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
Air-snowpack exchange of bromine, ozone and mercury in
2013
To provide a theoretical framework towards 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. In this paper, we describe 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. The model employs a chemical mechanism adapted from the one previously used for the simulation of multiphase halogen chemistry involving deliquesced sea-salt aerosols in the marine boundary layer. A common set of aqueous-phase reactions describe chemistry both in the liquid-like (or brine) layer on the grain surface of the snowpack and in "haze" aerosols mainly composed of sulfate in the atmosphere. The process of highly soluble/reactive trace gases, whether entering the snowpack from the atmosphere or formed via gas-phase chemistry in the snowpack interstitial air (SIA), is simulated by the uptake on brine-covered snow grains and subsequent reactions in the aqueous phase while being traveled vertically within the SIA. A "bromine explosion", by which, in a conventional definition, HOBr formed in the ambient air is deposited and then converted heterogeneously to Br 2 , is a dominant process of reactive bromine formation in the top 1 mm (or less) layer of the snowpack. Deeper in the snowpack, HOBr formed within the SIA leads to an in-snow bromine explosion, but a significant fraction of Br 2 is also produced via aqueous radical chemistry in the brine on the surface of the snow grains. These top-and deeper-layer productions of Br 2 both contribute to the Br 2 release into the atmosphere, but the deeper-layer production is found to be more important for the net outflux of reactive bromine. Although ozone is removed via bromine chemistry, it is also among the key species that control both the conventional and in-snow bromine explosions. On the other hand, aqueousphase radical chemistry initiated by photolytic OH formation in the liquid-like layer is 20342
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.
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