China's chewy air
I saw a picture today at Shakesville of air pollution in China that got me thinking. Here's the picture:
Update: My Chinese coworker thinks the visibility reduction in the picture might be a mixture of smog (which this post is about) and a dust storm, which are common this time of year in Beijing. He says that dust comes from Mongolia and blows through the Beijing area on its way to Japan and the United States (there's a group at the University of Iowa that measures China dust in their area). The dust storms apparently happen during spring and summer, so they could be a factor during the Olympics in addition to the smog.
It's a picture of some guys standing on the roof of a stadium in Beijing, and you can hardly see them because of all the pollution. One of the commenters in that thread asked a question:
The answer is yes, with a "but". The "but" is that to stop that air pollution from forming, the Chinese government would basically have to shut down all their factories and power plants, and tell people not to drive their cars during the Olympics. Somehow I doubt that this will happen.
The reason that shutting off their industry would stop the smog is that the kind of pollution in the picture is photochemical smog, or LA-type smog (so called to differentiate it from London-type smog, which is formed from smoke + fog, hence the name). Photochemical smog forms when an area has high enough levels of volatile organic compounds (VOCs) and oxides of nitrogen (NOx), and also when temperature inversions are present.
When the mixture of VOCs and NOx gets hit with sunlight, the products are ozone (which is used to set smog alerts in California), particles (which account for most of the visibility reduction) and some other stuff that can be toxic. Some of the brown color comes from the particles and some comes from NO2 (a component of NOx). Temperature inversions basically put a lid on an area, so that pollutants can't be convectively transported to higher levels of the atmosphere. In practice, this gives the VOCs and NOx time to be turned into photochemical smog.
You can see why the LA area is so screwed. We're in a basin surrounded by mountain ranges, so pollutants don't get removed by horizontal transport, and we get temperature inversions so that there's no vertical removal either. We also get a lot of sun. Overhead photographs of Beijing show a similar geography, and they get tons of sun (100° days are frequent in summer).
So where do VOCs and NOx come from? In general the answer is fossil fuel burning, but the largest source is usually vehicle traffic. Areas with strict pollution control standards (like California) have mandated the 3-stage catalytic converter, which converts NOx into nitrogen gas and turns VOCs into carbon dioxide and water. I don't know if China has those kind of emissions standards, but even if they do, catalytic converters aren't perfectly efficient and their sheer number of cars will probably make for a significant source anyway. Back in the days before emission controls, Los Angeles looked much worse than that picture of China above.
Probably more troublesome is the huge number of coal plants in China. Coal plants emit large amounts of NO (one component of NOx), which gets turned into NO2 via some complicated chain-reaction chemistry involving hydroxyl radicals (OH) and VOCs. When NO2 gets hit with sunlight, it gets photolyzed into ozone, which is a pollutant near ground level. Ozone itself gets destroyed by the most energetic parts of sunlight, leading to 2 OH molecules for every one ozone molecule. Ozone can also react with NO to re-form NO2. Every step of the reaction generates more of the starting material for other steps of the reaction, in other words. So as long as there's sunlight, VOCs and NOx, you're going to get a ton of ozone and other pollutants.
So what's going to happen during the Olympics? Well, the government actually does plan to limit vehicle traffic and close gas stations, which will help. They also want to try to use cloud seeding to control the weather, which will remove smog via rain-out if it works (which it probably won't). My prediction is that the government won't take the single most effective step and drastically limit their industry (they've done this to some extent, but to do it as much as would be necessary would probably cost a ton of money) or their power generation (because they can't), and there will still be plenty of VOCs and NOx in the air to cause photochemical smog during the Olympics. For a country that got awarded the Games because they promised to put on an environmentally-friendly show, that should be a huge black eye. The best case scenario is that such a debacle will spur emissions controls throughout China. I guess we'll see.
Update: My Chinese coworker thinks the visibility reduction in the picture might be a mixture of smog (which this post is about) and a dust storm, which are common this time of year in Beijing. He says that dust comes from Mongolia and blows through the Beijing area on its way to Japan and the United States (there's a group at the University of Iowa that measures China dust in their area). The dust storms apparently happen during spring and summer, so they could be a factor during the Olympics in addition to the smog.
It's a picture of some guys standing on the roof of a stadium in Beijing, and you can hardly see them because of all the pollution. One of the commenters in that thread asked a question:
When do the Olympics start? It's not that far away, right? Can they really clean up that much pollution in so short a time?
The answer is yes, with a "but". The "but" is that to stop that air pollution from forming, the Chinese government would basically have to shut down all their factories and power plants, and tell people not to drive their cars during the Olympics. Somehow I doubt that this will happen.
The reason that shutting off their industry would stop the smog is that the kind of pollution in the picture is photochemical smog, or LA-type smog (so called to differentiate it from London-type smog, which is formed from smoke + fog, hence the name). Photochemical smog forms when an area has high enough levels of volatile organic compounds (VOCs) and oxides of nitrogen (NOx), and also when temperature inversions are present.
When the mixture of VOCs and NOx gets hit with sunlight, the products are ozone (which is used to set smog alerts in California), particles (which account for most of the visibility reduction) and some other stuff that can be toxic. Some of the brown color comes from the particles and some comes from NO2 (a component of NOx). Temperature inversions basically put a lid on an area, so that pollutants can't be convectively transported to higher levels of the atmosphere. In practice, this gives the VOCs and NOx time to be turned into photochemical smog.
You can see why the LA area is so screwed. We're in a basin surrounded by mountain ranges, so pollutants don't get removed by horizontal transport, and we get temperature inversions so that there's no vertical removal either. We also get a lot of sun. Overhead photographs of Beijing show a similar geography, and they get tons of sun (100° days are frequent in summer).
So where do VOCs and NOx come from? In general the answer is fossil fuel burning, but the largest source is usually vehicle traffic. Areas with strict pollution control standards (like California) have mandated the 3-stage catalytic converter, which converts NOx into nitrogen gas and turns VOCs into carbon dioxide and water. I don't know if China has those kind of emissions standards, but even if they do, catalytic converters aren't perfectly efficient and their sheer number of cars will probably make for a significant source anyway. Back in the days before emission controls, Los Angeles looked much worse than that picture of China above.
Probably more troublesome is the huge number of coal plants in China. Coal plants emit large amounts of NO (one component of NOx), which gets turned into NO2 via some complicated chain-reaction chemistry involving hydroxyl radicals (OH) and VOCs. When NO2 gets hit with sunlight, it gets photolyzed into ozone, which is a pollutant near ground level. Ozone itself gets destroyed by the most energetic parts of sunlight, leading to 2 OH molecules for every one ozone molecule. Ozone can also react with NO to re-form NO2. Every step of the reaction generates more of the starting material for other steps of the reaction, in other words. So as long as there's sunlight, VOCs and NOx, you're going to get a ton of ozone and other pollutants.
So what's going to happen during the Olympics? Well, the government actually does plan to limit vehicle traffic and close gas stations, which will help. They also want to try to use cloud seeding to control the weather, which will remove smog via rain-out if it works (which it probably won't). My prediction is that the government won't take the single most effective step and drastically limit their industry (they've done this to some extent, but to do it as much as would be necessary would probably cost a ton of money) or their power generation (because they can't), and there will still be plenty of VOCs and NOx in the air to cause photochemical smog during the Olympics. For a country that got awarded the Games because they promised to put on an environmentally-friendly show, that should be a huge black eye. The best case scenario is that such a debacle will spur emissions controls throughout China. I guess we'll see.
Labels: chemistry, learnin', science, your impending doom
posted by The Disgruntled Chemist at
10:34 AM
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