Article: Bromine explosions and Antarctic ozone

 

Towing the instrument across the sea ice in McMurdo Sound with Mt Erebus in the background.(Photo: Tim Hay)

Tim Hay, Karin Kreher, and Katja Riedel have tested a new instrument system that can measure important atmospheric chemistry where the action is – out on the sea ice.

Everyone’s heard about the ozone hole: it’s a severe reduction of the ozone layer in the stratosphere and in recent years it has developed over Antarctica each spring. However, few people realise that sudden and rapid ozone depletion also occurs in the lowest part of the polar atmosphere, in the marine boundary layer. These springtime ozone depletion events can happen in just a few hours, with ozone concentrations dropping from normal levels of 25–40 parts per billion (ppb) to as low as 0.05 ppb. Even though the small amount of ozone this close to the ground has no significance for filtering harmful UV rays, it is important to understand the processes involved in its depletion.

Bromine explosions

The main mechanism for this sudden loss of ozone involves release of bromine compounds from sea salt, in what’s known as a ‘bromine explosion’. Sea salt is exposed to the atmosphere in various ways: from young sea-ice surfaces, frost flowers, snow pack, sea water, and marine aerosols. We know from satellite remote sensing that extensive areas of enhanced atmospheric bromine oxide (BrO) form over the Antarctic sea ice in the spring. This formation often coincides with ozone depletion measured by ground-based instruments. We don’t fully understand the conditions that trigger this depletion, but the presence of first-year sea ice and an inversion layer (which acts as a barrier to exchange with the air above) are known to be significant.

Atmospheric bromine is important, not only in the polar marine boundary layer, but also in the free troposphere above the boundary layer. It is involved in many critical chemical reactions. When BrO oxidises gaseous mercury, harmful water-soluble mercury is deposited on the sea ice and then released into the sea during the spring melt. Bromine can potentially influence global climate by removing ozone (which absorbs solar radiation), by oxidation of greenhouse gases such as methane, and by altering the production of cloud condensation nuclei, the minute particles upon which clouds form. Conversely, the potential impact of climate change on the chemistry of bromine, ozone, and mercury – by altering the extent of sea ice cover through warmer temperatures – is uncertain and needs further investigation.

First measurements on the sea ice

Katja Riedel and the instrument on the beach at Cape Bird. (Photo: Tim Hay)

NIWA scientists have been measuring tropospheric BrO since 1995 and surface ozone since 1997 at the Arrival Heights observatory near Scott Base. They have observed several bromine explosions there each spring. However, because the marine boundary layer is often lower than the observatory and prevailing wind at Arrival Heights is from inland, some bromine explosions have probably gone undetected. To obtain measurements out on the sea ice for the first time in Antarctica, we mounted a three-month field campaign with a prototype instrument system developed at NIWA in Lauder.

The insulated, weatherproof, battery-powered system includes a spectrometer to measure BrO, a webcam to keep an eye on cloud cover, a monitor to measure surface ozone, and a weather station. The batteries are charged by a combination of solar power and a small generator.

How bromine explosions are detected

After two weeks of testing and calibration at Arrival Heights, we towed the instrument to a site on the sea ice in McMurdo Sound, downwind of Arrival Heights. This first measurement period from mid-September to mid-October provided a useful comparison of the air chemistry over the sea ice and at Arrival Heights.

The spectrometer repeatedly scanned from the horizon to the zenith, completing one full scan every 20 minutes. By comparing the spectra from a series of set elevation angles, we were able to retrieve a relative measure of BrO in the boundary layer throughout daylight hours (see ‘Scanning the sky’).

Elevated BrO should generally coincide with low ozone during bromine explosions, although various causes, such as pollution and transport of air masses from other regions, can negate this relationship. The origin of the air that is sampled is important, since air masses passing over sea ice are more likely to carry ozone-depleted air from a bromine explosion than air masses arriving from inland. However, ozone-depleted air masses can sometimes travel several hundred kilometres before normal ozone levels are restored through mixing with ozone-rich air. We used a combination of local wind direction measured at the instrument site and back-trajectory calculations from a chemical-transport model to determine air mass origins.

High BrO and low ozone

This satellite image shows elevated BrO levels (red and orange) in the regions of the Ross Sea (R) and Weddell Sea (W).

We detected at least two ozone depletion events on the sea ice during our campaign, including one that the instruments at Arrival Heights missed. Preliminary analysis indicates elevated BrO levels in the boundary layer during these events. Satellite images also show elevated BrO over the Ross Sea region on these days (see image at right). The occurrence of polar tropospheric ozone depletion events is unpredictable and infrequent, so it is rewarding to have observed these two with the remote instrument, especially the one on the sea ice that went undetected up at the observatory.

Measurements at Cape Bird

The second half of the measurement campaign, from mid-October until late November, was at Cape Bird on the northern end of Ross Island. The sea ice there had recently broken out and refrozen, and the generally calm conditions were favourable for sampling ozone from air masses coming from the sea ice zone. We detected slightly elevated BrO in the troposphere at the end of October when the temperature was around –20 °C and the freshly opened tide crack and leads refroze. Delicate ice crystals called frost flowers had also formed on the refreezing surfaces.

Plans for the International Polar Year

Our prototype instrumentation had a very successful first field campaign in Antarctica. Having completed initial testing and calibration in the first season, we plan to take the instrument to more remote sites downwind of large areas of young sea ice. Some modifications, such as reducing power consumption and increasing the wind and solar energy collection, will prepare the instrument for greater autonomy at more remote sites in the spring of 2007. This next campaign also marks the first austral spring of the International Polar Year that runs from March 2007 to March 2009 with a burst of Antarctic research activities.

Scanning the sky

Multi-axis differential optical absorption spectroscopy (MAX-DOAS) uses the unique spectral absorption characteristics of different trace gases to determine their abundance in the atmosphere. Slightly different horizon viewing angles all have a similar light path through the stratosphere; the light path in the troposphere and boundary layer is extended and significantly increases with decreasing viewing elevation angle.

If the trace gases of interest are present in the lowest part of the atmosphere, they will show up as greater quantities at low elevations relative to high viewing angles of the instrument. This is due to the greater absorption of light by molecules of the trace gas along extended light paths. Detection of trace gases in the stratosphere, on the other hand, is much more sensitive to the angle of the sun and not to the viewing angle of the instrument.

Teachers’ resource for NCEA Achievement Standards or Unit Standards:
Science Level 1 US6349
Technology Level 1 AS90051
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