Tuesday, 19 February 2013

Recipe for Obtaining Carbon Dioxide Emission Estimates - First Ingredient: Measurements

So...what does one need to estimate carbon dioxide emissions for a particular region? Of course there is more than one way, but I'm going to be focussing on the method referred to as "atmospheric inversion". This term is actually a little confusing because those involved with atmospheric or weather modelling also use the word "inversion" to refer to an atmospheric phenomenon, where usually temperature, changes with altitude, and the layer in which this change occurs as the "inversion layer". I'm not talking about that inversion. I'm referring more to its meaning as "back-to-front" or "wrong way round".

Think of the carbon dioxide concentration at a particular point as a credit card balance. It's the sum of all the spending and payments made on that account. If we only knew what the credit card balance was, we wouldn't know how much had been spent at department store A, at the movies, or on petrol, and we wouldn't know how much had been paid back on the credit card. But, if we knew which places had been visited and how long they had been visited for, and what the average spending (or paying) was at each of these places in the past, we could start making a pretty good guess about what each transaction amount was that resulted in the current credit card balance. So we have to "back solve" what the transaction amounts were - hence the word "inversion". OK ... that's a terrible analogy, but it's the best one I could come up with. I'll try to think of a better one and add it in later.

Using this recipe, we know what the carbon dioxide concentration was at a particular point, we know the way the air travelled before it got to that point, and we know something about what's happening on the ground where that air has travelled. So we have carbon dioxide measurements, an atmospheric transport model, and prior information.

The first ingredient, the atmospheric measurements, need to be extremely precise. This is because, as far as the overall make up of the atmosphere is concerned, carbon dioxide is an extremely small proportion. It makes up less than one percent of the total composition of the air we breathe (well most of the time - more about that later!). Small increases in the concentration of carbon dioxide are actually the result of very large carbon dioxide emissions. If we want to make reasonable estimates of the emissions that are taking place, then we need to know, to the parts per billion precision level, what the change in carbon dioxide concentration is over time and between two different points.

There are only a handful of instruments that can measure to the precision required for an atmospheric inversion study. One of these instruments is the Picarro Cavity Ring Down Spectroscopy Analyser (Picarro CRDS analyser http://www.picarro.com/gas_analyzers/co2_ch4_h2o/). Three of these instruments are located around the Cape Town area.
The Picarro's voyage to Robben Island on the Robben Island Museum staff boat.  Ahh...love those 4:30am wake ups to get to the harbour in time.
Getting the the Picarro's to their measurement stations was a challenge in itself. On the Picarro website, they refer to the CRDS analyser as a "portable spot measurement tool" and to its "compact portability". Well, I'm not sure if they've ever tried to carry one of these things up the seven flights of stairs leading to the top of the Hangklip lighthouse. Since I started this job, I've had to join a gym - because a woman's gotta get the job done! But I wasn't alone ... I've had to enlist the aid of family and friendly Transnet lighthouse keepers. As anyone doing a PhD will know - you don't do it alone, your whole family does it with you, whether they like it or not. Given the fact that I was able to get the analyser up there at all means it is pretty portable, as far as gas analysing equipment goes.

Picarro CRDS gas analyser installed at Robben Island
Now if you ever decide to do anything involving air measurements, you better train up on plumbing. Or make friends with a plumber. It seems like a simple thing - suck air in through a tube and sample it at the other end, but there's a lot more too it than that. Most importantly - there can't be any leaks. You don't want to be sampling room air. You also can't just use any old tubing. It has to be a material that will not react with the air and alter the concentration of the air in anyway - what goes in the inlet should be what comes out the outlet. The analysers also don't like get water or particulate matter inside them, so there has to be a special filter in the line which prevents this. And then there are the connectors. These are potentially the weakest points in the plumbing. They come in all shapes and sizes, straight or bent, and with different threads. These have to be suited to the tubing and need to connect to the analyser. The Picarro CRDS analyser has a 1/4 inch Swagelok male connector, and so the plumbing would need to terminate in a 1/4 inch Swagelok female connector. Oh yes...plumbing has gender as well.
The more advanced installation including calibration gas, with the VICI rotary valve on the left and its control unit to the right
Just having a inlet tube connected to the analyser is the simplest way to go, but to match up with international standards in measuring atmospheric constituents, it's necessary to check the analyser against a known calibration gas standard on a regular interval, to make sure that it's not drifting off anywhere. That's when the plumbing gets really complicated. An important component in this system is a rotary valve. Basically, it rotates to a new position when told to, and then opens up a different side of the plumbing, and closes the other side off. The first side of the plumbing is the sampling side, and the second part of the plumbing is the calibration gas. Just to make things complicated, the tubing required for the rotary valve is 1/8 inch stainless steal tubing, so there have to be adaptors between 1/4 inch and 1/8 inch.

The Picarro CRDS gas analyser measures approximately once every second, and these measurements are then stored in hourly files. The data gets checked for any anomalies, and then gets converted into an hourly average, which is what finally ends up getting used for the atmospheric inversion.

And that's the more adventurous side of obtaining carbon dioxide emission estimates. It's usually quite fun - but not so much when things don't work out. It's rather disheartening after spending months planning where every little tube and connector is going to go, and then finding out there was a giant flaw in the plan, and needing to go back to the drawing board in the field. But such is the nature of field work and instrumentation.

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