Air-Ice Chemical Interactions (AICI) Task Team

This task team was approved as a joint SOLAS-IGAC task in June 2003. You can read the task proposal below or download a pdf of the document.

For queries about AICI, please contact Dr. Eric Wolff (ewwo AT bas.ac.uk)

 

  1. Introduction
  2. The Scientific Context
  3. Natural Variability and Climate Change
  4. Key science questions, IGAC and SOLAS
  5. Research Activities
  6. Task Co-ordination
  7. Timetable and other activities
  8. Education and capacity building
  9. References
Introduction

It is now recognised that changes in one compartment of the Earth System can strongly affect the state of other compartments. Feedbacks can amplify or mitigate trends. As the Earth and its climate change, particularly in response to phenomena such as greenhouse gas increases, and stratospheric ozone depletion, it becomes increasingly important to understand the interactions between different parts of the system. This has been the basis for the new structure of IGBP, which studies the three main compartments (land, ocean, and atmosphere) and the exchanges and interactions between them. The cryosphere - ice, snow and permafrost - occurs in all three of these compartments. It forms a large proportion of the surface separating the land and ocean from the atmosphere: a seasonal maximum of 40% of land is covered by snow or ice, while several percent of the ocean is sea-ice covered.

The physical processes that involve ice are now being studied by the WCRP project CliC (Climate and Cryosphere). Until recently, it was generally assumed that the main biogeochemical role of ice was that it restricted exchanges between the more active ocean and land surfaces and the atmosphere. However, studies in recent years have revealed evidence that important chemical exchanges also occur between ice and the atmosphere, and it makes sense to study these processes and their consequences generically. AICI aims to do this, forming a bridge between scientists working on ice-covered surfaces in both polar regions and in the mid-latitudes, atmospheric scientists interested in processes occurring on ice particles, laboratory scientists studying the properties of ice, and modellers who need to parameterise processes that involve ice.

This proposal is aimed particularly at the projects IGAC (International Global Atmospheric Chemistry) and SOLAS (Surface Ocean Lower Atmosphere Study), although it is also relevant to the planned IGBP Land-Atmosphere project, to PAGES (Past Global Changes), as well as to WCRP-CliC. This proposal will start by outlining some of the issues that have made it apparent that the topic of air-ice chemical interactions is an important area of study within the context of global change. It will then focus on a subset of issues that are amenable to study within the three year timeframe of an IGAC-II project. We expect that the results from this first phase of AICI may lead to a further proposal, focusing on different issues, later in the IGAC-II period.

 

The scientific context

Until recently, the snow and ice covered regions of the world received little interest from the atmospheric chemistry community. However, a number of field programmes, principally in the Arctic and Antarctic have revealed many unexpected and interesting phenomena, and opened up the possibility that the chemistry of the ice may control aspects of boundary layer chemistry over large regions of the world, rather than the other way round. In addition, for some species the exchanges between snow and the atmosphere, as well as post-depositional ice processes, have a decisive impact on the signal that is retrieved from ice cores: our best hope of reconstructing atmospheric chemistry from the pre-instrumental period. In the following section, we outline, as examples, two of the issues that have become apparent in recent years, as a preamble to setting questions that can be attacked within the life span of an IGAC-II or SOLAS task.

Example 1 - tropospheric ozone depletion

The discovery of ozone depletion events within the polar marine boundary layer [Bottenheim et al., 1986; Oltmans and Komhyr, 1986] came as a surprise. Since the initial finding, we have learnt much about the phenomenon, but we are still far from being able to predict the spatial or temporal distribution of the depletion, or to model the effects. The surprising magnitude of the effect points to weaknesses in our understanding of tropospheric ozone chemistry. This must be remedied, given its central role with respect to the atmosphere's oxidizing capacity, and its significant contribution to radiative forcing.

