Posts Tagged ‘Carbon Dioxide’

Hazards of Flying

November 17, 2019

Radiation Dose

Radeye in Cabin

RadEye Geiger Counter on my lap in the plane.

It is well-known that by flying in commercial airliners, one exposes oneself to increased intensity of ionising radiation.

But it is one thing to know something in the abstract, and another to watch it in front of you.

Thus on a recent flight from Zurich I was fascinated to use a Radeye B20-ER survey meter to watch the intensity of radiation rise with altitude as I flew home.

Slide1

Graph showing the dose rate in microsieverts per hour as a function of time before and after take off. The dose rate at cruising altitude was around 25 times on the ground.

Slide2

During the flight from Zurich, the accumulated radiation dose was almost equal to my entire daily dose in the UK.

The absolute doses are not very great (Some typical doses). The dose on flight from Zurich (about 2.2 microsieverts) was roughly equivalent to the dose from a dental X-ray, or one whole day’s dose in the UK.

But for people who fly regularly the effects mount up.

Given how skittish people are about exposing themselves to any hazard I am surprised that more is not made of this – it is certainly one more reason to travel by train!

CO2 Exposure

Although I knew that by flying I was exposing myself to higher levels of radiation – I was not aware of how high the levels of carbon dioxide can become in the cabin.

I have been using a portable detector for several months. I was sceptical that it really worked well, and needed to re-assure myself that it reads correctly. I am now more or less convinced and the insights it has given have been very helpful.

In fresh air the meter reads around 400 parts per million (ppm) – but in the house, levels can exceed this by a factor of two – especially if I have been cooking using gas.

One colleague plotted levels of CO2 in the office as a function of the number of people using the office. We were then able to make a simple airflow model based on standard breathing rates and the specified number of air changes per hour.

Slide5

However I was surprised at just how high the levels became in the cabin of an airliner.

The picture below shows CO2 levels in the bridge leading to the plane in Zurich Airport. Levels around 1500 ppm are indicative very poor air quality.

Slide3

Carbon dioxide concentration on the bridge leading to the plane – notice the rapid rise.

The picture below shows that things were even worse in the aeroplane cabin as we taxied on the tarmac.

Slide4

Carbon dioxide concentration measured in the cabin while we taxied on the ground in Zurich.

Once airborne, levels quickly fell to around 1000 ppm – still a high level – but much more comfortable.

I have often felt preternaturally sleepy on aircraft and now I think I know why – the spike in carbon dioxide concentrations at this level can easily induce drowsiness.

One more reason not to fly!

 

 

 

Critical Opalescence in Carbon Dioxide

January 27, 2016

One feature of the teaching at Dalhousie University’s Physics Department is a laudable emphasis on demonstrations.

Visiting Professor Tom Duck there, I was delighted to be shown a demonstration I had heard of, but never seen: the phenomenon of critical opalescence in carbon dioxide.

I have written about critical opalescence previously on this blog (here) and with more pictures (here), so I won’t repeat most of that.

In my previous articles I described the phenomenon in two immiscible liquids which is an exact analogy for the physics of critical opalescence in a pure substance. But it’s not what physics students read about in text books.

Michael: What are you going on about?

The phenomenon occurs when one heats a liquid in a container with a small amount of free space.

  • As the liquid heats up, it expands causing its density to fall.
  • The liquid also evaporates causing the vapour (gas) pressure to increase.
  • The critical point is where the density of the liquid matches that of the vapour.

Above this ‘critical’ temperature and pressure, the substance forms a single fluid with no distinct liquid state. In the movie you can see that the meniscus at the top of the liquid just gradually disappears – there is now no ‘surface tension’.

At the critical point, the density of the fluid is typically one third of the liquid density at atmospheric pressure. Because there is no difference between liquid and gas, the latent heat associated with evaporation (when molecules move from the liquid to the gas) and condensation (when molecules move from the gas to the liquid) falls to zero.

Critical Opalescence can be seen when cooling just below the critical temperature.

The random motion of the molecules in the fluid causes some regions to transiently have densities that are slightly greater than the average (more typical of the liquid) – and others to have densities more typical of the gas.

Because the latent heat and the surface tension are very close to zero, these microscopic fluctuations can grow dramatically. Spontaneous fluctuations can cause regions as large as a thousandth of a millimetre – containing thousands of billions of molecules -to fluctuate into and out of the liquid state – forming droplets.

