Photochemical Smog (HL) (DP IB Environmental Systems & Societies (ESS))

Formation of Photochemical Smog

What is photochemical smog?

• Photochemical smog is a type of air pollution formed when sunlight triggers chemical reactions between primary pollutants

  • These reactions produce secondary pollutants

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

Primary pollutants

• Nitrogen oxides (NOx):

  • Released from combustion of fossil fuels, such as in vehicles and power plants

  • Includes nitrogen dioxide (NO₂) and nitric oxide (NO)

  • Reacts in sunlight to form secondary pollutants like tropospheric ozone

• Volatile organic compounds (VOCs):

  • Emitted from vehicle exhausts, industrial processes, and chemical solvents

  • Includes hydrocarbons like benzene and methane

  • These can form secondary pollutants in the presence of sunlight

Secondary pollutants

• Peroxyacyl nitrates (PANs):

  • Formed from the reaction of VOCs and NOx under sunlight

  • Irritate eyes and respiratory systems

  • Harm plants by reducing growth and photosynthesis

• Tropospheric ozone (O₃):

  • Created when NOx reacts with oxygen in sunlight

  • A major component of photochemical smog

  • Causes respiratory problems and damages crops

Factors Intensifying Smog Formation

• Photochemical smog formation is affected by meteorological and topographical factors

Meteorological factors

Abundant insolation (high sunlight levels)

• Strong sunlight leads to photochemical reactions between pollutants like nitrogen oxides (NOx) and volatile organic compounds (VOCs)

  • Long sunny days increase smog intensity

  • E.g. cities in warm climates, such as Los Angeles and New Delhi, experience severe smog due to high levels of insolation

Reduced wind

• Stagnant air prevents dispersion of pollutants

  • This traps them near ground-level

  • This allows pollutants to accumulate and react to form smog

Temperature inversion

• Occurs when a layer of warm air traps cooler air near the ground

  • Pollutants accumulate in the trapped cool air, leading to high concentrations

  • This typically occurs during winter nights or early mornings

• How temperature inversion occurs:

  • During the day:

    • The ground heats up, warming the air above

    • Warm air rises, allowing pollutants to disperse

  • During the night:

    • The ground cools rapidly, cooling the air near the surface

    • A layer of warm air above traps the cooler air and pollutants

  • Inversions are more likely in valleys or areas with minimal wind

Topographical factors

Mountainous terrain

• Cities surrounded by mountains (e.g. Mexico City) are prone to smog formation

  • Mountains block wind, trapping pollutants in the valley

Urban infrastructure

• High-rise buildings can create "urban canyons"

  • These restrict air movement, reducing pollutant dispersal

  • E.g. Hong Kong experiences smog intensified by dense urban development