Tropospheric ozone depletion occurs in both the Arctic, sub-Arctic and the Antarctic [Wessel et al., 1998]. In the Arctic marine boundary layer in springtime, depletion occurs from the surface up to ~1.5km [Bottenheim et al., 2002b], and occurs over periods of days to weeks, with concentrations as low as 50 ppt at times. The depletion events are associated with halogens, with Br/BrO appearing to play an especially important role [e.g. Barrie et al., 1988; Foster et al., 2001]. Satellite observations of BrO, showing particularly high concentrations over sea ice zones in springtime [Wagner et al., 2001], have reinforced current ideas about the general nature of the reactions leading to depletion, and suggested that sea ice is implicated. Recently it has been suggested that concentrated brines on new sea ice and in frost flowers [Rankin et al., 2002] might be the source of the halogens. This would explain why a similar phenomenon is seen over salt lakes [Stutz et al., 2002] and the Dead Sea [Matveev et al., 2001] where very salty deposits are also available. Brines and frost flowers may also be responsible for a significant part of sea salt aerosol concentrations over the polar regions [Rankin et al., 2002]. Among the chemical effects of the ozone depletion, it is perturbing the biogeochemistry of mercury, leading to enhanced atmospheric removal, and deposition to Arctic ecosystems [Lindberg et al., 2002; Schroeder et al., 1998]; this process has also been reported from the Antarctic [Ebinghaus et al., 2002]. Finally, it is also noteworthy that ozone depletion occurs in cirrus clouds; it is conceivable that these chemistries could be related [Roumeau et al., 2000].

This work, gives a picture of a phenomenon for which we have a skeleton understanding, but as yet no answers to some important questions. We do not yet understand why depletion occurs in the spring but not in the autumn; we have no confirmation that frost flowers are involved; we have not explored the likely effects on atmospheric boundary layer chemistry in full; we cannot completely identify the precursors to halogen atoms; and we have not established whether the phenomenon is involved in significant climate feedbacks.

Example 2 - snow photochemistry

Until recently, no consideration was given to the concept that the snowpack could itself be affected by photochemistry. However, a series of field [e.g. Honrath et al., 1999; Jones et al., 2000] and laboratory [e.g. Cotter et al., 2003; Dubowski et al., 2001; Honrath et al., 2000] experiments have established beyond doubt that NOx, as the first example, is produced photochemically from snow, with nitrate as the main precursor. In retrospect, this is not surprising - light penetrates at least the top few cm of snowpack, and many hydrophilic UV-absorbing chemicals are present in concentrations that are high relative to those in the atmosphere [Wolff et al., 2002].

Comparisons with model results have shown that emissions out of the snowpack can overwhelmingly control the chemistry of the sunlit summer boundary layer over snow, particularly in cases where there is little mixing due to a surface inversion. For example, NO was present at South Pole in concentrations over 200 pptv during December 1998 [Davis et al., 2001], when model calculations excluding a snow source would have suggested 1-5 pptv. Very high OH concentrations [Mauldin et al., 2001] were also observed, and the NOx rich environment may have been responsible for this, in turn influencing the entire range of chemistry. The full implications have not yet been explored, but ozone production over a large part of the Antarctic plateau is one probable consequence [Crawford et al., 2001].

Many other chemicals seem to be produced photochemically from the snowpack - there is evidence so far for at least aldehydes such as HCHO [Sumner and Shepson, 1999; Sumner et al., 2002], CO [Haan et al., 2001], C2H2 [Bottenheim et al., 2002a] and HONO [Zhou et al., 2001]. In addition, there is extensive physical exchange of important species such as HCHO and H2O2 [Hutterli et al., 2001; Hutterli et al., 1999]. Until now, only the existence of the phenomenon, and some tentative flux estimates [e.g. Beine et al., 2002; Honrath et al., 2002; Jones et al., 2001] are available. Much work is needed, in the laboratory and the field, to understand the factors controlling the release of each chemical. Only then will it be possible, through modelling studies, to assess whether the processes have any large-scale importance. However, it is already clear that such snowpack chemistry will significantly affect both ice phase and boundary layer concentrations for photo-labile species. With our current state of knowledge, we assume that similar emission processes are occurring over mid-latitude and seasonal snowcovers, so that there is the potential for significant alteration of the boundary layer chemistry over substantial parts of the globe, including urban areas.

 

Natural variability and changing climate

As far as we know, the phenomena described above occur under natural conditions and have always been present. However, changes in UV radiation, in concentrations of pollutants, in snow cover, and in temperature, could all affect the extent of the processes. For example, the tropospheric ozone production reported at the South Pole [Crawford et al., 2001] seems to have increased over the last 30 years; this could be partly a result of increased NOx production that would be predicted from increased UV radiation reaching the surface (because of stratospheric ozone depletion) [Jones and Wolff, Submitted]; indeed we would expect all the photochemical reactions in snow to be enhanced in this way. The chemistry of halogen activation, as well as emission of species such as HONO may be pH dependent, and thus impacted by acidic deposition from anthropogenic sources.

