Screen Shot 2018-08-14 at 5.24.40 PM.png

by Emily Hays


Smog - you might recognize it as the grey or yellowish-brown haze that often covers big cities around the world, or maybe you've noticed the throat-burning chemical smell of city air on a windless, summer day or a foggy, cool day. But what exactly is smog? You may be surprised to learn that it is more than a mix of smoke and fumes from automobiles and smokestacks, and it is just one of a number of types of air pollution that human activities can produce.

Today, we have a wealth of scientific evidence indicating that smog is a direct threat to human and environmental health, and many countries around the world have taken successful steps toward reducing smog production in their industrialized areas. However, smog is still a major problem in many parts of the world, especially in developing countries like China, Pakistan and India, where the livelihoods of urban dwellers are increasingly reliant on industrial processes that pollute the air. In many ways, then, smog reduction is both one of our planet's greatest, ongoing environmental success stories, and yet it is still an intricate and dynamic issue that will require both innovation and global awareness to be successfully eliminated from our atmosphere in the future.

This article will address a few introductory questions about smog, drawing upon meteorology and technology:

  • What are the two types of smog, and what is the atmospheric science behind their formation?

  • What meteorological and geographical phenomena favor smog formation?

  • What technological and regulatory advancements have been successful at mitigating smog formation, and what more can be done?

  • Finally, what is the broader context of smog in environmental and atmospheric science?

 Figure 1. The effect of acid rain on limestone sculpture over time.  Chemistry@Elmhurst

Figure 1. The effect of acid rain on limestone sculpture over time. Chemistry@Elmhurst

Atmospheric Science

Simply put, smog is a type of air pollution that occurs when chemicals and particles from human activities accumulate in the atmosphere. Under the right weather conditions, the air pollutants in smog can become so concentrated that they partially block out the sun, greatly reducing visibility and creating hazardous conditions for drivers and pedestrians. But even under less extreme conditions, smog poses a number of other risks to both humans and the planet. Numerous scientific studies show that the prolonged inhalation of certain components of smoggy air is detrimental to respiratory and cardiac health, and may even lower the functioning of our immune systems. In fact, residents in some of the world's most polluted cities wear face masks outdoors to protect their lungs from the toxic air. Moreover, smog can damage croplands, and contaminate ecosystems

and bodies of water several miles downwind of its source; it's even known to contribute to the erosion of limestone statues and architecture (See Figure 1.) To understand how these things happen, we need to look first at the atmospheric science behind smog formation, and then at the meteorological conditions that determine its severity.

Industrial Smog

Industrial smog is generated when coal is burned for fuel or heating, and has been a feature of the urban landscape ever since Western Europe and the United States underwent an industrial revolution in the mid to late nineteenth century. While these regions have since greatly decreased their reliance on coal energy - and subsequently the presence of concentrated industrial smog in their urban environments - industrial smog continues to be a problem anywhere that large amounts of coal are burned. (See Figure 2)

 Figure 2 - Atsushi Kodera, "Photochemical Smog Creeps Back on Hot, Still Days,"  The Japan Times, August 10, 2015

Figure 2 - Atsushi Kodera, "Photochemical Smog Creeps Back on Hot, Still Days," The Japan Times, August 10, 2015

The most common components of industrial smog from coal burning are sulfur dioxide and particulate matter, or soot. These substances are known as primary pollutants because they are the initial chemical ingredients in smog, and they have yet to be chemically converted by interactions in the atmosphere.

Once the primary pollutants enter the atmosphere, they mix with the water vapor droplets in the air to create smog - this is where the term smog originates, it is a combination of the words "smoke" and "fog." As it mixes with the water vapor, the sulfur dioxide molecules undergo a number of chemical reactions, and ultimately become sulfuric acid aerosols. These aerosols are called secondary pollutants because they have undergone chemical transformations in the atmosphere. Together, the aerosols and soot remain in the air until they are dissipated by wind or precipitation.

This brings us to a further by product of coal burning. When water molecules combine with the sulfuric acid aerosols, they can condense, form clouds, and eventually fall as precipitation. While acid rain is usually around a pH of 6, it is acidic enough to wilt crops, acidify bodies of water, and damage the delicate balance of both terrestrial and aquatic ecosystems. Over the course of decades, acid rain can even dissolve exposed marble and limestone architecture and statues.

Many countries around the world have made progress in reducing production of industrial smog by phasing out coal as a fuel source, implementing emission regulations and clean air standards, and making technological advancements. Still, industrial smog is a major problem in many parts of the developing world.

