Abstract. We use the GEOS-Chem chemical transport model to examine theinfluence of bromine release from blowing-snow sea salt aerosol (SSA) onspringtime bromine activation and O3 depletion events (ODEs) in theArctic lower troposphere. We evaluate our simulation against observations oftropospheric BrO vertical column densities (VCDtropo) from the GOME-2 (second Global Ozone Monitoring Experiment)and Ozone Monitoring Instrument (OMI) spaceborne instruments for 3 years (2007–2009), as well asagainst surface observations of O3. We conduct a simulation withblowing-snow SSA emissions from first-year sea ice (FYI; with a surface snowsalinity of 0.1 psu) and multi-year sea ice (MYI; with a surface snowsalinity of 0.05 psu), assuming a factor of 5 bromide enrichment of surfacesnow relative to seawater. This simulation captures the magnitude ofobserved March–April GOME-2 and OMI VCDtropo to within 17 %, as wellas their spatiotemporal variability (r=0.76–0.85). Many of the large-scalebromine explosions are successfully reproduced, with the exception of eventsin May, which are absent or systematically underpredicted in the model. Ifwe assume a lower salinity on MYI (0.01 psu), some of the bromine explosionsevents observed over MYI are not captured, suggesting that blowing snow overMYI is an important source of bromine activation. We find that the modeledatmospheric deposition onto snow-covered sea ice becomes highly enriched inbromide, increasing from enrichment factors of ∼5 inSeptember–February to 10–60 in May, consistent with composition observations of freshly fallen snow. We propose that this progressive enrichment indeposition could enable blowing-snow-induced halogen activation to propagateinto May and might explain our late-spring underestimate in VCDtropo.We estimate that the atmospheric deposition of SSA could increase snow salinityby up to 0.04 psu between February and April, which could be an importantsource of salinity for surface snow on MYI as well as FYI covered by deepsnowpack. Inclusion of halogen release from blowing-snow SSA in oursimulations decreases monthly mean Arctic surface O3 by 4–8 ppbv(15 %–30 %) in March and 8–14 ppbv (30 %–40 %) in April. We reproduce atransport event of depleted O3 Arctic air down to 40∘ Nobserved at many sub-Arctic surface sites in early April 2007. While oursimulation captures 25 %–40 % of the ODEs observed at coastal Arctic surfacesites, it underestimates the magnitude of many of these events and entirelymisses 60 %–75 % of ODEs. This difficulty in reproducing observed surfaceODEs could be related to the coarse horizontal resolution of the model, theknown biases in simulating Arctic boundary layer exchange processes, thelack of detailed chlorine chemistry, and/or the fact that we did not includedirect halogen activation by snowpack chemistry.
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A review of air–ice chemical and physical interactions (AICI): liquids, quasi-liquids, and solids in snow
Abstract. Snow in the environment acts as a host to rich chemistry and provides a matrix for physical exchange of contaminants within the ecosystem. The goal of this review is to summarise the current state of knowledge of physical processes and chemical reactivity in surface snow with relevance to polar regions. It focuses on a description of impurities in distinct compartments present in surface snow, such as snow crystals, grain boundaries, crystal surfaces, and liquid parts. It emphasises the microscopic description of the ice surface and its link with the environment. Distinct differences between the disordered air–ice interface, often termed quasi-liquid layer, and a liquid phase are highlighted. The reactivity in these different compartments of surface snow is discussed using many experimental studies, simulations, and selected snow models from the molecular to the macro-scale.
Although new experimental techniques have extended our knowledge of the surface properties of ice and their impact on some single reactions and processes, others occurring on, at or within snow grains remain unquantified. The presence of liquid or liquid-like compartments either due to the formation of brine or disorder at surfaces of snow crystals below the freezing point may strongly modify reaction rates. Therefore, future experiments should include a detailed characterisation of the surface properties of the ice matrices. A further point that remains largely unresolved is the distribution of impurities between the different domains of the condensed phase inside the snowpack, i.e. in the bulk solid, in liquid at the surface or trapped in confined pockets within or between grains, or at the surface. While surface-sensitive laboratory techniques may in the future help to resolve this point for equilibrium conditions, additional uncertainty for the environmental snowpack may be caused by the highly dynamic nature of the snowpack due to the fast metamorphism occurring under certain environmental conditions.
