Of Smog and Satellites

In an article (pay wall) in Geophysical Research Letters, Lamsal (and others) measure the amount of NOx (NO + NO2) at the Earth's surface using an instrument on board a satellite. NOx is an important gas to track in the Earth's atmosphere despite the relatively low amount present, because it ends up making smog and acid rain. Before this paper, scientists and regulators had to rely on crude estimates based on indirect data about NOx formation from combustion in car engines, traffic, car usage, power plants and electricity usage, and so on. Because of extremely rapid economic growth in China, though, these estimates are quickly outdated and need to be updated with more real-time data. The authors of this paper were able to use satellite data along with computer models of the Earth's atmosphere to provide forecasts until these estimates are updated with current data. They are able to get the same forecast for 2003 that was predicted using a "bottom-up" approach described above. They were also able to predict an increase in NOx over East Asia in 2009, while correctly predicting a decrease in NOx over North America due to increased regulation. These results provide useful benchmarks for updating estimates of NOx emissions more frequently as conditions change.


Photo David McNew, Getty Images
The color of the LA skyline shown here is actually caused by NO2, which is a brown gas

Basic Concepts
Photochemical smog has some fairly complex chemistry that creates numerous compounds with negative health consequences for humans and other life, such as important food crops. The most important of these compounds is ozone. Although ozone is a good thing in the ozone layer 20 miles into the atmosphere since it absorbs harmful radiation from the sun, at the surface it is toxic to humans. Furthermore, ozone can also react with other forms of pollution, such as hydrocarbons from incomplete combustion of gasoline or diesel, to form other harmful compounds. How can NOx compounds end up producing ozone though? This is where the "photo" (light) in photochemical smog comes in. During the day time, any NO2 produced, from say automobiles, is broken down by light to form NO and a highly reactive loose oxygen atom. This oxygen atom goes on to combine again with an O2 molecule to form ozone (O3).

Some more informed readers may ask, "If NOx is the problem in creating smog, then why is it still a problem since catalytic converters have been around for decades now?" It's true that catalytic converters have greatly helped the problem, as the smog in Los Angeles was much, much worse in the 70's and 80's than it is today. However, even small amounts of NOx can cause a lot of ozone production. After NO2 is converted to NO by light, the NO2 can be created again by NO reacting with hydrocarbons from unburned fuel. If you've ever gotten your car smogged, they're likely to reject it if the engine is not running optimally because this ends up releasing a lot of unburned hydrocarbons into the air. Catalytic converters and exhaust filters can help deal with this problem, and indeed have helped reduce the pollution that causes smog. Because of the complex chemistry of smog, though, it still remains a difficult problem to keep at manageable levels.

Other interesting and related topics:
Website for the SCHIAMACHY instrument from the European Space Agency
Website for the equivalent NASA satellite
Any of the Wikipedia entries linked above

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. :)]

Summary
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.

Concepts
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. 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