Photochemical Smog

Just like industrial smog, photochemical smog emerged as a product of widespread reliance on fossil fuels as our main energy source, particularly for transportation. In the first half of the twentieth century, the prevalence of personal automobiles in the United States expanded dramatically, and the vast highway system that we know today was developed to accommodate them. By the late 1960's and 70's, major cities such as Los Angeles, New York, and Chicago were often choked by a blanket of air pollution so dense and noxious that it threatened the health of millions of urbanites. 

It is largely thanks to our scientific research and technological advancements that have allowed us to greatly improve air quality in these and other cities since the 1970's.

The Internal Combustion Engine

So, how exactly is photochemical smog different from industrial smog? First, it requires the energy of sunlight rather than water vapor to produce it's secondary pollutants. The process of photochemical smog production begins when an automobile burns gasoline in its internal combustion engine, or ICE.

During the chemical process of combustion, the hydrocarbons composing the gasoline are combined with oxygen that is pulled into the engine from the atmosphere. While "complete" combustion of fuel only produces water vapor and carbon dioxide, there are almost always other products left over, which are also emitted into the atmosphere as exhaust. This "incomplete" combustion of gasoline generates unburned hydrocarbons and carbon monoxide, also known as volatile organic compounds, or VOCs. On the other hand, when an ICE is burning hot enough to combust most of the hydrocarbons, it also produces pollutants known as thermal NOx's, or nitrous oxides.

Screen Shot 2018-08-15 at 4.10.40 PM.png

Clearly, ICE combustion entails a trade off; no matter what they emit an air pollutant that contributes to smog formation. In an ideal world, ICE's would be able to burn hydrocarbons in such a way that they produce the minimum amount of primary pollutants, but this is not the case. In order to run smoothly, an ICE must maintain an optimal air-fuel ratio, which varies widely based on how the car is performing at the time. When a car is cruising down the freeway or accelerating, the engine will heat up and consume more oxygen than a car that is idling or decelerating. So, a car that is running at a lower temperature will emit CO and hydrocarbons in its exhaust, while a car running at higher temperatures will emit more NOx's. 

Internal combustion engines are not the only source of the volatile organic compounds in photochemical smog. Other sources of VOCs include paints and solvents containing petrochemical byproducts, diesel and organic waste combustion, fracking and oil drilling processes. The largest source of VOC’s is vegetation. (See Figure 4 for an illustration of the ICE combustion trade off.)

So, how do the emission of VOCs and NOxs lead to the production of secondary pollutants in photochemical smog? Without VOCs, NOx makes ozone and then destroys it, so ozone doesn't accumulate. VOCs allow the NOx to be recycled, so that it can produce ozone without destroying it. Normally, a series of chemical reactions between oxygen and nitrous oxide gases keep the relative amounts of ozone in equilibrium. However, when large amounts of NOxs and VOCs enter the atmosphere, they set off a chain reaction that prevents ozone from being removed from the lower atmosphere, while also producing excessive ozone and other secondary pollutants. This process relies upon the light energy of UV rays from the sun, which is why primary pollutants like VOCs and NOx's predominate in earlier hours of the day as people drive to work in the morning. Likewise, ozone levels rise later in the afternoon once hours of UV radiation have allowed atmospheric reactions to consume the primary pollutants in the production of secondary pollutants (See Figure 5.)

 Figure 5. Stanley E. Manahan, Fundamentals of Environmental Chemistry (Boca Raton, FL, Lewis Publishers, 1993), 630.

Figure 5. Stanley E. Manahan, Fundamentals of Environmental Chemistry (Boca Raton, FL, Lewis Publishers, 1993), 630.

The dominant secondary pollutant in photochemical pollution is ozone (O3.) You may know it as the protective layer of gas in the stratosphere, which absorbs large amounts of dangerous, incoming UV radiation from the sun. However, while the ozone layer in the upper atmosphere is extremely beneficial to life on Earth, ozone in the troposphere is a toxin to human and ecological life. Known to damage lung tissue, weaken the immune system, and damage crops and other plant and animal life, this powerful oxidizing agent is one of the deadliest components of photochemical smog. Other secondary pollutants that form when NOx's react with sunlight include peroxyacetyl nitrate and aldehydes.

Finally, just like industrial smog, the primary pollutants may react with atmospheric water vapor to form acid rain. When nitrous oxides encounter water, they form nitric acid, which has similarly damaging implications for environmental and human health to those posed by sulfuric aerosols. Ultimately, the secondary pollutants in photochemical smog are among the most hazardous components of smog, and the science behind their formation and their health effects is still under investigation.

