COBRA - Background Information


  1. Introduction
  2. Rationale
  3. Field site, Measurements and techniques, and Deliverables

1 - Introduction

Throughout the Arctic and Antarctic coastal regions, episodes of rapid ozone and mercury depletion occur every spring. These events can be sustained (occurring over periods of days to weeks) and can extend from the surface up to ~2.5 km (Roscoe et al., 2001; Bottenheim et al., 2002). Tropospheric ozone is a significant greenhouse gas, whilst mercury is a toxic, persistent and bioaccumulative pollutant, thus these phenomena have implications for the atmospheric oxidative capacity, climate and health. The observed rapid and dramatic ozone and mercury depletion at polar sunrise (with less frequent and dramatic events during autumn) is mediated by exchange of halogen species between the snowpack and sea-ice surface and the overlying atmosphere, yet many aspects of the underlying chemistry and physics are not well understood. The halogen species may also impact Arctic haze by enhancing photochemical reactions which produce fine aerosol. Since the nature and extent of snow and ice cover in polar regions is now changing at a significant rate (Yu et al., 2004), and this appears to be reflected in the magnitude and extent of ozone and mercury depletion events (Hollwedel et al., 2004), it is imperative to develop a quantitative understanding of the relevant ocean-snow-ice-atmosphere interactions, in order to predict the future state of the Arctic (and Antarctic) atmosphere and its relation to climate, health, and environmental change. Such improved knowledge is also relevant for processes occurring in lower-latitude winter environments.

2 - Rationale

2.1 - Previous work on polar bromine release and ozone and mercury depletion events

The key role played by bromine in ozone and mercury depletion events has been described in numerous studies (Bottenheim et al., 1986; Schroeder et al., 1998; Bottenheim et al., 2002; Spicer et al., 2002, and references therein). The suggested autocatalytic heterogeneous mechanism involves uptake of HOBr into slightly acidic and concentrated brines (containing enhanced Br- and Cl-) on sea-ice/snow-pack/sea-salt aerosol surfaces resulting in release of gaseous Br2 and BrCl, which are rapidly photolysed to bromine atoms:

 
aqueous phase, ice
   
HOBr + X- + H+
 → 
BrX + H2O,            where X = Br, Cl.

Every HOBr molecule entering the liquid phase has the potential to release two bromine atoms to the gaseous phase, resulting in an exponential increase of Br, the so-called "Bromine explosion".

The resulting photolysable BrX molecules rapidly produce Br atoms that enter various ozone-destroying cycles:

Cycle I      Cycle II
2Br + 2O3  →  2BrO + 2O2      Br+O3  →  BrO + O2
BrO + BrO  →  2Br + O2      BrO + HO2  →  HOBr + O2
         HOBr + hv  →  Br + OH
         OH + CO (or VOC)  →  HO2 + CO2 (or VOC products)

Cycle I will dominate at high BrO levels, and cycle II at low BrO levels. The re-formation of soluble HOBr(g) in Cycle II, which re-enters the aqueous phase either through uptake on sea-salt aerosol or direct deposition to the snow-pack/sea-ice, results in an autocatalytic mechanism.

Schroeder et al. (1998) first observed the rapid decrease in Hg atoms, now referred to as "MDEs" (mercury depletion events) in the Arctic. The MDEs are accompanied by increases in oxidised, but so far unspeciated, mercury in the snowpack, potentially leading to bioaccumulation of soluble and toxic forms of mercury. Recent measurements of the rate coefficients for reactions of a number of halogen gases with Hg suggested that Br atoms could be a major reactant with Hg in the polar atmosphere (Ariya et al., 2002), and subsequent studies have supported this (e.g. Calvert and Lindberg 2004a; Goodsite et al., 2004; Skov et al., 2004). BrO may also be an important reactant, but there are insufficient studies on its rate of reaction with Hg vapour and its concentration in the polar boundary layer. There has been a recent increase in Hg in the Arctic biosphere, and it has been hypothesized that this results primarily from increases in atmospheric oxidant levels and/or reactive bromine rather than from an increase in Hg emissions (OASIS science plan).

It has been suggested that ozone and mercury depletion events apparently operate almost exclusively in polar spring either (a) because only this season has the combination of strong surface inversions (i.e. isolation of boundary layer air from free tropospheric air), sunlight and sea ice (Lehrer et al., 2004), or (b) because the asymmetry of sea ice/frost flower production compared to the amount of sunlight leads to a strong spring maximum in their product (Kaleschke et al., 2005). However, both of the proposed drivers for this seasonality, and the initial source of bromine atoms required to form HOBr and thus initiate the chain destruction of ozone, are controversial.

