One of the important pieces of science that goes on down here is the measuring of atmospheric conditions by the Atmospheric Research Observatory team. Run by the National Oceanic and Atmospheric Administration (NOAA), two of the scientists are in charge of measuring important factors such as particulates in the atmosphere, how many parts per million of CO2 are in the atmosphere, and take samples for further analysis back in the USA of rarer gases such as fluoridated species that are important for depleting ozone.
One of the most important, and highly variable, conditions to monitor is the level of ozone in the outer layers of the atmosphere, particularly the stratosphere, where it varies on a highly seasonal basis.
Ozone is critically important for protecting us from the harmful ultraviolet B light (UV-B) (280-315 nm wavelength, as different to UV-A which is 315-400 nm) that is emitted from the sun. Its effects on us are well known in New Zealand, where our lax sun protection and tendency to enjoy long periods outdoors coupled with proximity to the ‘ozone hole’, result in us having the highest rates of melanoma in the world, as well as massive amounts of squamous and basal cell skin cancers. Having done a year of General Surgery covering skin cancers in a provincial centre in NZ, the burden on people health and the system is remarkable.
Ozone is a molecule containing three oxygen atoms, and its name derives from ozien (οζειν) the greek work ‘to smell’, due to its pungent odour. It is the same smell you get when using an electric drill, as the electrical discharges in a motor generate small amounts. The vast majority of it (90%) hangs out in the stratosphere, which begins at 10-16km up, and extends to 50km. Within that, there is a band of higher density, known as the ozone layer. The other 10% is in the troposphere, the lowest layer, in which we reside. Even in the areas of highest density, it is still only at densities around a couple of molecules per million molecules of O2 or N2. Despite its low concentration, it’s pretty important when in the stratosphere. The tropospheric bit is little help in UV-B protection, and itself is harmful to us, plants, and is even a greenhouse gas. So when using your electric drill, try and be 20km above the earth’s surface. You’ll need a long extension cord though.
Ozone is made by the UV degradation of O2 into two O atoms. Some of these highly unstable oxygen atoms combine with O2 molecules to form ozone. This therefore is a sun dependent process, and most occurs in the tropics where the sun’s energy is most concentrated. Some is also made due to polluting processes in the troposphere, but this contributes minimally. It is broken down by hydrogen, nitrogen oxides and substances containing chlorine and bromine.
The behaviour of ozone above the Antarctic continent is unique compared to elsewhere in the stratosphere. Ozone-depleting substances (ODSs) are dispersed throughout the stratosphere by circulating air currents, from where they are predominantly made by the more industrial northern hemisphere, to relatively symmetric distributions throughout the stratosphere. In the Antarctic, the extreme cold of winter (due to lack of sunlight, and circumpolar vortex that traps the cold air) and early spring facilitate the development of polar stratospheric clouds (PSCs) – when temps fall below -78C in the stratosphere, water condenses together and freezes, sometimes with nitric acid. These clouds in the stratosphere act as an surface for the conversion of the more destructive chlorine monoxide (ClO) to be formed from its source chlorofluorocarbons or other ODSs. In the Antarctic, these clouds are present for around 5-6 months over winter, whilst in the arctic, the winter does not get cold enough, and the PSCs can often only be present for 1 to 2 months.
However, whilst ClO is made throughout winter in abundance on PSCs, that is not enough for ozone destruction. ClO is a catalyst for ozone destruction, and each ClO molecule can destroy hundreds-thousands of ozone molecules. The reaction needs sunlight which is only present in late winter and early spring. This allows ClO to catalyse destruction of O3 to O2 and causes rapid loss of ozone, and subsequent development of the ozone hole over Antarctica. Due to minimal PSC formation in the Arctic winter, little ClO is formed, and therefore little ozone destruction is catalysed, and the loss is therefore much more minimal.
By late spring, the stratosphere has warmed enough for PSCs to dissipate, ceasing production of ClO, and the ozone depletion tapers off. Over the summer, there is a lag in restoration of ozone levels, as it has to diffuse in from the stratosphere elsewhere in the world, and this takes time.
Ozone levels are measured at multiple places around the world, and in particular, multiple stations in Antarctica. At the South Pole, it is measured by two main methods – using a Dobson ozone spectrophotometer and by using ozonesondes attached to balloons that are launched up into the stratosphere on a regular basis.
Dobson, the pioneer of ozone measurements in the UK in the 1920s and 1930s, developed ways to measure the density of ozone in the atmosphere. The units of ozone are named after him – normal amounts of ozone are 200-500 Dobson Units. The spectrophotometer works on the same principle as a pulse oximeter that we use to measure oxygen saturations in a medical environment, as I found out during recent Part one ICU exam study. Both systems work according to the Beer-Lambert Law, which states that the transmitted light = incident light x e^(extinction coefficient for certain substance x distance travelled x concentration of substance). The extinction coefficient differs for different wavelengths of light. By knowing the distance travelled, and a known extinction coefficient, the concentration of a substance can be calculated. In a finger sats probe, we measure the concentration of oxygenated haemaglobin by comparing transmission of light 660 nm (red) and 940nm (Infra-red light). In a Dobson spectrophotometer, we use two wavelengths of ultraviolet light which have different absorptions by ozone to calculate ozone density. Therefore, a light source that is incident on the stratosphere is needed. In summer, the sun is used, and in winter, the reflected light of the moon is used. Factors like how cloudy it is need to be worked around (cloudy nights – well it is always night – suck for measurement). Similarly, the height of the moon above the horizon is used to measure the length of stratosphere the light is passing through – if the moon was straight above, the column of stratosphere passed through would be much shorter than when it is just above the horizon. The more stratosphere, the more light absorption.