Meteorological and Geographic Factors in smog formation
While smog can form any place where enough primary pollutants are produced, a number of meteorological and geographical factors dictate the density and duration of its presence. Urban areas located in valleys and basins such as Mexico City are especially prone to smog formation because they trap air, allowing smog to build up without circulation (See Figure 6, showing smog trapped in the Mexico City valley.)

Weather also dictates smog formation, and it can do this in two major ways. First, the weather conditions over a city must be conducive to the accumulation of secondary pollutants. This is why photochemical smog is typically densest on cloudless summer days when incoming solar radiation, or insolation, is most direct, and also why the sunny climes of Los Angeles and Denver are known for their photochemical smog. From a meteorological perspective, such fair weather often occurs under a high pressure system or anticyclone that forms over the region.

The second way weather affects smog is by how effectively it traps pollution and allows it to accumulate in the atmosphere. High relative humidity - or the presence of dense water vapor in the atmosphere - is extremely effective at trapping particulate matter, as well as sulfuric and nitric acid aerosols. Unless dissipated by wind or precipitation, the pollutants in smog will remain in the lower atmosphere, concentrated over their source. Smog is often densest when a temperature inversion traps a layer of cool air below a layer of warm air, the inversion process also favors fog formation.

Innovation and Regulation

 Left: A smoggy haze lingers over the L.A. skyline in the 1980s. Right: The city's air is clear after a winter storm in March, 2015.  As Smog Thins in L.A., Dramatic Evidence of Kids' Healthier Lungs  National Geographic  PHOTOGRAPHS BY PHILIP JAMES CORWIN, CORBIS (LEFT) AND RINGO CHIU, ZUMA WIRE/CORBIS (RIGHT)

Left: A smoggy haze lingers over the L.A. skyline in the 1980s. Right: The city's air is clear after a winter storm in March, 2015. As Smog Thins in L.A., Dramatic Evidence of Kids' Healthier Lungs National Geographic


Just like any other environmental issue, the key to reducing smog lies in our ability to innovate and raise public awareness. In the decades following the 1970's, numerous cities in the United States have seen a substantial improvement in their air quality, largely thanks to the federal, regulatory measures implemented by the Environmental Protection Agency. The Clean Air Act of 1970 mandated tighter federal control over how much air pollution could be produced, particularly in the industrial sector. One of the most effective measures against industrial smog is the installation of particulate filters and scrubbers in any new coal-fired power plants to remove much of the soot and sulfur oxides from emissions. (See Figure 7, showing Los Angeles in 1980s vs 2015.)

In addition to air quality regulations, we have also made great technological progress in reducing smog production. For instance, many developed countries have shifted away from coal as a main fuel source, which has substantially reduced the presence of industrial smog in their urban areas. Many countries in Europe have even been able to begin phasing out fossil fuels altogether by adopting renewable energy sources like solar, wind, and hydroelectric power.

Still, photochemical smog is a prevailing issue in most developed countries due to the high traffic volume in densely populated urban centers. As such, some of the most important technology for combatting photochemical smog lies in a few, particular ICE innovations.

First, the installation of catalytic converters in all new cars has been a major step toward reducing primary pollutant production. These devices use rare metals to turn some of the CO, VOCs, and NOxs in exhaust into water vapor and carbon dioxide before they are emitted into the atmosphere. A second key innovation is the implementation of oxygen sensors and fuel injection systems in newer cars, which are calibrated to regulate the air-fuel ratio in a way that reduces exhaust pollution.

Ultimately, these technologies can only mitigate the production of primary pollutants by internal combustion engines; a vast reduction in photochemical smog will require both the technological advancements and public commitment necessary to dramatically reshape our current relationship with fossil fuel-powered transportation.


Industrial and photochemical smog are just a few of the many forms of air pollution that pose environmental risks to our planet. Some of the most environmentally destructive air pollutants produced by human activity are greenhouse gases like carbon dioxide and methane, and even smog pollutants like ozone and nitrous oxides. While the atmosphere naturally contains certain levels of greenhouse gases - mainly methane, carbon dioxide, and water vapor - any substantial increase in their amounts has the potential to raise Earth's average, surface temperature above its homeostasis of roughly 14 degrees Celsius. Climate scientists agree that even a few degrees difference in global temperature average would have devastating and comprehensive impacts on both ecological and human well-being.

Clearly, then, our shared responsibility for practices that produce air pollution extend beyond the local or regional threats posed by smoggy landscapes. As citizens of our shared planet, our ability to make the technological advancements and establish regulatory measures that will curtail air pollution will impact all of us, not just our cities. This global perspective - the understanding that we all live under the same sky - is what will catalyze the change we need.

See all our Science Explainers here

Back to SkyDayProject


Emily Hays created this while a student in the Department of Earth and Planetary Sciences at Northwestern University