2.2. - Potential role of frost flowers and other surfaces for halogen release

There is substantial evidence that sea salt, either on the sea-ice or snow-pack surface or contained in aerosol, is almost exclusively the source of bromine liberation in polar regions. The high salinity and surface area of frost flowers, which are ice crystals that grow on newly forming sea ice, along with their widespread occurrence as part of the normal process of new sea ice formation, has recently led to proposals that they may be an important part of this process (e.g. Rankin et al., 2002). From an analysis of satellite BrO data and sea ice coverage, Kaleschke et al. (2004) showed that areas potentially covered in frost flowers (FF), i.e. with cold temperatures and open leads, are plausibly the dominant source of bromine in both the Arctic and Antarctic. The exact nature of emissions from frost flowers is not well established and is not yet explicitly treated in global models, and so far there are no in situ field observational studies to confirm or otherwise the important role of FF in bromine release. It is vital that field data is obtained on the nature and magnitude of emissions of halogens as a function of age and type (air/sea-ice differential temperature) of frost-flowers and other sea-ice surfaces, in order to predict the spatial and temporal variability of halogen release, and how this might change in the future.

Brines and frost flowers themselves may be responsible for a significant fraction of sea salt aerosol concentrations over the polar regions (Rankin et al., 2002). However, under typical Arctic conditions, even a flat sea-ice surface (i.e., no frost flowers) provides a factor of ~1000 higher surface area than sea salt aerosol (Lehrer et al., 2004). The latter modelling study of Lehrer et al. (2004), although highly parameterised, found that sea salt aerosol alone was not sufficient to sustain the elevated levels of gas phase bromine observed in polar spring, although is an efficient recycler of more stable bromine species such as HBr. However, the study of Evans et al. (2003) was able to adequately simulate the chemistry occurring within an ODE through the use of aerosols alone. Thus there are competing hypotheses for the relative role of the surface and aerosol phase in the production and recycling of bromine. The full suite of analytical tools we will deploy during COBRA will allow these hypotheses to be tested more thoroughly than previous studies have allowed.

2.3 - Potential sources and role of iodine in the Arctic

Little is known about the role and sources of iodine in polar boundary layer chemistry. Iodinecontaining aerosol has been associated with ozone depletion at polar sunrise but also appears in autumn. A well-known and obvious source of iodine compounds are under-ice diatoms, which grow there particularly well because of the benevolent light conditions through less than 1 m of ice, compared to those of open water where they are mixed downward to depths of tens of metres. Many diatoms synthesise methyl iodide and other organic iodine compounds, and the chlorophyll they produce is observable from space in large quantities in open water in the sea ice zone in early summer. However, recent work has suggested abiotic/photochemical sources of iodine in the sea ice zone. In a 10-month campaign at Halley station, Antarctica, the iodine oxide radical, IO, was observed using a boundary layer DOAS (Differential Optical Absorption Spectrometer) (A. Saiz-Lopez and J. M.C. Plane, part of the COBRA team, unpublished data). A clear seasonal cycle was observed: IO was below the detection limit (~1.5 ppt) during polar night, and reached levels of several ppt during spring and summer. Preliminary modelling indicates that the source of the iodine during summer is likely to have been emission from the snowpack, arising from photochemical or thermal processes, rather than directly from the ocean.

Further evidence of iodine in the polar atmosphere comes from recent observations of reactive organoiodines including di-iodomethane (CH2I2) during spring at Kuujjuarapik on the south-east shore of Hudson Bay, at the highest concentrations ever observed in air and associated with trajectories across the frozen Bay with no observable leads (L. J. Carpenter et al., COBRA PI, submitted to ES&T, 2005). The absence of local ice leads coupled with the extremely short atmospheric lifetime of CH2I2 indicated that production was from the surface of the sea-ice/snowpack over Hudson Bay. We proposed an abiotic mechanism for the production of polyhalogenated iodo- and bromocarbons, via reaction of HOI and/or HOBr with organic material on the quasi-liquid layer above sea-ice/snow pack, and reported laboratory data to support this mechanism. The presence of this mechanism in the natural world (as opposed to in the lab) is at present entirely speculative, however, it might explain the so far puzzling enrichment in CHBr3 observed in upper sea-ice (enhanced in salinity but low in chlorophyll) of the Canadian Arctic (). It also suggests the possibility for photochemical inorganic iodine release from sea-ice, snow-pack and/or FF, analogous to that recently observed by Zingler and Platt (2005) over the Dead Sea, and consistent with the Antarctic IO observations of Saiz-Lopez and J. M.C. Plane described above. Photochemical reactions in the snow are known to release a wide range of trace gases to the polar atmosphere (e.g. Sumner and Shepson., 1999). Photochemistry within sea-ice is also perfectly feasible, with ice potentially more transparent to the UV than to visible radiation (Belzile et al., 2000), yet has not been well studied. In sea-ice, fresh-water ice co-exists with solid salts, concentrated brines, gases and marine-derived organic material, thus starting material is present for both organic and inorganic halogen release.

