkyl nitrates that are typically a factor of 2-10 higher in concentration within the firn air than in the ambient air 1-10 m above the snow. Profiles of concentration to a depth of 2 m into the firn show that maximum values of these trace gases occur between the surface and 60 cm depth. The alkenes show highest pore mixing ratios very close to the surface, with mixing ratios in the order ethene > propene > 1-butene: Mixing ratios of the alkyl iodides and alkyl nitrates peak slightly deeper in the firn, with mixing ratios in order of methyl > ethyl > propyl: These variations are likely consistent with different near-surface photochemical production mechanisms. Diurnal mixing ratio variations within the firn correlate well with actinic flux for all these gases, with a temporal offset between the solar maximum and peak concentrations, lengthening with depth. Using a snow-filled chamber under constant flow conditions, we calculated production rates for the halocarbons and alkenes that ranged between 10 3 -10 5 and 10 6 molecules cm À3 s À1 , respectively. Taken together, these results suggest that photochemistry associated with the surface snowpack environment plays an important role in the oxidative capacity of the local atmospheric boundary layer, and influences post-depositional chemistry, which in turn may affect the interpretation of certain aspects of the ice core records collected previously at Summit. r
Evaluating the impact of blowing snow sea salt aerosol on springtime BrO and O3 in the Arctic
We use the GEOS-Chem chemical transport model to examine the influence of bromine release from blowing snow sea salt aerosol (SSA) on springtime bromine activation and O3 depletion events (ODEs) in the Arctic lower troposphere. We 15 evaluate our simulation against observations of tropospheric BrO vertical column densities (VCDtropo) from the GOME-2 and OMI spaceborne instruments for three years (2007-2009), as well as against surface observations of O3. We conduct a simulation with blowing snow SSA emissions from first-year sea ice (FYI, with a surface snow salinity of 0.1 psu) and multi-year sea ice (MYI, with a surface snow salinity of 0.05 psu), assuming a factor of 5 bromide enrichment of surface snow relative to seawater. This simulation captures the magnitude of observed March-April GOME-2 and OMI VCDtropo to within 20 17%, as well as their spatiotemporal variability (r=0.76-0.85). Many of the large-scale bromine explosions are successfully reproduced, with the exception of events in May, which are absent or systematically underpredicted in the model. If we assume a lower salinity on MYI (0.01 psu) some of the bromine explosions events observed over MYI are not captured, suggesting that blowing snow over MYI is an important source of bromine activation. We find that the modeled atmospheric deposition onto snow-covered sea ice becomes highly enriched in bromide, increasing from enrichment factors of ~5 in September-25 February to 10-60 in May, consistent with freshly fallen snow composition observations. We propose that this progressive enrichment in deposition could enable blowing snow-induced halogen activation to propagate into May and might explain our late-spring underestimate in VCDtropo. We estimate that atmospheric deposition of SSA could increase snow salinity by up to 0.04 psu between February and April, which could be an important source of salinity for surface snow on MYI as well as FYI covered by deep snowpack. Inclusion of halogen release from blowing snow SSA in our simulations decreases monthly mean 30 Arctic surface O3 by 4-8 ppbv(15-30%) in March and 8-14 ppbv (30-40%) in April. We reproduce a transport event of depleted O3 Arctic air down to 40º N observed at many sub-Arctic surface sites in early April 2007. While our simulation captures a few ODEs observed at coastal Arctic surface sites, it underestimates the magnitude of other events and entirely misses some events. We suggest that inclusion of direct snowpack activation, which is a strong local source of Br radicals in the shallow
Journal of Geophysical Research, 1995
Experiments were performed during the period May-July of 1993 at Summit, Greenland. Aerosol mass size distributions as well as daily average concentrations of several anionic and cationic species were measured. Dry deposition velocities for SO,:' were estimated using surrogate surfaces (symmetric airfoils) as well as impactor data. Real-time concentrations of particles greater than 0.5 gm and greater than 0.01 gm were measured. Snow and fog samples from nearly all of the events occurring during the field season were collected.
