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 Because of extremely rapid economic growth in China, though, these estimates are quickly outdated and need to be updated with more real-time data.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