IB ESS HL 8.3 — Urban Air Pollution (Photochemical Smog and Tropospheric Ozone) Notes

Topic overview: Urban air pollution (IB ESS HL 8.3)

• Guiding question: How can urban air pollution be effectively managed?
• Focus: Photochemical smog and tropospheric ozone as key issues in urban environments; their formation, drivers, impacts, and economic consequences.

Photochemical Smog: definition and composition

• Photochemical smog forms when sunlight acts on primary pollutants causing their chemical transformation into secondary pollutants.
• Major components:
  • Tropospheric ozone (O$3$) and nitrogen dioxide (NO$2$)
  • Complex mixture of about $ext{approximately } 100$ different primary and secondary air pollutants
• Primary contributors in cities: motor vehicle exhausts; forest burning can also contribute significantly.
• Process: sunlight drives reactions among NOx, volatile organic compounds (VOCs), and hydrocarbons to produce secondary pollutants including O$_3$.

Major pollutants and sources

• Nitrogen dioxide (NO$_2$): brown hue; strong oxidizer; irritates eyes; can reduce concentration; main source is combustible vehicle emissions.
• Nitrous oxides (NOx): include NO and NO$2$; precursors to O$3$ and PANs.
• Volatile Organic Compounds (VOCs): compounds that evaporate and participate in photochemical reactions; examples include methane (CH$4$), ethane (C$2$H$_6$), and alcohols.
• Peroxyacyl nitrates (PANs): secondary pollutants produced when oxidized VOCs combine with NO$_2$.
• Tropospheric ozone (O$_3$): secondary pollutant formed from the reaction of NOx and VOCs under sunlight.
• Key takeaway: Photochemical smog is a complex mix of primary and secondary pollutants, with NOx and VOCs from vehicle emissions as central ingredients.

Formation mechanisms: tropospheric ozone (O$_3$)

• Tropospheric ozone is a secondary pollutant formed in the presence of sunlight.
• Simplified sequence: 1) NO reacts with oxygen to form NO$2$ (NO + O$2$ → NO$_2$).
  • Note: a common concise representation is $ext{NO} + frac{1}{2} ext{O}<em>2 ightarrow ext{NO}</em>2 ext{,}$ reflecting that O atoms come from O$2$. 2) NO$2$ absorbs sunlight and dissociates: $ext{NO}<em>2 + h\nu ightarrow ext{NO} + ext{O}.$ 3) The free oxygen atom reacts with molecular oxygen: $ext{O} + ext{O}</em>2 <br /> ightarrow ext{O}<em>3.$ 4) VOCs and hydrocarbons accelerate the formation by generating radicals that propagate radical chain reactions, sustaining higher O$3$ production.
• Additional detail: VOCs can be oxidized to form reactive intermediates that drive the cycle, promoting NO$2$ conversion to O$3$ even when NO is present.
• PANs example: oxidized VOCs plus NO$2$ lead to PAN formation: $ext{VOCs} + ext{NO}</em>2 <br /> ightarrow ext{PANs} ext{ (peroxyacyl nitrates)}.$

Tropospheric ozone: key facts

• Tropospheric ozone is a secondary pollutant formed via photochemical reactions involving NOx and VOCs under sunlight.
• Only about $10\%$ of atmospheric ozone is in the troposphere: $\%<em>{\text{troposphere}}(O</em>3) \approx 0.10.$
• Ozone is also a greenhouse gas with a global warming potential per unit mass roughly $GWP(O<em>3) \approx 2000 \times GWP(CO</em>2)$, i.e., far more impactful per unit mass than CO$_2$ on a 100-year timescale.

Relevance of VOCs and PANs

• VOCs include methane (CH$4$), ethane (C$2$H$_6$), and alcohols; main sources are road transport and solvent releases from paints, glues, inks, petrol handling, and distribution.
• PANs form via oxidation chemistry and act as reservoir species for NOx, influencing ozone formation and transport.

Photochemical smog formation conditions: timing and intensity

• Local primary pollutants (NOx and hydrocarbons) peak during morning and evening rush hours due to traffic.
• Photochemical smog peaks in the early afternoon when sunlight is strongest, driving photochemical reactions to their maximum.