Due to these gaps in knowledge the first snow chemistry models have attempted to reproduce certain processes like the long-term incorporation of volatile compounds in snow and firn or the release of reactive species from the snowpack. Although so far none of the models offers a coupled approach of physical and chemical processes or a detailed representation of the different compartments, they have successfully been used to reproduce some field experiments. A fully coupled snow chemistry and physics model remains to be developed.
- Award ID(s):
- 1255290
- NSF-PAR ID:
- 10107229
- Author(s) / Creator(s):
- ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; more »
- Date Published:
- Journal Name:
- Atmospheric Chemistry and Physics
- Volume:
- 14
- Issue:
- 3
- ISSN:
- 1680-7324
- Page Range / eLocation ID:
- 1587 to 1633
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Reactive chlorine and bromine species emitted from snow and aerosols can significantly alter the oxidative capacity of the polar boundary layer. However, halogen production mechanisms from snow remain highly uncertain, making it difficult for most models to include descriptions of halogen snow emissions and to understand the impact on atmospheric chemistry. We investigate the influence of Arctic halogen emissions from snow on boundary layer oxidation processes using a one‐dimensional atmospheric chemistry and transport model (PACT‐1D). To understand the combined impact of snow emissions and boundary layer dynamics on atmospheric chemistry, we model Cl2and Br2primary emissions from snow and include heterogeneous recycling of halogens on both snow and aerosols. We focus on a 2‐day case study from the 2009 Ocean‐Atmosphere‐Sea Ice‐Snowpack campaign at Utqiaġvik, Alaska. The model reproduces both the diurnal cycle and high quantity of Cl2observed, along with the measured concentrations of Br2, BrO, and HOBr. Due to the combined effects of emissions, recycling, vertical mixing, and atmospheric chemistry, reactive chlorine is typically confined to the lowest 15 m of the atmosphere, while bromine can impact chemistry up to and above the surface inversion height. Upon including halogen emissions and recycling, the concentration of HO
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Abstract. The Snow, Ice, and Aerosol Radiative (SNICAR) model has been used in various capacities over the last 15 years to model the spectral albedo of snow with light-absorbing constituents (LACs). Recent studies have extended the model to include an adding-doubling two-stream solver and representations of non-spherical ice particles; carbon dioxide snow; snow algae; and new types of mineral dust, volcanic ash, and brown carbon. New options also exist for ice refractive indices and solar-zenith-angle-dependent surface spectral irradiances used to derive broadband albedo. The model spectral range was also extended deeper into the ultraviolet for studies of extraterrestrial and high-altitude cryospheric surfaces. Until now, however, these improvements and capabilities have not been merged into a unified code base. Here, we document the formulation and evaluation of the publicly available SNICAR-ADv3 source code, web-based model, and accompanying library of constituent optical properties. The use of non-spherical ice grains, which scatter less strongly into the forward direction, reduces the simulated albedo perturbations from LACs by ∼9 %–31 %, depending on which of the three available non-spherical shapes are applied. The model compares very well against measurements of snow albedo from seven studies, though key properties affecting snow albedo are not fully constrained with measurements, including ice effective grain size of the top sub-millimeter of the snowpack, mixing state of LACs with respect to ice grains, and site-specific LAC optical properties. The new default ice refractive indices produce extremely high pure snow albedo (>0.99) in the blue and ultraviolet part of the spectrum, with such values only measured in Antarctica so far. More work is needed particularly in the representation of snow algae, including experimental verification of how different pigment expressions and algal cell concentrations affect snow albedo. Representations and measurements of the influence of liquid water on spectral snow albedo are also needed.more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.5194/acp-14-1587-2014