The other method for ozone measurement is using an ozonesonde on a helium- filled balloon. These disposable units are released from the BIF – the Balloon Inflation Facility at regular intervals over the season, and increased frequency during late winter and early summer when the ozone depletion is greatest. A large plastic balloon is filled about 1% with helium at ground level (~3200m atmospheric pressure level). To it is attached a small insulated unit, containing a platinum electrode in a potassium iodide solution that reacts with ozone, creating a small electrical current. There is also an altimeter and radiotransmitter to get the info back to a receiving station, where it is processed by some pretty 1980s computing. After careful calibration and prewarming to ensure they don’t freeze during the period of data collection, they are bundled into an insulated box and slung beneath the large balloon. Balloon release is hugely dependent on wind, with a strong crosswind causing the releaser to have to run with the sonde to prevent it from smashing into the ground as it slews sideways before the balloon generates enough lift to allow it to clear the ground. As the balloon rises, the atmospheric pressure steadily falls, and as per Boyle’s law (more exam study coming in handy…) the volume increases proportionally, with the volume of helium eventually taking up the whole contents of the balloon, and lifting it up to 30km above the earth where it measures the ozone concentration on its way, transmitting levels back to earth. Eventually, the balloon disintegrates and falls to the polar plateau, or out to sea. Just one more piece of litter in the name of science. The sonde gives much more info that a simple Dobson measurement, as it gives a breakdown of ozone as the balloon passes through different altitudes.
The balloons are visible for varying amounts as they ascend through the atmosphere. Christian, one of my colleages got a fantastic series of shots showing the balloon as it launched and was caught by the wind. The majority of the time, they head off in one direction and are never seen again, but a couple of weeks ago, the nature of the polar air currents meant that the balloon recircled back overhead at the height of its path. It was before the sun had risen here on the ground, but the balloon was illuminated up in the atmosphere as it was high enough to catch the sun’s rays. The white balloon has a high albedo (reflects light well), and even though it was too small to resolve on its own, a small white dot was visible in the deep blue sky, at an altitude of 29km (26km above the surface).
In the mid 1980s, amongst the flares, peace signs and flower power, scientists discovered that the ozone level in the stratosphere over Antarctica was starting to be depleted on a seasonal basis. In what should be regarded as one of the better forms of human co-operation, people got together, actually listened to scientists and did something meaningful about it. It was noted that there were increasing rates of ozone depleting substances (ODSs) in the atmosphere, in particular chloroflurocarbons. Here was a group of chemicals that were pretty handy to us. Compress them under pressure, and they heat up (as do all gasses). Then let them expand and they cool down. This principle was nice and handy for doing things like keeping food cold, and keeping us nice and temperate when we decide as a species to inhabit places like Arizona, or Queensland, or if we drive our car through Death Valley. They could withstand repeated compression and expansion without degrading, and their rate of cooling was efficient for energy expended. But unbeknownst to us (before we listened to scientists), they were destroying the sky above us, and subjecting future generations to increased risk of malignancy, as well as other environmental damage to crops and species.
So we did what rational people do, we listened to evidence, and came up with better solutions. That triumph for logic was the 1987 United Nations Montreal Protocol, which with a series of subsequent amendments over the next two decades, enforced a reduction and then cessation of production and use of the most harmful ODSs, and a transition to less harmful substances. The onus was on developed countries that had the resources to make the shift to lead the way, with developing countries to follow behind.
Chlorofluorocarbons (CFCs) were transitioned to hydrochloroflurocarbons (HCFCs) for refrigeration, making insulating foam and as solvents. The presence of a hydrogen atom means that they are more reactive in the troposphere on their way to the stratosphere, and are removed in the process, preventing damage to the ozone layer. Further subsequent transition to hydroflurocarbons (HFCs) has been further progress, as the absence of chlorine or bromine means there is no ozone destruction potential for HFCs.
Wow, mankind working together, using global legislation to protect our environment. It’s great we are replicating this effort with the much bigger atmospheric environmental challenge in controlling CO2 – oh wait, we’re languishing in gridlock, giving unnecessary respect and weighting to fringe scientists who oppose the 98% of scientists, picking petty fights about how best to make change, trading in fraudulent carbon credits (cough * NZ government * cough), printing glossy brochures about superficial government ruminations without making meaningful progress towards carbon neutral options. In the meantime, CO2 continues to rise, the majestic Antarctic Ice sheets wither away, and weather systems become more and more variable leading to more flooding and famine. Shame we can’t replicate the co-ordination of the 90’s. Just as mankind has forgotten how, and now can’t work out how to replicate the Saturn V rocket that powered the Apollo missions to the moon, it seems we have regressed in our ability to co-operate on significant environmental issues.
Thanks to our curtailed use of ODSs, ozone levels are back on the rise, and the seasonal Antarctic ‘ozone hole’ is gradually diminishing. The lag is due to the long lifespan of chlorine-containing substances in the stratosphere, as well as the residual use of older technology that still contain banned substances. But at least we can make a difference to some of the havoc mankind wrecks on this planet, even if it means a balloon or three strewn every couple of hundred kilometers across the Antarctic plateau.