shanghai smog

The news last week contained many pictures like the one above of a monumental smog enveloping the city of Shanghai, China. The smog resulted in many problems: construction projects were halted, public events were cancelled, and the city’s reputation suffered. Chinese authorities were ridiculed for trying to find a silver lining in the smog by suggesting that it would protect Shanghai from an aerial attack. In addition to being unsightly, smog makes life very difficult for people with asthma and other respiratory problems, and long-term exposure can result in lung cancer.

The term smog was coined in 19th century London to describe a smoky fog. Here is the definition of smog from

A photochemical haze caused by the action of solar ultraviolet radiation on atmosphere polluted with hydrocarbons and oxides of nitrogen, especially from automobile exhaust.

Hazes are created by particles which are small enough to remain suspended in the air but still large enough to scatter light. Light has a wavelength of approximately 1 µm (1 one-millionth of a meter, commonly called a micron). If light passes through particles which are smaller than this, such as the oxygen and nitrogen molecules in the air between you and the computer screen, then the medium is transparent. When light passes through a suspension of particles which are larger than 1 µm it scatters, which creates a haze. Smog is composed of particles which are on the order of 1 to 20 µm in diameter.

The potential health effects of smog particles are inversely related to size. Here is what happens when you inhale a breath full of smog: Particles which are larger than 10 µm are trapped by your nose, mouth and throat and, therefore, are not harmful. Particles smaller than 10 µm reach your lungs, a more troubling result, but one which your body can manage. The smallest of particles, those with diameters of less than 2.5 µm, can penetrate into the nooks and crannies of your lungs and are responsible for causing respiratory distress and disease.

The units which are used to measure smog are called PM 2.5, which corresponds to the micrograms of particles less than 2.5 µm in diameter per cubic meter of air. (Having read the paragraph above, you now know why that diameter was selected.) The World Health Organization (WHO) recommends that exposure to smog be limited to a PM 2.5 of no more than 25. The PM 2.5 count in Shanghai last week was over 600, which is 2,400% of the WHO limit!

The primary culprits in the formation of smog are car exhaust and smoke from coal-fired power plants. The emissions from these tailpipes and smokestacks undergo chemical reactions upon exposure to sunlight on their way to becoming smog particles. While the details of chemistry involved in this process are too lengthy to review here, there are two points which need further exploration in order to understand what is happening in Shanghai.

If you burn a mixture of hydrocarbons such as gasoline at high temperature and with a sufficient supply of oxygen, the only combustion products are water and carbon dioxide. While carbon dioxide is the primary culprit in Global Warming, it does not contribute to smog. If combustion of hydrocarbons occurs in an environment which is either too cold or has insufficient oxygen supply, the emissions will contain significant amounts of partially combusted hydrocarbon fragments. These fragments are what lead to smog. Car engines are colder when first started and also while idling. As a result, cars generate more smog in stop-and-go city environments than during highway driving. Thus, Shanghai’s legendary traffic jams are a key factor in their smog troubles.

The vast majority of the electricity supply for Shanghai’s 14 million residents comes from coal-fired power plants. The two key parameters which determine the smog production from these facilities are the composition of the coal being used and the quality of the emissions control systems. As I have discussed in previous columns, as time has passed humanity has used up earth’s high-purity deposits of coal, relegating us to sources which contain impurities and are composed of more-difficult-to-burn hydrocarbons. Burning these lower-quality coals, as is being done in Shanghai, generates a significant amount of smog-producing hydrocarbon fragments. To some extent, emissions of these hydrocarbon fragments, as well as other smog-producing components from the combustion chamber, can be reduced by a process called scrubbing, in which the emissions pass through a shower of water droplets before reaching the smokestack. Unfortunately, the power plants in the Shanghai area do not employ state-of-the-art scrubbing technology.

Finding the answers to Shanghai’s smog problem is not the hard part. They need to install better scrubbing systems on the power plants and avoid traffic jams by providing public transit. The difficulty, as is nearly always the case, is the implementation. These changes for Shanghai will be expensive and require the residents to turn away from the car-based culture that they have embraced. When enough days like the one pictured above occur, I suspect they will start moving in the right direction.

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