Smog and Acid Rain

Core Concepts

In this article, you will learn the basic introductory concepts necessary to understand smog and acid rain. You will learn the necessary conditions and fundamental chemical reactions that create and destroy these atmospheric phenomena. You will also learn the health and environmental impacts of these pollutants.

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Smog is a form of air pollution most frequently found in urban areas which refers to any type of atmospheric pollution that reduces visibility. There are two types of smog: sulfurous and photochemical. Sulfurous and photochemical smog are commonly referred to as London smog and Los Angeles smog, respectively. 

Photochemical Smog

Photochemical smog is primarily a combination of nitrogen dioxide and ozone. Nitrogen dioxide is released from vehicles via exhaust and is broken down (photolyzed) by solar radiation. This results in nitrogen oxide and a lone oxygen atom which will, in turn, combine with an oxygen molecule and create ozone. Ozone then reacts with nitrogen oxide creating oxygen and nitrogen dioxide. This reaction can be seen below:

NO2 + hv → NO +O⦁

O⦁ + O2 → O3

O3 + NO → O2 + NO2

This reaction on its own is relatively manageable and results in a small amount of ozone created near ground level. However, when VOCs (volatile organic compounds) are present, nitrogen oxide oxidizes into nitrogen dioxide without any ozone. Since this ozone is not used in the conversion of nitrogen oxide to nitrogen dioxide, it remains and accumulates near ground level resulting in photochemical smog as seen in the image below.

Smog on central Tokyo

This reaction can be seen below:

RH + OH⦁ → R⦁ + H2O

R⦁ + O2 → RO2

RO2 + NO → RO-⦁ + NO2

This chain of reactions completely eliminates the final reaction in the previous cycle:

O3 + NO → O2 + NO2

The elimination of this reaction results in residual ozone at ground level. This is the chemical formation of photochemical smog. 

Since smog formation is a reaction that heavily relies on photochemical factors, increased sun and heat in the atmosphere will increase the formation of smog at ground level. In Los Angeles, California photochemical smog is particularly abundant. This is due to the large population in the city, high temperatures, and the low altitude of the basin city surrounded by large mountain ranges. Smog formation in Los Angeles can be seen below.

Smog in Los Angeles, California

Sulfurous Smog

Sulfurous smog is formed as a result of burning large amounts of sulfur containing coal. However, sulfurous smog has become less and less of a problem since the passing of Clean air laws in the 1950s. As it is a result of burning sulfur containing coal, the main substance of sulfurous smog is soot or smoke. However, there are other components of sulfurous smog including sulfur dioxide, sodium chloride, and calcium sulfate particles. Sulfurous smog is often intensified by large concentrations of suspended particulate matter and increased humidity. It is commonly referred to as London smog as it was prevalent in London before the passing of Clean air laws in the 1950s.

The formula below is the production of sulfuric acid from sulfur dioxide. This can be one component of sulfurous smog.

2SO2 + O2 →2SO3

SO3 + H2O → H2SO4

Acid Rain

Acid rain is an umbrella term defining any precipitation with acidic components. These components can include nitric acid (HNO3) and sulfuric acid (H2SO4). The formation of acid rain begins with released emissions of sulfur dioxide (SO2) and nitrogen oxides (NOx), often from the burning of fossil fuels like coal and oil. Sulfur dioxide and nitrogen oxide are additionally emitted through natural sources including volcanoes and biological decay. The sources of (SO2) and (NOx) are both naturogenic and anthropogenic. However, it is estimated that 98 percent of all sulfur dioxide emissions are produced anthropogenically.

Sulfur dioxide and nitrogen oxides can combine with water and other compounds to form sulfuric acid (H2SO4) and nitric acid (HNO3). The formation of sulfuric acid can be seen below:

SO2 (g) + H2O (l) → H2SO3 (aq)

2H2SO3 (aq) + O2 (g) → 2H2SO4 (aq)

Nitric acid is formed through the reaction seen below:

3NO2 + H2O → 2HNO3 + NO

There are two different kinds of acid precipitation: wet deposition and dry deposition. Wet deposition is acid rain. It can take the form of rain, snow, fog, or hail. Dry deposition is precipitation that happens in the absence of moisture. Dry deposition occurs when gas and/or dust particles become acidic, and these particles can accumulate on surfaces including buildings/infrastructure, bodies of water, and biotic cells (vegetation). When the next rainfall occurs, these acidic particles are moved off the surface and flow with the water into the ground. This cycle can be seen in the image below.

Both wet deposition and dry deposition can be harmful to plant, animal, and human life. Potential human hazards include respiratory diseases, worsening of pre-existing conditions, and permanent lung damage. When acid precipitation reaches the ground and soil, it can dissolve and destroy nutrients found in soil that are necessary for plant growth. The destruction of these necessary nutrients causes diminished or even halted plant growth additionally making vegetation more susceptible to cold weather damage and insects. When acid precipitation reaches bodies of water, it lowers the pH of the water, making it more acidic. This can be deadly for marine life including reefs, vegetation, and animals. Below is a table highlighting the critical pH levels for select aquatic organisms.

As can be seen, the critical pH for some aquatic organisms is just below the threshold for a solution to be considered basic. This means that even a slight change in pH can negatively impact some species.

Acid rain, in combination with increased absorption of CO2 from the atmosphere, is resulting in ocean acidification. Ocean acidification can often result in coral bleaching and a major loss of marine habitat.

Healthy coral (left side) vs. bleached coral (right side)

Remediation Steps

The EPA has an Acid Rain Program which is decreasing the amount of NOx and SO2 emissions by major industrial plants. This program was created in 1995 and has dramatically reduced these emissions with the support of the government.


Smog and acid rain are two common atmospheric pollutants that have negative impacts on humans and the environment. Smog occurs often in urban areas and can present as photochemical smog (Los Angeles smog) or sulfurous smog (London smog) based on chemical composition. Acid rain is any acidic form of precipitation; this includes both wet deposition and dry deposition. Acid rain, ozone depletion, and smog have negative impacts on the environment and human life.