EGEE 101
Energy and the Environment

What is Smog?

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What we typically call smog is primarily made up of ground-level ozone (photochemical smog).  While ozone in the stratosphere, high above the Earth protects human health and the environment, ground-level ozone is the main harmful ingredient in smog.

Ozone is a secondary pollutant - it is not released into the atmosphere directly, but is formed by other released pollutant interactions. The equation that is important is as follows:

NOx + unburned hydrocarbons or any volatile organic compounds (VOCs) sunlight over the arrow O3 

Smog over LA
Here the layer of warm air helps contain the world-famous LA Smog. The mountains help to contain the pollution and the cold air layer also.
Credit: CA ARB

Watch the following video that talks about the science of smog. It is almost 6 minutes long.

Click here for a transcript of The science of smog video.

On July 26, 1943, Los Angeles was blanketed by a thick gas that stung people's eyes and blocked out the Sun. Panicked residents believed their city had been attacked using chemical warfare. But the cloud wasn't an act of war. It was smog.

A portmanteau of smoke and fog, the word "smog" was coined at the beginning of the 20th century to describe the thick gray haze that covered cities such as London, Glasgow, and Edinburgh. This industrial smog was known to form when smoke from coal-burning home stoves and factories combined with moisture in the air. But the smog behind the LA panic was different. It was yellowish with a chemical odor. Since the city didn't burn much coal, its cause would remain a mystery until a chemist named Arie Haagen-Smit identified two culprits: volatile organic compounds, or VOCs, and nitrogen oxides.

VOCs are compounds that easily become vapors and may contain elements, such as carbon, oxygen, hydrogen, chlorine, and sulfur. Some are naturally produced by plants and animals, but others come from manmade sources, like solvents, paints, glues, and petroleum.

Meanwhile, the incomplete combustion of gas in motor vehicles releases nitrogen oxide. That's what gives this type of smog its yellowish color. VOCs and nitrogen oxide react with sunlight to produce secondary pollutants called PANs and tropospheric, or ground-level, ozone.

PANs and ozone cause eye irritation and damage lung tissue. Both are key ingredients in photochemical smog, which is what had been plaguing LA.

So why does smog affect some cities but not others? Both industrial and photochemical smog combine manmade pollution with local weather and geography. London's high humidity made it a prime location for industrial smog. Photochemical smog is strongest in urban areas with calm winds and dry, warm, sunny weather. The ultraviolet radiation from sunlight provides the energy necessary to breakdown molecules that contribute to smog formation. Cities surrounded by mountains, like LA, or lying in a basin, like Beijing, are also especially vulnerable to smog since there's nowhere for it to dissipate. That's also partially due to a phenomenon known as temperature inversion, where instead of warm air continuously rising upward, a pollution-filled layer of air remains trapped near the Earth's surface by a slightly warmer layer above.

Smog isn't just an aesthetic eyesore. Both forms of smog irritate the eyes, nose, and throat, exacerbate conditions like asthma and emphysema, and increase the risk of respiratory infections like bronchitis. Smog can be especially harmful to young children and older people and exposure in pregnant women has been linked to low birth weight and potential birth defects. Secondary pollutants found in photochemical smog can damage and weaken crops and decrease yield, making them more susceptible to insects. Yet for decades, smog was seen as the inevitable price of civilization. Londoners had become accustomed to the notorious pea soup fog swirling over their streets until 1952 when the Great Smog of London shut down all transportation in the city for days and caused more than 4,000 respiratory deaths.

As a result, the Clean Air Act of 1956 banned burning coal in certain areas of the city, leading to a massive reduction in smog. Similarly, regulations on vehicle emissions and gas content in the US reduced the volatile compounds in the air and smog levels along with them.

Smog remains a major problem around the world. Countries like China and Poland that depend on coal for energy experience high levels of industrial smog. Photochemical smog and airborne particles from vehicle emissions affect many rapidly developing cities, from Mexico City and Santiago to New Delhi and Tehran. Governments have tried many methods to tackle it, such as banning cars from driving for days at a time. As more than half of the world's population crowds into cities, considering a shift to mass transit and away from fossil fuels may allow us to breathe easier.

Credit: TEDEd, Kim Preshoff

We also normally think of smog as something that occurs in cities, due to excessive traffic and emissions outputs.

What Does Smog Require?

Smog requires:

  • Warm temperatures (thus smog is a summer issue in the US).
  • Sunlight (thus it does not form at night).
  • Some form of volatile organic compound, such as gasoline but also natural VOCs emissions from forests, etc., are a significant source
  • NOx – there are both natural and anthropologic emissions.
Photograph of smog in London, the city is only partially visible in the haze.
Smog is a city problem (mostly) because lots of cars generate the pollutants that form smog. It appears in the morning hours and can last late into the evening. Not only does it reduce visibility — it is damaging to human health and can even kill! This example is from LA where there is pollution from vehicles, electricity generation, and factories combining.
Credit: Artinun/adobe.stock.com

Other Components of Smog

Ozone (O3) is the main component of photochemical smog. Particulates from combustion can also be present. The most dangerous smog is when both ozone and sulfate aerosols are present. The famous Killer Fog of London during the mid 1900s and the late 1800s was comprised of sulfate aerosols and smoke (particulates) from the coal fires used to heat homes and businesses. Over a 4-day period in 1952, the London Smog was probably responsible for about 3,000 or so "extra deaths" that year. The English expression would be "kick the bucket." And it would have been the old, ill, and infirm who died, not healthy Londoners. In New York City during the late 1800s, improving the air quality took the passing of a law that required anthracite coal (little smoke and low sulfur, so no sulfate aerosols) as the fuel of choice. This law, ultimately, was responsible for the rapid growth of anthracite mining in NE Pennsylvania at the turn of the century (1900's).

Temperature Inversion

Smog is a problem because it is concentrated pollution. In the normal events of the average day, our pollutants are released to rise above our cities to be blown elsewhere (this upsets the Swedish when British pollution kills their forests (acid deposition related)). However, we can also have a temperature inversion. This is when a layer of warm air acts like a lid containing pollutants sitting over a layer of cold air. The situation is more common in valleys as the cold air can collect in the bottom of the valley (this is how fog forms, stagnant cold air sitting over river valleys).

Temperature inversion.  pollution from traffic is stuck under a warm inversion layer which is stuck under cooler air.
Temperature Inversion
Credit: The Amazing World of Science is licensed under a CC BY-SA 4.0 License.

At night, the ground level temperature cools faster than the air above it. Pollutants become trapped under the layer of warm air. As the sun rises in the morning, the ground level temperature warms up faster than the air above it, pushing the air upwards, which breaks up the warm air layer, allowing the pollutants to escape. However, if there is no wind, the air can become stagnant.

-Marconia County Web Site.