Causes, Effects & Managing Photochemical Smog (College Board AP® Environmental Science): Study Guide

What is photochemical smog?

• Photochemical smog is a type of air pollution formed when primary pollutants, nitrogen oxides and volatile organic hydrocarbons, react with heat and sunlight to produce secondary pollutants of tropospheric ozone, aldehydes and peroxyacyl nitrates (PANs)

• The reaction is often seen as a brown or grey haze over urban areas

• Industrial smog is a thick, yellowish fog created by the burning of fossil fuels

  • Combined with other chemicals in the air, sulfur dioxide, particulates, and nitrogen oxides generate secondary pollutants of sulfuric acid, particulates, and nitrogen dioxide

What affects the formation of smog?

• Many environmental factors affect the formation of photochemical smog

• The frequency and severity of smog depend on the local topography, climate conditions, population density, and the amount of fossil fuel use in an area

• Areas with geographical features that inhibit air movement, such as valleys or basins, can experience higher levels of smog due to the accumulation of pollutants

  • Mexico City lies 2,240 m above sea level in an extinct volcano crater

  • Air oxygen decreases at this altitude, and fuel burns incompletely, emitting more carbon monoxide and other chemicals which are trapped within the crater

• Deforestation, forest fires and burning of biomass release large amounts of particulate matter, volatile organic compounds, carbon monoxide and other pollutants into the atmosphere

• Regions with hot and sunny climates are more prone to smog formation as sunlight contributes to the chemical reactions that produce ozone

  • Cities such as Los Angeles and Mumbai experience severe smog due to high levels of insolation

• Stagnant air prevents dispersion of pollutants

  • This traps them near ground-level

  • This allows pollutants to accumulate and react to form smog

• High-rise buildings can create 'urban canyons' and reduce air movement

When is smog formed?

• Nitrogen dioxide (NO₂) is from both natural and anthropogenic sources

• Sunlight is essential to break the chemical bonds and release one oxygen molecule to form nitrogen oxide (NO)

• The single oxygen molecule then bonds with atmospheric oxygen (O₂) to form tropospheric ozone (O₃)

• Lightning strikes convert nitrogen gas (N₂) in the atmosphere into nitrogen oxide (NOₓ)

Early morning

• NOx and VOCs are produced as people drive to work

  • These begin to react in the atmosphere, forming nitrogen dioxide (NO₂)

Early afternoon

• Ozone levels are highest in the afternoon and during the summer due to higher levels of insolation

• NO₂ reacts with water vapor to form nitric acid (HNO₃) and nitric oxide (NO)

• NO₂ also reacts with VOCs released by vehicles, refineries, and gas stations to produce toxic PANs (peroxyacyl nitrates)

Late afternoon

• Late afternoons and evenings, ozone production slows and eventually stops

• O₃ then reacts with the newly formed NO to form NO₂ and O₂

Awaiting Image

The production of photochemical smog in the presence of insolation

Volatile organic compounds (VOCs)

• VOCs are a group of chemicals that easily evaporate or sublimate into the air at room temperature from things like paints, cleaning products, new furniture, gasoline and formaldehyde

• Photochemical oxidants are made when VOCs mix with NO in the air, which disrupt the breakdown of O₃

• Photochemical smog (brown smog) forms when the remaining ozone and photochemical oxidants mix

• Trees release VOCs, which means that forested places can also contribute to brown smog, even if they are cooler in temperature

Where does smog form?

• Smog can form in any climate where industries or urban areas emit significant amounts of air pollution, such as gases, smoke and vehicle exhaust

  • Traffic increases nitrogen oxide (NO) and VOC emissions, leading to photochemical smog

• It is more severe during warm, sunny weather when the upper air is warm enough to stop vertical circulation

  • Smog forms when when nitrogen oxides and VOCs compounds react with sunlight, producing ozone and PAN

  • Urban ozone levels tend to be low because ozone reacts with nitric oxide (NO) from vehicle emissions to form nitrogen dioxide

  • Rural ozone levels are higher because NO₂ catalyzes ozone formation in sunlight. NO₂ converts to nitric acid vapor, reacting with atmospheric ammonia to create fine particles

• It is particularly common in low-lying basins surrounded by mountains because the smog is trapped in the valley and cannot be carried away by wind

• Smog frequently remains over densely populated cities or urban areas for extended periods and can build-up to hazardous levels

• VOCs from traffic and plants oxidize under sunlight, forming organic particles that reduce visibility, especially on sunny days with little wind

Reducing smog

• Photochemical smog can be reduced through the reduction of nitrogen oxide and VOCs through:

• Reducing the use of fossil fuels by driving less, carpooling, using public transport and making homes and cars more energy efficient

• Maintaining vehicles by having regular servicing, scheduled oil changes and keeping tires inflated to the proper levels to improve gas mileage to reduce emissions

• Reducing VOCs by

  • fueling up during cooler times of the day to prevent gas fumes from heating up and producing ozone

  • using low-VOC paints

  • using electric appliance instead of gas-powered ones

• Reducing waste through recycling and reusing and avoiding unnecessary packaging

• Not burning any trash that will release particulates into the atmosphere

Impacts of smog on people

• Photochemical smog can negatively affect human health in different ways, including respiratory problems, eye irritation, cardiovascular dysfunction, neurological disorders, and cancer

• Children and pregnant women are at risk to the hazards of smog

Respiratory issues

• Tropospheric ozone irritates the respiratory system, causing coughing, throat irritation, and contributes to asthma and other respiratory conditions

• Prolonged exposure to high ozone levels can lead to chronic respiratory illnesses and reduced lung function

Eye irritation

• Exposure to smog can irritate the eyes, leading to inflammation, redness, tearing and discomfort

• Long-term exposure can lead to blindness