Although the difference in density (and hence refractive index) between the liquid droplets and the gas is tiny – it is just enough to scatter light – like a fog – a phenomenon which someone poetically named ‘opalescence’ rather than fog.

Critical Opalescence is mentioned in Physics course, but  it is rarely seen. The high pressures involved (more than 73 atmospheres in this case) present a hazard that few people are prepared to tackle. I suspect that students at Dalhousie may not appreciate how lucky they are!

———————

P.S. It turns out that carbon dioxide above its critical point is an excellent non-toxic solvent for caffeine and so when you sip your de-caff latte tomorrow – you can now imagine the physics that describes the fluid which took away the caffeine.

Watching Earth Breathe

May 12, 2013
Daily carbon dioxide concentration measurements for the year to May 2013. Daily measurements are shown as black dots, weekly averages as red lines, and monthly averages as blue lines.  On May 9th 2013, the daily value exceeded 400 ppm.

Daily carbon dioxide concentration measurements for the year to May 2013. Daily measurements are shown as black dots, weekly averages as red lines, and monthly averages as blue lines. On May 9th 2013, the daily value exceeded 400 ppm. Click for larger graph: Graphic is from NOAA Mauna Loa Observatory

On May 9th 2013, the observatory at Mauna Loa in Hawaii recorded a single daily reading of carbon dioxide concentration of 400.3 parts per million (ppm) – the highest value since human beings have existed as a distinct species.

This is a bit depressing for reasons we are all familiar with, but on the bright side, the annual average won’t exceed 400 ppm until 2015 🙂

I took the opportunity to look at the Mauna Loa data again – it is freely available – because I found the annual cycle rather curious. The graph at the head of the page shows the Earth ‘breathing’ – absorbing CO2 from May to October (Northern hemisphere summer) and then emitting it again from November to May.

The daily variations are interesting showing lots of systematic increases and decreases, presumably reflecting imperfect mixing of CO2 in the weather in the central Pacific ocean

You can see that the concentration from May 2012 to May 2013 has increased by around 3 ppm – that’s our carbon dioxide emissions – but I wondered if the annual cycle of  ‘breathing’ had changed over the years. After a little bit of Excel jiggery -pokery I found to my relief and surprise that there was no evidence of this. I think this means that Earth is still ‘breathing’ OK.

The annual cycle of carbon dioxide measurements at Mauna Loa plotted with the trend subtracted against month of the year. The data is colour-coded.

The annual cycle of carbon dioxide measurements at Mauna Loa plotted with the trend subtracted against month of the year. The data is colour-coded and stupidly I have used the same colour for the 2010’s as the 1970s (Doh!). But even so it looks to me like this cycle is unchanged

However the rate at which we are emitting CO2 is increasing. No news here 😦

The annual increase in annually averaged CO2 concentration. Back in eth 1970s the annual incerase were just over 1 ppm per year. Now they are 2 ppm per year and above. The rate of increase is around 0.23 ppm per year per decade.

The annual increase in annually averaged CO2 concentration. Back in the 1970s the annual increase about 1.5 ppm per year. Now it is more typically 2 ppm per year and above. The rate of acceleration is around 0.23 ppm per year per decade.

Contrails: where they come from and what they do…

April 7, 2011
Condensation Trails - Contrails for short.

Condensation Trails - Contrails for short.

WARNING: This blog contains maths. It’s interesting! But it does contain maths. Sorry.

 Updated on April 17 to use fuel consumption figures as suggested by Stephen Skinner in his comment.

Boeing 747 fuelled for a journey from London to Buenos Aires – a flying distance around 6900 miles – might take off with just over 200 cubic metres of fuel – kerosene. Some of this is held in reserve – but let’s imagine a hypothetical flight in which all the fuel was burned.