One of the key expectations of a warming world (under increased greenhouse gas concentrations) is that the cryosphere will shrink - in the near-term, seasonal snow cover and sea ice extent are expected to diminish. This will both reduce the scope for the snow-atmosphere interactions (such as those described above) to occur, and enhance the interactions of the atmosphere and the increased land and ocean areas. As an example, if the springtime depletion of tropospheric ozone is indeed linked to brine layers on new sea ice, then any change in the production rate of new sea ice will alter the locations and extent of this phenomenon. There are indications that Arctic ozone depletion events, and concomitant deposition of Hg, have increased in recent decades [Lindberg et al., 2002; Tarasick and Bottenheim, 2002]. We would also anticipate a change in the concentration of sea salt aerosol, with further possible direct and indirect climate effects.

Surface uptake by ice is highly temperature dependent, so that climate change can effect the deposition of contaminants (such as persistent organic pollutants (POPs)) to the polar regions. Even the nature of the surface of ice is highly temperature dependent, and this may affect the nature of ice surface chemistry [Cho et al., 2002]. Assessments of possible feedbacks such as these are central to understanding the effects of global change.

 

Key science questions, IGAC and SOLAS

IGAC has two over-arching science questions, and for each of them, we have formulated an overall AICI question.

  • What is the role of atmospheric chemistry in amplifying or damping climate change?
    • How will changing amounts of sea ice, snow cover, and atmospheric ice alter atmospheric chemistry and composition and are there important feedbacks to climate?
  • Within the Earth System, what effects do changing regional emissions and depositions have on air quality and the chemical composition of the planetary boundary layer?
    • What are the present regional emissions and losses over snow, ice and sea ice, and how could they alter with changing climate?

For the sea ice component, these same issues address various SOLAS activities, particularly 1.1 (sea-salt particle formation and transformations), 1.2 (trace gas emissions and photochemical feedbacks) and associated issues in activity 2. Understanding halogen release from sea ice is specifically identified as a goal in the SOLAS science plan.

Obviously the two science questions we have highlighted in bold, as well as the associated IGAC questions, are similarly large, and unlikely to be satisfactorily answered in three years. We have therefore prepared some more limited goals that can be addressed seriously in this first phase of AICI:

  • To document the full range of processes and emitted trace gases that arise at the air-ice interface, and how they depend on environmental conditions.
  • To quantify the fluxes of trace gases and aerosol between atmosphere and ice under a range of field experimental conditions
  • To determine the main factors that control the fluxes of trace gases between air and ice, using both field data and carefully designed laboratory experiments
  • To build simple models that include processes in the upper firn and lower atmosphere, to assess the effects of air-ice fluxes
  • To make a first assessment of the scale of these processes, and assess the significance for climate or atmospheric chemistry within the global troposphere, the atmospheric boundary layer, or regionally, and in urban environments.
  • To develop simple parameterisations of the fluxes and processes studied for incorporation into sophisticated global chemical models

Based on the outcome of this first phase of AICI, we envisage a second AICI project in which more quantitative fluxes and parameterisations will be made, and the effects of changing climate and cryosphere will be rigorously assessed, through a combination of regional scale field studies, with associated laboratory and modeling activities.

 

Research activities

Addressing the goals outlined above will require a combined programme of field measurements, laboratory measurements, satellite data retrieval, and modelling studies. The field work poses a particular challenge because of the remoteness of the polar sites, so that only through a range of international co-operative activities can sufficient data be obtained. The organisation of Arctic and Antarctic logistics has previously dictated that most studies were led by a single country, with participation from others; we regard the future integration of the different experiments and leveraging of multinational resources to be an important added value that AICI can provide. The following elements will be essential, some of which are already in place, and some of which we will encourage:

