Friday, March 4, 2011

Can isotopes be used to track regional sources of greenhouse gas pollution?

[Note: This is an attempt to try to write about recent science articles for a lay audience. I don't think many people even follow this, but any feedback would be appreciated. My plan is to try to break down a current journal article related to my research every week or so. We'll see how that turns out in practice. :)]

In a recent article in the open access (i.e. free) journal Atmospheric Chemistry and Physics, Tuszon and colleagues explore this idea in the mountains of Switzerland. They measure the stable isotope ratios of both carbon and oxygen in CO2 using a laser device that allows for rapid real-time measurements. Previously, measurement of the oxygen isotope ratios of CO2 (but not carbon) used a much more time-consuming technique that requires hours of sample preparation and half an hour of measurement time. (I have to do it for my research; it's not fun.) Because of the high quality and sheer number of the measurements, the authors of this paper could estimate the sources of the elevated CO2 by comparing the isotope ratios to the concentration. Using the carbon-13 to carbon-12 ratio, they find that three of the events with high CO2 concentration can be linked to burning gasoline or other petroleum products, while one of the events is the result of coal or wood burning. They use a computer model to estimate the potential sources in the following colorful plot (Fig. 7 in the paper):

Essentially, the authors were able to detect not only the source of the high levels of CO2 but the region the CO2 was emitted from. Such measurements may be important for verifying or enforcing any regulations limiting greenhouse gas emissions since the isotope ratios of CO2 in polluted air could be used to estimate the potential source.

Although when most people think of the word 'isotope', they probably associate it with radioactivity, the isotopes measured in this study do not undergo radioactive decay. (thus 'stable isotope') So unfortunately, nobody in that laboratory in Switzerland is going to be bit by a radioactive spider and turn into Spider-man. Copyright Marvel Characters, Inc. 2008 So just what is an isotope then? Recall that an atom is made up of protons and neutrons in the central nucleus and electrons in outer orbitals. Because the positively-charged protons determine the chemistry of the atom while the neutrons for the most part do not affect the chemistry, each element is named for the number of protons. Thus, an atom with one proton, regardless of the number of neutrons, is always a hydrogen atom. An atom with two protons is always helium, an atom with 6 protons is always carbon, and so on. An isotope then is an atom of an element with a specific number of neutrons that is named for the total amount of protons and neutrons in the nucleus. (e.g. carbon-13 has 6 protons and 7 neutrons) A certain number of neutrons are required for a nuclei to remain stable and not radioactively decay, however - these are the 'stable isotopes' of an element. Most of the time there is one major stable isotope with one or more rare stable isotopes. For the article here, the isotopes discussed are carbon-12 (major) and carbon-13 (rare, ~1% of carbon atoms) along with oxygen-16 (major) and oxygen-18 (rare, ~0.2% of oxygen atoms). (Aside: There's also an even rarer but stable oxygen-17 isotope that makes up 0.04% of all oxygen atoms, but for various reasons it is usually not studied. There are some highly unusual isotope effects related to oxygen-17 in the middle atmosphere that are beyond the scope of this article, but are the focus of my research.)

Alright, so if the difference in the number of neutrons doesn't really affect the chemistry, how can the scientists in this paper tell what the sources of the greenhouse gas pollution are? Well, the main factor in determining the chemistry of an element is the number of protons and electrons, but the neutrons can affect the chemistry slightly because they change the mass of the nucleus. The change in mass makes it harder (or even easier in some cases) for any chemical containing a heavier isotope such as carbon-13 to react. Consider a chemical reaction like a "hill" that the reactants have to climb such as shown below. In a sense*, atoms and molecules that contain a heavy isotope are "harder" to push up the hill than lighter isotopes, so they react more slowly. Thus, unless all of the reactants are converted into products, the reactants will tend to contain more of the heavy isotope, and the products will tend to contain more of the light isotope. Because the "steepness" of the hill depends on the reactants, different chemicals will tend to have different ratios of heavy to light isotopes. Thus, by looking at the carbon-13 content of the CO2, the scientists here can tell what kind of process produced it based on what they already know about the isotope distribution for those processes in the lab. In practice, the differences are small, around 3%. This has a number of other uses, as well, including (my personal favorite) verifying the region wine was grown in by comparing the known carbon-13 content of regional soils to that in wine. I like to imagine scientists drinking some wine and then pouring some into an instrument. :) Pretty cool, huh? (Well, at least I think so... but I'm getting my Ph.D. in chemical physics...)

* The real picture is more complicated than this (the isotopes actually affect the "shape" of the hill, for example) but it's fine for the purposes of this post

1 comment:

  1. Very interesting. I feel like I learned something.