The blog posts on the experiences around measuring carbon in the atmosphere around the City of Cape Town. The triumphs and the setbacks, and the unexpected.
Follow the link to the Science article to get the full story!
Marcott, S.A., Shakun, J.D., Clark, P.U., Mix, A.C., 2013. A Reconstruction of Regional and Global Temperature for the Past 11300 years, Science, vol.339, DOI: 10.1126/science.1228026 http://www.sciencemag.org/content/339/6124/1198.full
Also an update! Hangklip monitoring station is back online!! Follow on twitter at @CapeCarbon.
If you’ve been following the twitter feed (@CapeCarbon) –
thanks to Laurie Butgereit of Meraka, CSIR, for getting it up and running –
then you might have noticed, that not only is the carbon dioxide concentration
at the two sites being tweeted, but under certain conditions, the carbon
dioxide emissions from Cape Town and surrounds as well.
So how am I doing this exactly? As my last post revealed,
normally a super computer, or at least a pretty amazing desktop, is needed to
carry out an atmospheric inversion for obtaining estimates of carbon emissions.
Well that certainly is the ultimate goal, and it’s the only estimate that’s
really publishable. But in the mean time, to give people a taste of what’s
possible with atmospheric measurements of carbon dioxide, I came up with a
method to get a “ballpark” figure of what the carbon dioxide flux is. This is
not a statistical approach. This is not a physics approach. But rather an “applied
maths” approach. The same type of approach you need to use when your lecturer
asks you to estimate how many ping pong balls can fit inside the lecture
theatre.
Now the name comes from one of my favourite movies,
Armegeddon (which probably popped into my mind because of all the meteors that
have been crashing down to Earth lately - http://www.geekosystem.com/nasa-explains-russian-meteor/),
specifically from the Russian cosmonaut, Lev Andropov, where he calls the
America astronauts a “bunch of cowboys” after they've just caused his space
station to blow up. There’s a scene where he bashes a spanner against the space
ship to get it to work, and it springs to life.
Well this is my way of smashing a spanner against the measurements to get them
to give emission estimates.
The first thing that needs to happen is that the wind needs
to be blowing in the right direction. It needs go from one of the measurement
stations, over Cape Town, and directly to the next measurement station. So this
is either when the wind is blowing from the South East or from the North West. We
also need to know how fast the wind is blowing. Fortunately we can get all of
this from the South African Weather Service (SAWS Cape Town Weather).
Map showing the required wind directions in order to make flux estimates based on the difference in CO2 concentrations between the Robben Island and Hangklip sites. Map generated using Google Earth.
Then we need to get the difference in carbon dioxide concentration
between the two sites. If the wind is coming from the North West, then we want
Hangklip – Robben Island, and if it’s from the South East, then we want Robben
Island – Hangklip. This difference then needs to be converted into mg of carbon
dioxide per cubed m. This can be done using the ideal gas law.
mg CO2 per m3 = CO2 ppm × 44.01/(8.3145 ×
Temperature/Pressure)
Now this is where the serious wangling comes in.
The first major assumption that we need to make is that the
wind is travelling in a straight line from station A to station B. Then we need
to assume that the wind speed is constant. This won’t be 100% correct, but we’re
not going to try and model the wind fields for this exercise (that’s what the
super computer is used for).
Then my thinking is: imagine this concentration of CO2 as a cylinder
with a 1m2 base and 1m in height. The cylinder starts off at station A and then
travels towards station B at the speed of the wind in a straight line, a total
distance of 77.4km. Along the way, the cylinder is going to collect or lose CO2
due to processes taking place on the surface. There is also not just one
cylinder at a particular point, but cylinders stacked up on top of each other
until they reach the planetary boundary layer. To start off with, just to keep
things simple and manageable, we’re going to assume that all the cylinders are
moving at the same speed. This is not true, because the higher up in the atmosphere
you go, the faster the wind is going. What I plan to use in the future is the wind
profile power law (http://en.wikipedia.org/wiki/Wind_profile_power_law).
The planetary boundary layer extends from the surface to
height of up to 3km, depending on the temperature and other factors (http://www.sciencedaily.com/articles/p/planetary_boundary_layer.htm).
It’s where the air is most influenced by what’s happening on the surface, and
it’s pretty hard to model. This height is required in order to make the
estimation of the CO2 fluxes possible, and so, for starters we’re just going to
assume that the height of the planetary boundary layer is 1000m, which I
grabbed from a study on North America, but for an area of similar latitude (http://www.meteor.iastate.edu/~jdduda/portfolio/605_paper.pdf).
As the tweets get a bit more advanced, I will try to change the PBL depending
on the time of the day and the current season.
An illustration of the approach used to estimate the CO2 fluxes and the assumptions made
We can now come up with an empirical relationship
The difference between the two CO2 concentrations = ΔCO2 = The amount of CO2 emitted or absorbed per m2 per second ×
the total distance travelled × the total amount of time spent per m2 / the
total number of cylinders in the stack.
We know the difference in concentration between the two
sites, we know the total distance travelled which is 77400m, the amount of time
spent at each m2 is the inverse of the wind speed, and the total number of
cylinders is 1000. So all that’s left is the emission (or absorption) of CO2:
CO2 flux = ΔCO2 × wind speed × 1000 / 77400
So far the values that I've tested this on are in the right range, so I'm happy that the estimates are at least of the right order of magnitude.
During the mornings we have seen that there are negative fluxes observed. This is because photosynthesis has kicked in. During the morning, the stomata open up, and allow CO2 to passes into the plant cells. At the same time water vapour can also leave the plant cells. If it starts to get too hot, the plant needs to be careful that it doesn't dessicate, and so it has to control how open it's stomata are going to be, which then limits the amount of CO2 that can be absorbed through photosynthesis. If there's one thing I hope people take away, it's the important role that our natural systems play in regulating the atmosphere in which we exist.