  • Comprehensive studies in snow/ice covered areas of a range of types, locations, and time periods. Some studies will focus on particular times of year (such as springtime), but year-round studies will also be important so that we can be sure all relevant processes are being observed. Profiling studies, involving aircraft, tethered balloons, and towers, will be needed, as well as ground-based studies. The range of environments should include
    • Clean permanent inland ice sheet locations, where snow photochemistry can most simply be studied (ANTCI at South Pole and Geosummit in Greenland are examples of studies that are already funded, although additional activities may be added to get full value from them
    • Coastal marine ice sheet sites, where both snow photochemistry under different climatic and snow conditions, as well as gas phase and aerosol processes associated with new sea ice surfaces can be studied (the year-round CHABLIS project at Halley (Antarctica), and ongoing studies at sites such as Neumayer and Dumont d'Urville (Antarctica), Alert, Barrow and Ny Ålesund (Arctic) are examples of this category; again additional activities could be added to these sites)
    • Studies within the sea ice zone, using for example a ship or platform, ideal for studying processes over sea ice (the proposed US SOLAS OASIS study falls into this category
    • Studies over Alpine and seasonal snowcover environments, necessary to assess how widely applicable the polar findings are, and to determine the importance of regional scale anthropogenic inputs to the snowpack and the resultant impact of the subsequent photochemical processes
    • Studies in urban snow-impacted environments, which are likely to have much higher concentrations of key ice-surface reactants, e.g. nitrate, organic matter, etc
  • Field studies of ice in the upper troposphere (e.g. cirrus), which can also benefit from and contribute to advances in the other areas discussed here.
  • Laboratory experiments to assess the fundamental processes and controlling factors. They are likely to include use of idealised artificial ice experiments, to determine quantum yields and uptake coefficients, diffusion coefficients, reaction rates and equilibrium constants, as well as controlled studies of natural snow to isolate the factors controlling fluxes.
  • Use of satellite data to determine the spatial and seasonal distribution of key species and environments. For example, it is expected that far more specific indicators of sea ice type (including algorithms for frost flower extent) will be developed, which can be correlated with the occurrence of BrO or of ozone loss.
  • The field studies above will necessarily be limited to fairly local areas, and the satellite data will mainly look at columns and will be less able to assess the extent of phenomena (such as boundary layer ozone depletion) at ground level. To gain a full picture of the spatial extent, it will be necessary to develop autonomous instrument packages that could produce a low resolution year-round record of key species at unattended sites. Such platforms would be a highly appropriate contribution to the planned International Polar Year (IPY; 2007-08). While IPY will not fall into this phase of AICI, the package development, which could be used by many nations, will.
  • Improvements in instrumentation will be needed to make the necessary measurements at the low concentrations, and under the difficult conditions, encountered at some sites
  • Simple models of the processes within the upper snowpack and the lower atmosphere will be used to assess the full range of likely impacts of the fluxes observed, and the possible influence on the larger-scale atmosphere, and on the material finally preserved in the ice core record.
  • 1D and 3D chemical transport models will need to incorporate ice-atmosphere interactions

 

Task co-ordination

The task coordinators will be:

Dr Eric Wolff, British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK. E-mail: ewwo AT bas.ac.uk

Prof. Paul Shepson, Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907-2084, USA. E-mail: pshepson AT purdue.edu

AICI should coordinate international research and education activities along the lines described, and efficiently leverage multinational research, human, and logistics resources. We propose to create an AICI Steering Committee, with representation from key participating countries, and critical science areas. The function of the SC will be to disseminate information about activities falling within the AICI remit, to coordinate such activities, and to encourage activities that are not currently funded or planned.

Timetable and other activities

Some of the necessary field activities are already funded and planned, which is fortunate given the long lead time of polar field studies, and the need for many of them to run for a year or more. We propose that AICI officially commences at the start of 2004. The data from several of the main field campaigns should become available throughout the task period. In order to stimulate linking activities between them, as well as further field and modelling studies, we will take advantage of the plan to have an air-ice session at the IGAC conference in Christchurch in September 2004, and will hold AICI meetings in conjunction with it. We also plan to hold small workshops where the data from the campaigns, laboratory and other work can be discussed, during 2005-6. The task will close with an AICI symposium and a related journal special issue late in 2006, at which future plans, and the need for a further task, will be discussed.

A tentative timetable for AICI is:

Jan 2004 Start of AICI
2004-06 Major field campaigns, laboratory experiments, satellite data retrievals and model development
Sept 2004 AICI sessions at Christchurch IGAC symposium
2005-06 Workshops to disseminate and co-ordinate data
Late 2006 Journal special issue and AICI symposium

 

Education and capacity building

Many educational institutes would be involved in the proposed work. The effect of a concerted series of funded campaigns, linked by additional laboratory and modeling studies, will be to create a group of students with a strong interest in this topic, and our workshop and symposium activities will bring them together. The planning for an international polar year campaign will allow other nations, including the many with Antarctic presence, to develop a capacity in this field of work. An AICI web site, linked upwards to the IGAC website, and downwards to individual campaign web sites, will be maintained throughout the period of the task. We will also coordinate wherever appropriate with other IGAC and IGBP efforts to educate the public about the impacts of climate change on environmental systems, and in particular about the role of the cryosphere in the biogeochemistry of the Earth System.

 

References

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