With a density of 814 kg per cubic metre, this corresponds to a total fuel load of around 175  tonnes. Kerosene is a poorly specified mixture of alkanes – compounds of carbon and hydrogen – with between 6 and 16 carbon atoms per molecule. Here we will approximate the effect of this mixture of alkanes by considering an ‘average’ alkane – dodecane – with a chemical formula,  C12H26. Each molecule of dodecane consists of 12 atoms of carbon, each with a relative molecular mass of 12, and 26 atoms of hydrogen, each with a relative molecular mass of 1. So the relative molecular mass of a dodecane molecule is 12 × 12 + 26 × 1 = 170

So 1 mole of kerosene – the Avogadro number of kerosene molecules – weighs 170 grams or 0.17 kg. The fuel load of the plane therefore consists of 175,000 kg ÷ 0.17 kg which is just over 1 million moles of kerosone. When burned in the jet engine,

  • Each of the 12 carbon atoms within each kerosene molecule  combines with oxygen from the air to make 12 molecules of CO2
  • The 26 hydrogen atoms within each kerosene molecule combine with oxygen from the air to make 13 molecules of H2O

So the 1 million moles of kerosene in the original fuel load are turned into:

  • 12 million moles of CO2 each weighing 12 + 2 × 16 = 44 grams or 0.044 kg. So the CO2 emitted weighs 12 million × 0.044 kg = 528,000 kg or 528 tonnes
  • 13 million moles of H2O each weighing 1 × 2 + 16 = 18 grams or 0.018 kg. So the H2O emitted weighs 13 million × 0.018 kg = 234,000 kg or 234 tonnes

So even though the plane took off with only 175 tonnes of kerosene, by combining this carbon- and hydrogen-rich fuel  with the oxygen from the atmosphere, en route to Buenos Aires, the aeroplane produces 234 tonnes of water (H2O) and 528 tonnes of carbon dioxide (CO2) and removing 587 tonnes of oxygen from the atmosphere.

  • The effect of the carbon dioxide has been discussed at length. However, I still find these numbers shocking – something between 1 and 1.5 tonnes per passenger. For a shorter trip such as the 3,500 mile ‘hop’ to New York, these figures would represent the combined carbon emissions for the 7,000 mile return journey.
  • The effect of the removal of oxygen is not widely discussed but you can see the data here. Don’t worry – we’re not running out.
  • The effect of the water is to, sometimes, produce contrails.

Contrails: The air at cruising altitude (≈10 km) is typically close to – 60 °C and usually very dry. When the water vapour concentration exceeds ≈50 parts per million of the nitrogen and oxygen, then the water vapour condenses into ice crystals – and we see wispy ‘cirrus’ clouds. The plane typically releases its 234 tonnes of water over a flight distance of around 11,500 km, or around 20 kg per kilometre. if we guess that the exhaust gases form 4 tubes, each 10 metres in diameter, then the exhaust gases occupy a volume of 314,000 cubic metres per kilometre of flight.

  • The water density is thus 20 kg ÷ 314,000 m3 = 0.000064 kg per cubic metre. One mole of water has a mass of 0.018 kg and so the molar density of water is 0.0035 moles per cubic metre.
  • The density of air at this altitude is much less than at sea level, because the pressure is only around 30% of that at sea level. The low temperature slightly compensates yielding an air density of around 0.5 kg per cubic metre (compared with 1.2 kg per cubic metre at sea level). One mole of air has a molar mass of 0.029 kg and so the molar density of air is 17 moles per cubic metre.

Comparing these two figures we estimate that the water vapour concentration in the exhaust plume is around 0.0035/17 ≈ 200 parts per million – a factor 4 more than the 50 parts per million required to saturate the air. Thus the water vapour condenses into tiny ice crystals – the condensation trail – or contrail.

The effect of contrails is complex, but on a clear day we can see that they frequently drift across the sky, appearing to nucleate the growth of wispy white ‘cirrus’ clouds. I wrote about a particularly graphic example of this previously. This tends to reduce the amount of solar radiation reaching the Earth, somewhat compensating for the effect of the carbon dioxide emitted along with the water. But at night, the clouds will have a warming effect, and overall the balance is hard to estimate. A recent paper in Nature Climate Change, suggests the cooling effect of contrails actually exceeds the warming effect of the carbon dioxide, but considerable uncertainty still surrounds their net effect. But it would be nice to think that  perhaps the solution to the global warming problem is more foreign holidays – not less! Sometimes things work out like that.

CDIAC: The Keeling Curve and more.

November 27, 2010
Sites of atmospheric carbon dioxide measurements

Sites of atmospheric carbon dioxide measurements

Forgive me: I am re-blogging this from my old blog because
(a) its interesting and (b) someone at Protons asked a question about it.

