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about ozone, its production and depletion

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an Arctic polar stratospheric cloud, photographed in Sweden (filched from a website of NOAA’s Earth System Research Laboratory)

People will remember the ‘hole in the ozone’ issue that came up in the eighties I think, and investigators found that it was all down to CFCs, which were quite quickly banned, and then everything was hunky dory….

Or that’s how I vaguely recall it. Time to take a much closer look. 

I take my cue from ‘An ancient ozone catastrophe?’, chapter 4 of David Beerling’s The emerald planet, in which he looks at the evidence for a previous ozone disaster and its possible relation to the great Permian extinction of 252 millions years ago. I’ll probe into that matter in another post. In this post I’ll try to answer some more basic questions – what is ozone, where is the ozone layer and why does it have a hole in it?

Ozone is also known as trioxygen, which gives a handy clue to its structure. Oxygen can exist in different allotropes or molecular structures which are more or less stable. O3, ozone, is much less stable than O2 and has a very pungent chlorine-like odour and a pale blue colour. It’s present in minute quantities throughout the atmosphere but is most concentrated in the lower part of the stratosphere, 20 to 30 kilometres above the Earth’s surface. This region is called the ozone layer, or ozone shield, though it’s still not particularly dense with ozone, and that density varies geographically and seasonally. Ozone’s instability means that it doesn’t last long, and has to be replenished continually.

In 1928 chlorofluorocarbons (CFCs) were developed as a seemingly safe form of refrigerant, which, under patent as Freon, came to be used in air-conditioners, fridges, hair-sprays and a variety of other products. As it turned out, these CFCs aren’t so harmless when they reach the upper atmosphere, where the chlorine reacts with ozone to form chlorine monoxide (ClO), and regular O2. This reaction is activated by ultraviolet radiation, which then breaks up the unstable ClO, leaving the chlorine to react with more ozone in a continuing cycle.

By the eighties, it had become clear that something was going wrong with the ozone layer. Studies revealed that a gigantic hole in the layer had opened up over Antarctica, and without going into detail, CFCs were found to be largely responsible. There was the usual fight with vested business interests, but in 1987 the Montreal protocol against the use of ozone-depleting substances (ODS) was drawn up, a landmark agreement which has been successful in starting off the long and far from completed process of repair of the ozone shield.

As a very effective oxidant, ozone has many commercial applications, but the same oxidising property makes it a danger to plant and animal tissue. Much better for us to keep most of it up above the troposphere, where its ability to absorb UV radiation has made it virtually essential for maintaining healthy life on Earth’s surface. 

So here are some questions. Why does ozone proliferate particularly at the top of the troposphere, in the lower stratosphere? If it’s so reactive, how does it maintain itself at a particular rate? Has the thinning or reduction of that layer seriously influenced life on Earth in the past? From my reading, mainly of Beerling, I think I can answer the first two questions. The third question, which Beerling explores in the above-mentioned chapter of his book, is more speculative, and more interesting. 

Sidney Chapman, a brilliant geophysicist and mathematician of the early twentieth century, essentially answered the first question. He realised that ozone was both formed and destroyed by the action of sunlight, specifically UV radiation, on atmospheric oxygen. He calculated that this action would reduce and finally stop at a point approximately 15 km above sea level, because the reactions which had produced the ozone higher up had absorbed the UV radiation in the process. No activation energy to produce any more ozone. That explained the lower limit of ozone. The upper limit was explained by the lack of oxygen in the upper stratosphere to produce a stable layer – for production to exceed destruction. This was interesting confirmation of observations made earlier by the meteorologist and balloonist Léon-Phillippe Teisserenc de Bort, who noted that, contrary to his expectations, the air temperature didn’t fall gradually with altitude but reached a point of stabilisation where the air even seemed to become warmer. He named this upper layer of air the stratosphere, and the cooler more turbulent layer below he called the troposphere. It’s now known that this upper-air warming is caused by the absorption of UV radiation by ozone.

Our picture of ozone still had some holes in it, however, as it seemed there was a lot less of it around than the calculations of Chapman suggested. To quote from Beerling’s book: 

… there had to be some as-yet unappreciated means by which ozone was being destroyed. The fundamental leap required to solve the problem was taken comparatively recently, in 1970, by a then young scientist called Paul Crutzen. Crutzen showed that, remarkably, the oxides of nitrogen, produced by soil microbes, catalysed the destruction of ozone many kilometres up in the stratosphere. Few people appreciate the marvellous fact that the cycling of nitrogen by the biosphere exerts an influence on the global ozone layer: life on Earth reaches out to the chemistry of the stratosphere. 

Now to explain why the hole in the ozone shield occurred above the Antarctic. My understanding and explanation starts with reading Beerling and ends with this post from the USA’s National Oceanic and Atmospheric Administration’s Earth System Research Laboratory (NOAA/ESRL). 