These recent and so far unpublished IO and organoiodine observations of the COBRA team, in separate polar locations, suggest a widespread and likely abiotic/photochemical source of iodine to the polar atmosphere. The impact of such emissions may be significant. Theoretical studies indicate that iodine compounds added to a Br2/BrCl mixture in the Arctic atmosphere have a significantly greater ozone and mercury depletion effect than additional bromine molecules (Calvert and Lindberg, 2004a and 2004b). The role, if any, of iodine compounds on Hg-atoms is unknown. Direct reactions of I atoms with Hg may be unimportant (Goodsite et al., 2004), however a recent study by Calvert and Lindberg (2004b) points out a potential indirect effect of iodine through an enhancement in [Br] mediated by IO and BrO interactions. In mid-latitude coastal locations, release of iodine is known to result in new particle formation and there is a possibility that similar mechanisms operate in the Arctic.

2.4 - Potential for future change in Arctic halogen-related processes

The natural phenomena described above are likely to be affected by man-made changes such as increased UV radiation reaching the surface due to stratospheric ozone depletion, acidic deposition affecting pH-dependent halogen activation processes, and changes in the production rate of new sea ice, altering the locations and extent of frost flowers and brines. There are already indications that Arctic ozone depletion events and associated deposition of Hg have increased in recent decades (Lindberg et al., 2002; Tarasick and Bottenheim, 2002; Hollwedel et al., 2004). Possible feedbacks such as these must be investigated in order to understand the effects of global change.

3 - Field Site, Measurements and techniques, and Deliverables

3.1 - Field site

The measurement campaign took place during February 2008 and March 2008, at Kuujjuarapik, on the south east coast of Hudson Bay in Quebec, Canada. The project was based at the Centre d'Etudes Nordique Whapmagoostui-Kuujjuarapik research station operated by Laval University, Montreal with measurements being made at the research station, and a temporary site on the coast a few kilometres away. The coastal site consisted of two container labs on the shore with additional measurements being made approximately 300m out on the sea ice. The Whapmagoostui-Kuujjuarapik research station is equipped with research laboratories, a weather station, and accommodation blocks, and also has existing long term O3 and Hg measurements.

3.2 - Measurements and techniques

The major activity is a 5-week ground-based field campaign at Kuujjuarapik, Hudson Bay, during February-March 2008. Data generated will include in-situ measurements, remotely-sensed sea-ice and halogen oxide information, frost flower calculations, flux measurements of various ice surfaces, and aerosol production and evolution modelling (the latter will not be archived).


More photos available from COBRA Photo Album at the Centre for Atmospheric Science, The University of Manchester,

Details of measurements and techniques are given in the table below.

Kuuj = Kuujaparik campaign, Flux = flux measurements from lead, Lab = laboratory measurements of frost flower surrogates.

MeasurementsTechniqueGroupSite/s
I2/IO/BrO/OIO Long path DOAS, MAX-DOAS and CE-DOAS University of East Anglia Kuuj
Br2/BrCl APCI-MS Battelle Memorial Institute, US Kuuj, Flux
Reactive iodine and bromine organohalogens 2 x GC-MS University of York Kuuj, Flux, Lab
Offline I2Denuder tubes/ ICP-MS University of YorkFlux
Aerosol number-size distributions, size segregated particle fluxes UHSAS, CPCs and SMPSUniversity of ManchesterKuuj, Flux, Lab
Offline chemical (including iodine) analysis of aerosols TEM and EDX University of Manchester Kuuj, Flux
Micrometeorological techniquesEnclosure chambers, eddy correlation, multi-heightUniversity of ManchesterFlux
CO, VOCs, OVOCs Aerolaser GC-FID University of York Kuuj
O3, NO, NO2 ANNOXUniversity of York Kuuj, Lab
Total gaseous mercury, mercury speciation (Hg and reduced gaseous mercury) and total particulate mercury, O3 Tekran 1130, 1135 & 2537 (mercury suite) + ozone TECO Meteorological Service Canada, Montréal, Canada Kuuj
Dissolved Hg in snow (HgT, Hg reactive, Hg dissolved and particulate; methylmercury), microbiology in snow Mercury suite, Molecular microbiology techniques Meteorological Service Canada, Montréal, Canada Kuuj
UV Spectra 4 p spectral radiometer, j-O1D and j- NO2 spectral radiometers University of Leicester Kuuj
Wind, temperature and ozone profiles, plus height of top of boundary layer. Met. sonde on tethered kite or balloon. BASKuuj
Chemical, physical and biological characterisation of ice BET method for specific surface area, ion chromatography, microscope examination and analysis, chlorophyll assays BASKuuj, Lab
Meteorological data (air temp, sea-ice temp, wind direction, wind speed) Standard met package CEN station Kuuj

3.3 - Deliverables

COBRA seeks to deliver an increased understanding of the chemistry, biology and physics of Arctic ocean-atmosphere-ice interactions, in particular, the role and sources of iodine in concert with bromine.

The specific project deliverables are:

COBRA will strengthen the science of polar sea-ice/atmosphere exchange, and allow many UK atmospheric chemistry researchers so far not active in this field, but with highly relevant expertise, to establish themselves in polar research and to develop links with international researchers in the field.