Meteorological and topographical factors affecting smog

• Meteorological drivers that intensify smog:
  • Abundant insolation (lots of sunlight)
  • Reduced wind (stagnant air)
  • Temperature inversions (cool air trapped near the ground under a warmer layer)
• Topographical drivers:
  • Cities surrounded by hills and mountains are more vulnerable
  • Low-lying and/or valley cities trap pollutants; surrounding hills reduce wind movement
• Urban form factors: high building density can contribute to smog by reducing dispersion

Frequency and severity of smog

• Determined by:
  • Local topography
  • Climate
  • Population density
  • Fossil fuel use
• Smog tends to be most severe over large cities that are low-lying or in valleys, where wind is restricted and hills/mountains hinder dispersion.
• Hot, calm days exacerbate smog formation.

Thermal inversion and smog

• Inversion scenario: warm air overlays cooler air near the ground, trapping pollutants at ground level.
• Mechanism: a dense, cool air layer is beneath a lighter, warm air layer, reducing vertical mixing.
• Consequences: pollutant concentrations build up near the ground instead of dispersing with normal air movement.
• Inversions are common in warm, dry climates and can be temporary or persistent depending on weather.

Weather and smog dynamics

• Weather elements influence smog clearance:
  • Rain can wash pollutants from the air (air cleansing effect).
  • Winds can disperse pollutants and reduce concentrations.
• Diurnal patterns of temperature and wind, plus sunshine, drive daily cycles in smog levels.

Task prompts and ancillary resources

• Diurnal changes in air pollution: tasks on the website guide analysis of Mexico City's diurnal patterns (refer to the task for specifics).
• Related video resource: "The Science of Smog" (TED-Ed) for conceptual understanding and visuals.

Direct impacts of tropospheric ozone

• O$_3$ is highly reactive and can directly cause:
  • Damage to plant tissues (cuticles) and chlorophyll degradation, reducing photosynthesis and productivity.
  • Irritation of eyes and respiratory tract (nose, throat, lungs) in humans.
  • Damage to fabrics and rubber (e.g., tires).
• Smog is a complex mixture; tropospheric ozone is the main pollutant with wide-ranging effects on health, ecosystems, and materials.

Deforestation, burning, and regional haze

• Deforestation and burning can contribute to smog and haze in certain regions.
• Example: annual haze over Singapore and Malaysia linked to forest burning in Sumatra, Indonesia.

Economic losses and social implications

• Economic costs of urban air pollution are significant:
  • Increased risk of heart disease, respiratory illnesses, and lung cancer among individuals.
  • Higher healthcare costs for families, companies, and governments.
  • Reduced earnings due to staff absences and impacts on GDP.
  • Impacts disproportionately borne by poorer communities.

Connections to broader principles

• Photochemical smog illustrates interactions among emissions, sunlight, atmospheric chemistry, and meteorology.
• Addresses the balancing of energy needs (transport) with air quality, health, and economic costs.
• Concepts link to greenhouse gas effects (O$_3$ as a pollutant and a greenhouse gas with high GWP).

Key formulas and numerical references (recap)

• NO oxidation to NO$2$ (simplified): $ext{NO} + frac{1}{2} ext{O}</em>2 <br /> ightarrow ext{NO}_2 ag{1}$
• Photolysis of NO$2$ under sunlight: $ext{NO}</em>2 + h\nu <br /> ightarrow ext{NO} + ext{O} ag{2}$
• O atom reaction to form ozone:
  $ext{O} + ext{O}<em>2 ightarrow ext{O}</em>3 ag{3}$
• PAN formation (illustrative):
  $ext{VOC} + ext{NO}_2 <br /> ightarrow ext{PANs} ag{4}$
• Tropospheric ozone share:
  $ext{Fraction of O}_3 ext{ in troposphere} o \approx 0.10$
• Global warming potential relationship (relative to CO$2$): $ext{GWP}(O</em>3) \approx 2000 imes ext{GWP}(CO_2)$
• Approximate pollutant diversity: $\approx 100$ different primary and secondary pollutants in smog

Summary takeaways

• Urban photochemical smog arises from interactions between NOx, VOCs, and sunlight, producing O$_3$, PANs, and other secondary pollutants.
• Ozone in the troposphere is a key pollutant with strong health and climate relevance due to its high GWP and reactivity.
• Health, ecological, and economic impacts are substantial, with poorer communities often bearing higher burdens.
• Weather, topography, and human activities (notably vehicle emissions) dictate the frequency, severity, and distribution of smog in urban areas.
• Mitigation requires reducing emissions (especially NOx and VOCs), urban planning to improve dispersion, and policies addressing population exposure and health costs.