The ozone hole over Antarctica varies in size, and is largest in the months of winter and early spring. During these months, due to the large and mountainous land mass there, average minimum temperatures can reach as low as −90°C, which is on average 10°C lower than Arctic winter minimums (Arctic temperatures are generally more variable than in the Antarctic). When winter minimums fall below around −78°C at the poles, polar stratospheric clouds are formed, and this happens far more often in the Antarctic – for about five months in the year. Chemical reactions between halogen gases and these clouds produce the highly reactive gases chlorine monoxide (ClO) and bromine monoxide (BrO), which are destructive to ozone. 

this graphic shows that the Antarctic stratosphere is consistently colder, and less variable in temperature, than the Arctic. Polar stratospheric clouds (PSCs) form at −78°C

Most ozone is produced in the tropical stratosphere, in reactions driven by sunlight, but a slow movement of stratospheric air, known as the Brewer-Dobson circulation, transports it over time to the poles, so that ozone ends up being more sparse in the tropics. Interestingly, although most ozone-depleting substances – mainly halogen gases – are produced in the more humanly-populated northern hemisphere, complex tropospheric convection patterns distribute the gases more or less evenly throughout the lower atmosphere. Once in the stratosphere and distributed to the poles, the air carrying the halogen-gas products becomes isolated due to strong circumpolar winds, which are at their height during winter and early spring. This isolation preserves ozone depletion reactions for many weeks or months. The polar vortex at the Antarctic, being stronger than in the Arctic, is more effective in reducing the flow of ozone from tropical regions. 

So – I’ve looked here briefly at what ozone is, where it is, and how it’s produced and destroyed, but I haven’t really touched on its importance for protecting life here on Earth. So that, and whether its depletion may have had catastrophic consequences 250 million years ago, will be the focus of my next post. 

References

The Emerald Planet, by David Beerling, Oxford Landmark Science, 2009

https://www.esrl.noaa.gov/csd/assessments/ozone/2010/twentyquestions/Q10.pdf

https://en.wikipedia.org/wiki/Ozone

https://en.wikipedia.org/wiki/Brewer–Dobson_circulation

Written by stewart henderson

October 3, 2018 at 9:24 pm

the ACCC, coal, renewables, arguments, and the future

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Well as I watch my readership reduce to almost zero in its usual ups and downs I wonder whether to write just for myself or to attract a readership, so I’ll just go ahead and write, but I was amused to listen to Senator Matt Canavan, our Minister for Resources and a member of the Nationals, responding to the ACCC’s recommendations for bringing energy prices down. At one point he remarked ‘who cares where we get our fuel from?’, and compared the different fuel varieties to different types of ice-cream in a sweet shop. Presumably he was referring to encouraging an energy mix, but for someone who presumably knows something about resources, since he holds that portfolio (though that’s hardly ever proof of expertise in government), it struck me as bizarre. Who cares where we get our fuel from or what type it is? The Chinese government cares, for one. It has worked hard in recent years to combat pollution in Beijing, largely  in response to adverse publicity. China’s capital, ranked as the fifth most polluted city in China in 2011, has since dropped out of the top twenty, largely due to the adoption of cleaner, greener energy and technology. Unfortunately, many other cities in China’s highly populated and industrialised north-western region still suffer from an environment which has reduced life expectancy there by some 5.5 years, according to a joint study by Chinese and American university teams in 2013. This sadly suggests that the Chinese government appears to be more concerned with its international image than with protecting its own citizens from hazardous emissions. On the bright side, Beijing’s improvement indicates what can be done to improve environments when governments and industry get their act together.

Just as oils aint oils, fuels aren’t just fuels. Remember kerosene? I remember huddling over a kerosene heater in the seventies, along with student housemates. But in other parts, kero isn’t a past-tense energy source. In many of the poorest countries, particularly in Africa, it’s used for lighting, even though it’s toxic, causes frequent burns and fires, and produces inferior light. It has proved difficult to wean consumers from kerosene in these countries, even though there are potentially cheaper options available. There’s an interesting article about the problems and possible solutions here.

But really, since energy generation (i.e. using x,y, or z as fuel) is the number one cause of air pollution and global warming emissions, it’s not like comparing caramel praline with black raspberry crunch. Coal is of course the worst in terms of emissions. As of 2016, some 44% of US electricity comes from coal, but it accounts for 80% of that country’s power plant carbon emissions. Australia has great reserves of coal, but it exports much of it to China and, more recently, South-East Asia. In fact Australia has experienced a recent boom in coal exports, earning a record $56.5 billion in 2017. Unsurprisingly many conservative pollies are clamouring for more coal mines and more local use of the resource as a solution to our seemingly ever-rising energy costs. Maybe we too can pull out of the Paris Agreement? Of course, our massive coal exports do tend to undermine that agreement, while the government can congratulate itself on keeping domestic use within more or less acceptable limits (see graph above). Currently, we’re the largest coal exporter and the third largest exporter of carbon pollution in the world, behind Saudi Arabia and Russia. But of course it’s not our fault that other countries want to pollute with our resources, is it? We just take the money and keep our country clean (as do Norway, Denmark and Indonesia).