Couldn't have done it without a little help from my
friends...
Getting the Picarros up and running has been a journey in
itself. As I've mentioned before, scientific equipment doesn't just up and
install itself. And boy, you better get it right, or else you might as well
just chuck the data or else spend the rest of your life trying to justify
corrections and tweaks to the data.
My journey started in Australia, where I was lucky enough
to spend a few weeks with my now co-supervisor, Dr. Peter Rayner. Peter taught
me all the in's and out's of atmospheric inversion and how to go about getting
one to run. This was my first experience with working on a super computer, and
I think I learnt to code in about three different languages during those six
weeks - IDL, Python and Fortran. As a bonus, Peter also took me on a tour of
Lygon Street, Melbourne. This is a delightful place, which has restaurants of
all kinds stretching from end to end. Every day we would visit a different
restaurant so I got to enjoy a culinary tour of Melbourne, Australia, as well. I had possibly one of the best burgers ever at a place called Grill'd (http://www.grilld.com.au/). Unfortunately I'm not a ginger, or otherwise I would have been eligible for the special.
As an aside - there is a wonderful little cinema in Lygon Street, Cinema Nova (www.cinemanova.com.au/), which I discovered while staying there. It's an arthouse cinema, but they do show some of the movies on the regular circuit. My normal experience at a cinema, which I do love going to in general, is that you get your ticket and then you can maybe get a popcorn and some soda, and not much else. But at Cinema Nova, not only can you get your box of popcorn (so you can have the full cinema experience), but you can order a glass of wine as well!! In a GLASS! This was an entirely new experience for me. There's nothing like a glass of wine to help you get thoroughly enthralled in a soppy movie.
The perfect movie combo
Just around the corner
from the University of Melboure are the Aspendale CSIRO (Commonwealth
Scientific and Industrial Organisation) offices. Peter has a long standing
relationship with CSIRO and works often in collaboration with the CSIRO Marine
and Atmospheric Research group. Lucky for me! This group manages several
atmospheric monitoring sites, including Cape Grim on Tasmania. Who better to
ask how to go about setting up my own system? I was hoping for a few pointers
and maybe a setup diagram or two. Well I got that and much more. On the day I
visited, Zoë Loh (http://www.youtube.com/watch?v=n6313xRIdck) and Ann
Stavert (http://www.youtube.com/watch?v=oYmJJerYi-w) graciously and
enthusiastically showed me around their labs, and then took me through their entire setup of the Picarro systems, and the do’s and don’ts of plumbing. And
on top of that, Zoë arranged for me to go into the
field with David Etheridge to see two Picarro monitoring sites in action at a
geosequestration site, as well as to get some hands on training in collecting
flask samples for isotope analysis.
Atmospheric monitoring station near a geosequestration site in Otway, Australia
Assistance from CSIRO didn't stop there. I also got to
visit the Marine and Atmospheric Research group based in Canberra. This was to
get some hands on experience with code related to the CABLE (CSIRO Atmosphere
Biosphere Land Exchange) model, which our group in South Africa had planned to
start using and adapting for our purpose. This model represents the
interactions between soil, vegetation and the atmosphere, and includes linkages
between hydrology, plant physiology and their micro climate (http://cawcr.gov.au/projects/access/cable/cable_technical_description.pdf).
Vanessa Haverd very kindly took me through all the code and explained how to
get it to run.
When I arrived back home, it was time to start ordering the
bits and pieces. If there was one thing that I had taken away from the trip, it
was that it was going to be a lot harder to set everything up than I had
anticipated. There are a lot of little parts that are needed to make the whole
measurement system run properly. We had decided to go with the same Picarro
CRDS measurement systems that our colleagues at CSIRO were using, and Picarro,
although thousands of kilometres away were very helpful in getting us all the
Picarro bits and pieces that we needed, and never hesitated to call if they
needed to explain anything.
Once the Picarro’s arrived, it was time for me to sort out
the plumbing, It was something that I had been dreading since the Australia
trip because I knew that it was more complicated than I had originally thought,
and that it could make or break the whole measurement system.
Fortunately, just after getting back from Australia, I had
the opportunity to attend the conference for the South African Society of Atmospheric
Sciences, where I met in person Ernst Brunke, of the Cape Point GAW station, run by the South African Weather Service.
Instead of treating me like someone who was trespassing onto his turf, Ernst was
happy to explain in any required detail what I needed to get for the plumbing
system to work, and also to share and collaborate with his lab at Cape Point.
The Robben Island and Hangklip Picarro's visiting Cape Point GAW station for a concurrent calibration
Lucky for me, the suppliers of Swagelok connectors (http://www.swagelok.com/johannesburg),
the same connectors used on the Picarro CRDS instrument, were literally just
around the corner from where I lived. Here I spent several hours while the
sales personal helped to identify all the correct components needed for my
plumbing diagram to turn into a reality. They even helped me to assemble the bits and pieces. I now know what a ferrule is.
When it came to the actual installation, I needed
permission from the Port Authority, Transnet, in Cape Town. Mr. Robin
Poggenpoel, regional manager, was glad to get on board with the research
project, and provided permission for me to access two of the lighthouses to
install the CO2 measurement instruments. With help from my family, and
from the lighthouse keepers at Robben Island and Hangklip - Peter and George - I
was able to finally get the instruments installed. After about a year of
planning, everything finally came together.
Peter, the Robben Island lighthouse keeper, accompanying me to Robben Island for installation
So as you can see, this research project would not be possible without the help of a lot people, who really didn't have to, but did anyway. Thank you!