Atmospheric Chemistry and Physics, 2011
Episodes of high bromine levels and surface ozone depletion in the springtime Arctic are simulated by an online air-quality model, GEM-AQ, with gas-phase and heterogeneous reactions of inorganic bromine species and a simple scheme of air-snowpack chemical interactions implemented for this study. Snowpack on sea ice is assumed to be the only source of bromine to the atmosphere and to be capable of converting relatively stable bromine species to photolabile Br2 via air-snowpack interactions. A set of sensitivity model runs are performed for April 2001 at a horizontal resolution of approximately 100 km×100 km in the Arctic, to provide insights into the effects of temperature and the age (first-year, FY, versus multi-year, MY) of sea ice on the release of reactive bromine to the atmosphere. The model simulations capture much of the temporal variations in surface ozone mixing ratios as observed at stations in the high Arctic and the synoptic-scale evolution of areas with enhanced BrO column amount ("BrO clouds") as estimated from satellite observations. The simulated "BrO clouds" are in modestly better agreement with the satellite measurements when the FY sea ice is assumed to be more efficient at releasing reactive bromine to the atmosphere than on the MY sea ice. Surface ozone data from coastal stations used in this study are not sufficient to evaluate unambiguously the difference between the FY sea ice and the MY sea ice as a source of bromine. The results strongly suggest that reactive bromine is released ubiquitously from the snow on the sea ice during the Arctic spring while the timing and location of the bromine release are largely controlled by meteorological factors. It appears that a rapid advection and an enhanced turbulent diffusion associated with strong boundary-layer winds drive transport and dispersion of ozone to the near-surface air over the sea ice, increasing the oxidation rate of bromide (Br-) in the surface snow. Also, if indeed the surface snowpack does supply most of the reactive bromine in the Arctic boundary layer, it appears to be capable of releasing reactive bromine at temperatures as high as -10 °C, particularly on the sea ice in the central and eastern Arctic Ocean. Dynamically-induced BrO column variability in the lowermost stratosphere appears to interfere with the use of satellite BrO column measurements for interpreting BrO variability in the lower troposphere but probably not to the extent of totally obscuring "BrO clouds" that originate from the surface snow/ice source of bromine in the high Arctic. A budget analysis of the simulated air-surface exchange of bromine compounds suggests that a "bromine explosion" occurs in the interstitial air of the snowpack and/or is accelerated by heterogeneous reactions on the surface of wind-blown snow in ambient air, both of which are not represented explicitly in our simple model but could have been approximated by a parameter adjustment for the yield of Br2 from the trigger.
Can We Model Snow Photochemistry? Problems with the Current Approaches
The Journal of Physical Chemistry A, 2013
Snow is a very active photochemical reactor that considerably affects the composition and chemistry of the lower troposphere in polar regions. Snow photochemistry models have therefore been recently developed to describe these processes. In all those models, the chemically active medium is a brine formed at the surface of snow crystals by impurities whose presence cause surface melting. Reaction and photolysis rate coefficients are those measured in dilute liquid solutions. Here, we critically examine the basis for these models by considering the structure of ice crystal surfaces, the processes involved in the interactions between impurities and ice crystals, the location of impurities in snow, and the reactivity of impurities in the various media present in snow. We conclude that the brine formed by impurities can only be present in grooves at grain boundaries and cannot cover ice crystal surfaces because of insufficient ice wettability. It is then very likely that most reactions in snow do not take place in liquids, but rather either on an actual ice surface highly different from a liquid or in particulate matter contained in snow, such as organic particles that are thought to contain most snow chromophores. We discuss why some snow models appear to adequately reproduce some observations, concluding that they are insufficiently constrained and that the use of adjustable parameters allows acceptable fits. We discuss the complexity of developing a snow model without adjustable parameters and with a predictive value. We conclude that reaching this goal in the near future is a tremendous challenge. Modeling attempts focused on snow where the impact of organic particles is minimal, such as on the east Antarctic plateau, represents the best chance of midterm success.
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