So considering our dubious status in terms of global emissions (but, as many experts point out, it’s a little arrogant to expect developing and transitioning countries like China, our biggest coal customer, to rapidly abandon a fuel that the developed world has used for so long, thereby gaining ascendancy), it’s interesting to note that AGL, Australia’s largest owner of coal fired power stations and biggest emitter of carbon dioxide, is continuing its push away from coal in spite of government pressure. Of course the government itself is divided on this, with Turnbull and Frydenberg largely at odds with the Nationals on the question of transition, but looking into the future, it seems inevitable that demand for coal will decline – the only question is the rate of that decline, which of course depends on how quickly other nations move away from coal. All of those nations have signed the Paris Agreement. Already, coal ports such as Newcastle, and Australia’s mining regions, are looking to diversify, and energy experts are debating the pursuance of a coal tax to support the industry as it transitions.

But Canavan and the Nationals are having none of this. They point to the above-mentioned boom and a currently accelerating demand, though Canavan is realistic enough to admit that future forecasts are reliably unreliable. Much will depend on cost declines and advancing technology in renewables, as well as various political scenarios.

Naturally the renewable energy sector is looking critically at what one of its experts calls the ‘series of scattershot proposals’ by the ACCC on reducing our electricity costs. The ACCC’s recommendation that the small-scale renewable energy scheme (SRES), a subsidy which mainly applies to rooftop solar, should be wound down, is seen as unfair if not counter-productive by the sector. The SRES is already slated to be wound down by 2030, and its earlier abolition (by 2021, according to ACCC recommendations) would mainly affect low-income and rental householders. There’s currently a new boom in rooftop solar, with rising energy costs being the main cause. So penalising future adopters of rooftop solar seems an odd way to reduce the problems they’re adopting solar to avoid. As to the possibility of new gas- or coal-fired power plants, a dream of the Nationals and renegade ultra-conservative Tony Abbott, that’s unlikely, considering changing public attitudes and the reasonable likelihood of a change of federal government by next year. The good thing about the ACCC’s analysis is that the behaviour of retailers, and the phenomenon of price gouging, have finally been criticised, and the idea of states writing down the value of their networks has been floated. Consumers shouldn’t have to bear the burden of extra energy infrastructure and errors in predicting future energy demand.

There have been many interesting responses to the report, to say the least. Danny Price, a leading analyst of the national energy market over three decades, regards the report as overly political in that it shies away from criticising the lack of a much-needed bipartisan approach to energy policy. Confusion and ideological squabbling over carbon pricing – the disastrous scrapping by the Abbott government of a carefully formulated carbon tax being the low point – has been a disincentive to major investment, and banks here are refusing to finance new coal-fired power stations, which would only be built via massive government subsidies. Consequently we’ve seen an upsurge in interest in renewables from consumers and business, which also reflects worldwide growth, with major oil companies like BP joining the fray.

Of course the problem of reliable back-up power remains, and analyst Ian Verrender has criticised the ACCC report for omitting his best solution – gas. Gas turbines are more flexible than coal generators as well as producing fewer emissions. Australia is a major exporter of gas, but our companies have been providing little for domestic consumption, a situation which was only partly remedied by recent federal intervention. Yet the Nationals are more interested in coal than gas, in spite of its many problems, and its inefficiencies in providing precise back-up supply. Gas, hydro and batteries are far more efficient in this respect.

A recent study by the Australian Energy Market Operator (AEMO) has also backed renewables (though apparently the current federal government isn’t listening). It has released its Integrated Systems Plan, reported on here by Giles Parkinson:

Based on its “neutral” scenario, which comprises existing federal and state government policies, the lowest cost replacement [for retiring coal-fired suppliers] will be solar (28GW), wind (10.5GW) and storage (17GW and 90GWh). Just 500MW of flexible gas plant will be needed, and no new coal. It says this portfolio in total can produce 90TWh (net) of energy per annum, more than offsetting the energy lost from retiring coal fired generation.

AEMO has also highlighted the need for new transmission infrastructure, as transformative and disruptive energy developments continue around the country. The need for forward planning should be obvious and governments – especially the federal government – ignore this at their peril. A change of federal government may be the answer, but only if the incoming government has a thorough-going plan to integrate and manage this clear and obvious national move away from fossil fuels. Such plans are already being drawn up – we just need the will, and some bipartisan support, to implement them.

 

 

 

 

Written by stewart henderson

July 17, 2018 at 5:01 pm

Posted in ACCC, gas

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