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.|
|Picarro CRDS gas analyser installed at Robben Island|
|The more advanced installation including calibration gas, with the VICI rotary valve on the left and its control unit to the right|
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.