I love the CDIAC (Carbon Dioxide Information Analysis Centre ) website. Yes, it is obscure and takes a lot of clicking to find what you want. But that is because you don’t really know what you want!

But after a bit of clicking you can find data – raw data that you can plot and analyse yourself – or simple graphics. I like this page which shows what the atmospheric CO2 record looks like from the Northern Pacific, southwards through the famous Mauna Loa data, and at successively further southward in the Pacific until one reaches the Antarctic. I have pasted graphs of the data below.

The vertical scale is the same in all the graphs, but not the horizontal scale. What the graphs show is the same rising trend, but the annual oscillations differ in character and magnitude. In particular  the further north one looks, the more dramatic is the effect of ‘summer’ in the northern hemisphere which causes a drastic fall in carbon dioxide concentrations as plants grow. Most of the Earth’s plants are in the northern hemisphere and so this effect is seen much more strongly here. And just to show that the site really does give access to the data, here is the data from these three sites plotted for the year 2007. Notice how weak the oscillation is in the antarctic and how it is out of phase with the Northern hemisphere summer.

Month by Month Data from 2007

Atmospheric Carbon Dioxide concentration versus month of the year for 2007. The data is for three different locations. Notice the seasonal variation for each location is quite different.

Atmospheric Carbon Dioxide concentration versus month of the year for 2007. The data is for three different locations. Notice the seasonal variation for each location is quite different.

Data from Alert, Alaska

Atmospheric Carbon dioxide data from Alert, Alaska

Atmospheric Carbon dioxide data from Alert, Alaska

Data from Mauna Loa, Hawaii

Atmospheric Carbon dioxide data from Muana Loa, Hawaii

Data from South Pole, Antarctica

Atmospheric Carbon Dioxide Concentration at the South Pole versus time

Atmospheric Carbon Dioxide Concentration at the South Pole versus time

Atmospheric CO2: Looking at the data

March 23, 2010
C13/C12 Isotope Ratio. The systematic decline in this ratio is due to the distinctive isotopic ratio of fossil fuel derived CO2

C13/C12 Isotope Ratio. The systematic decline in this ratio is due to the distinctive isotopic ratio of fossil fuel derived CO2

It might seem there was very little left to say about the rise in atmospheric CO2 due to anthropogenic emissions. But the other day my colleague Martin sent me a link to two presentations at the Scripps Institute ‘Home of the Keeling Curve‘, and I found out there was plenty left to say. Scientists there presented data which actually evaluated the things I have heard people chat about for years.

  • The first topic covered was changes in the isotopic composition of the carbon dioxide in the atmosphere. This change unambiguously links the rise in CO2 to anthropogenic emissions – in case anyone really doubted that.
  • The second topic covered was the corresponding decline in the oxygen concentration of the atmosphere caused by burning all that CO2.

Isotopic Composition

Carbon is made primarily from two stable isotopes: C12 with 6 protons and 6 neutrons, and C13 with 6 protons and 7 neutrons. The heavier C13 comprises only about 1% of natural carbon. As was first observed in 1961, plants preferentially build their cells from C12 based molecules. Thus fossil fuels, which are derived from plants, have a slightly reduced  ratio of C13 to C12. Thus if the increase in the atmospheric CO2 that we have observed is really due to the emissions from fossil fuels then we should observe a corresponding decline in the ratio of C13 to C12 in the atmosphere. The Figure at the head of this article shows the data, and more detailed graphs are available at the Scripps site which explains the different curves and dots on the graph.What I found pleasing was just to encounter this data that I have heard people chat about for years.

Oxygen Depletion

Graph showing the depletion of atmospheric oxygen due to fossil fuel burning

Graph showing the depletion of atmospheric oxygen due to fossil fuel burning

One natural consequence of the reaction C + O2 → CO2 is that the amount of oxygen in the atmosphere must be decreasing. Its a small effect but once again the people at Scripps have measured the effect and made calculations to determine that the effect observed is in line with the observed CO2 emissions. The relevant data shown above and can be found several slides into this pdf presentation.

And so…

And so it is 10:48 p.m. and time for bed. But I feel the need to comment, that what I really admire about the data in these presentations is that they are so basic. Somehow people have scrabbled together enough funding to keep the measurements going for long enough for someone to recognise their profound importance. This is IMHO really admirable science. Goodnight.


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