A while back, I was discussing the many ways that we can monitor greenhouse gases. One of these methods, the inventory method, involves estimating the greenhouse gases from human activity as an associated factor of how much emissions that activity generates. We can check that these estimates make sense in the context of the atmosphere by also taking measurements of greenhouse gases. If we want to do a really good job of checking the inventory estimates, we can apply both the inventories and some greenhouse gas measurements into a specialised transport model. This method is relatively new, and still under development.
In urban areas there are a number of obstacles to taking greenhouse gas measurements and to applying transport models. Firstly, cities are very rough, rather warm and have a lot of very concentrated emissions sources. The first two issues cause problems for accurately modelling how greenhouse gases are transported up into the atmosphere and also on how easy it is to place an instrument that can be representative of the whole city. We need both of these things to be done well if we are ever to produce results that are accurate enough to help us check our inventories. The third issue is of interest to the discussion here and I am going to focus particularly on CO2 for this. Cities have a lot of fossil fuel CO2 emissions sources such as traffic and local power generation, but they also have a lot of biological sources; such as plants and humans breathing. So the question is, how do we separate out the part of the measured greenhouse gas that comes from the fossil fuels from the background and from the biological sources?
The solution to this issue comes in the form of isotope analysis. In this case we can think of isotopes as a ‘tag’ for different types of CO2. You might not know this, but actually there are three slightly different types of carbon. Back to basic chemistry for a few moments now. All atoms are made up of protons, electrons and neutrons. Protons have a positive charge and make up the atom nucleus along with the neutrons which don’t have a charge. Electrons (negative charge) then buzz around the outside of the nucleus. The protons and the neutrons are what gives the atom most of its mass. Carbon always has 6 protons and 6 electrons to balance the charge. It usually has 6 neutrons too, and this form of Carbon with 6 of each is very stable. It is common and you’d expect to see it everywhere you see Carbon. But this is not the only type of Carbon there is. Sometimes an atom has more neutrons than it has protons and the more that it has (heavier it gets), the more it tries to decay back down to its stable form. Carbon can have 6, 7 or 8 neutrons. Add that to the 6 protons and you have carbon with a mass of 12, 13 or 14.
Now, because 12 is the stable form and 14 is the unstable form, you might expect that as time goes on, any Carbon 14 that exists will gradually decay away until it becomes Carbon 12. The time that this takes is called its radioactive half-life. The half life of Carbon 14 is about 5,730 years. This is important because the reason fossil fuels are called fossil fuels is because they are old, certainly older than 5,730 years. That means that the carbon that is contained in the fossil fuel will have already decayed to its stable form before it is combusted and releases CO2 to the atmosphere. So when we measure the isotopic composition of the carbon in the atmosphere, we can get a quite good indication of the fossil fuel contribution.
To take this a step further, we might want to attribute the fossil fuel we detect to one type of fossil fuel or another. We can do this using a tracer species, commonly carbon monoxide (CO) is used for this. CO is a tracer of ‘incomplete’ combustion, and usually the more incomplete the combustion, the more ‘dirty’ it is in terms of CO2 and air pollutants. We can use the ratio between the CO tracer and the isotopically derived ‘fossil fuel CO2’ to tell us something about how clean the combustion process is likely to have been. For example, a tar pit has a high CO/fossil fuel CO2 ratio of about 20 ppb/ppm, a car has a medium value of about 14 and a clean modern car has a CO/ fossil fuel CO2 ratio or 8 or 9 ppb/ppm. A clean power station has a CO/ fossil fuel CO2 ratio of about 3 ppb/ppm.
If we have an idea of what the CO/ fossil fuel CO2 ratio is in a sample of air, we can use measurements of CO to tell us how much of the total measured CO2 from the same place can be attributed to fossil fuels. This is important for modelling (which tries to estimate the fossil fuel CO2 from inventories) and for improving estimates from the inventories themselves.
I would like to talk about this topic again another time in more detail, as it is a very interesting area of science. Next time at Ground to Sky, I will discuss the balance of science between studying greenhouse gas emissions from natural and urban environments as a point of interest from my recent trip to the European Geosciences Union (EGU) General Assembly in Vienna.