Photochemical
Smog
Understanding how sunlight, nitrogen oxides, and volatile organic compounds react to form the hazy, harmful air pollution that blankets cities worldwide.
AP Environmental Science
What is Photochemical Smog?
Photochemical smog is a type of secondary air pollution formed when sunlight drives chemical reactions between nitrogen oxides (\(\text{NO}_x\)) and volatile organic compounds (VOCs) in the atmosphere. The result is a brownish haze containing ground-level ozone (\(\text{O}_3\)) and other harmful oxidants.
The Three Key Ingredients
Sunlight (UV)
Provides the energy that initiates photolysis of \(\text{NO}_2\), breaking it apart to start the reaction chain.
Nitrogen Oxides
\(\text{NO}\) and \(\text{NO}_2\) are primary pollutants released from vehicle exhaust and power plants.
VOCs
Volatile organic compounds from gasoline vapors, solvents, and vegetation interfere with the natural \(\text{NO}_x\) cycle.
Why Photochemical Smog Matters
Human Health
Ground-level \(\text{O}_3\) damages lung tissue, aggravates asthma, and increases respiratory infections.
Ecosystems
Ozone reduces photosynthesis in plants, damages crops, and weakens forest resilience.
Economics
Billions in healthcare costs, lost agricultural productivity, and infrastructure damage from smog-related acid deposition.
The Core Reaction Sequence
The \(\text{NO}_x\) Cycle in Detail
1
Sunlight splits \(\text{NO}_2\): UV radiation breaks nitrogen dioxide into nitric oxide (\(\text{NO}\)) and a free oxygen atom (\(\text{O}\)).
2
Ozone forms: The free oxygen atom combines with molecular oxygen (\(\text{O}_2\)) to create ozone (\(\text{O}_3\)).
3
Normal cleanup: \(\text{NO}\) reacts with \(\text{O}_3\), regenerating \(\text{NO}_2\) and \(\text{O}_2\). This would normally keep ozone levels low.
4
VOC disruption: VOCs react with \(\text{NO}\), removing it before it can break down ozone. Ozone accumulates: this is photochemical smog.
Photochemical vs. Industrial Smog
| Photochemical Smog | Industrial Smog | |
|---|---|---|
| Other Name | Los Angeles-type smog | London-type smog |
| Primary Cause | \(\text{NO}_x\) + VOCs + UV | \(\text{SO}_2\) + particulates from coal |
| Key Pollutant | Ground-level \(\text{O}_3\) | Sulfur dioxide (\(\text{SO}_2\)) |
| Appearance | Brown haze | Grey/black haze |
| Climate | Warm, sunny, dry | Cool, humid |
| Season | Summer | Winter |
Primary vs. Secondary Pollutants
Primary Pollutants
Emitted directly from a source. Examples: \(\text{NO}\), \(\text{NO}_2\), VOCs, \(\text{CO}\), particulates from tailpipes and smokestacks.
Secondary Pollutants
Formed in the atmosphere through chemical reactions. Examples: ground-level \(\text{O}_3\), PAN (peroxyacetyl nitrate), \(\text{HNO}_3\).
Factors That Worsen Smog
1
Temperature inversions: A warm air layer traps cool air and pollutants near the ground, preventing dispersion.
2
Urban heat islands: Cities absorb more heat, raising temperatures and accelerating reaction rates.
3
Geographic basins: Mountain-ringed valleys (like Los Angeles) trap polluted air with limited airflow.
4
High traffic density: More vehicles produce more \(\text{NO}_x\) and VOCs, the raw ingredients for smog formation.
Temperature Inversions Trap Smog
How to Analyze a Smog Scenario
1
Identify the pollutant sources: Are there vehicles, factories, or power plants? These emit \(\text{NO}_x\) and VOCs.
2
Check weather conditions: Is it sunny, warm, and calm? These conditions promote photochemical reactions.
3
Look for trapping mechanisms: Is there a temperature inversion or geographic basin that prevents pollutant dispersal?
4
Classify the pollutants: Distinguish primary (\(\text{NO}_x\), VOCs) from secondary (\(\text{O}_3\), PAN).
5
Propose solutions: Catalytic converters, public transit, emission standards, urban tree planting.
Example 1: Los Angeles Basin
Scenario: Los Angeles is ringed by mountains to the north and east. Millions of vehicles emit \(\text{NO}_x\) and VOCs daily. The city receives abundant sunshine year-round. Why does LA experience persistent photochemical smog?
Example 2: The Daily Ozone Curve
Example 3: Reducing Smog
Question: A city government wants to reduce photochemical smog. They can either target \(\text{NO}_x\) emissions or VOC emissions. Which approach is more effective, and why?
Reducing \(\text{NO}_x\)
Catalytic converters on vehicles convert \(\text{NO}_x\) to \(\text{N}_2\) and \(\text{O}_2\). Effective in VOC-limited areas where NOx is abundant.
Reducing VOCs
Vapor recovery systems, reformulated fuels, and solvent regulations reduce VOC emissions. Effective in \(\text{NO}_x\)-limited areas.
Common Mistakes on Smog Questions
✕ Confusing Ozone Types
Stratospheric \(\text{O}_3\) protects us from UV: it is beneficial. Ground-level \(\text{O}_3\) in smog is a harmful pollutant. Same molecule, different location.
✕ Calling \(\text{O}_3\) Primary
Ground-level ozone is always secondary: it forms from reactions in the atmosphere, not from direct emissions.
✕ Mixing Up Smog Types
Photochemical smog involves \(\text{NO}_x\) + VOCs + sun. Industrial smog involves \(\text{SO}_2\) + particulates from coal. Do not confuse their causes.
Summary: Key Pollutants in Smog
| Pollutant | Type | Source | Effect |
|---|---|---|---|
| \(\text{NO}_x\) | Primary | Vehicle exhaust, power plants | Precursor to \(\text{O}_3\) |
| VOCs | Primary | Fuel vapors, solvents, vegetation | Scavenge NO, enable \(\text{O}_3\) buildup |
| \(\text{O}_3\) | Secondary | Photochemical reactions | Lung damage, crop damage |
| PAN | Secondary | VOC + \(\text{NO}_x\) reactions | Eye irritation, plant damage |
| CO | Primary | Incomplete combustion | Reduces blood oxygen transport |
Question 1 of 5
Which of the following is the primary oxidant in photochemical smog?
Ground-level ozone (\(\text{O}_3\)) is the primary oxidant in photochemical smog. It forms as a secondary pollutant when sunlight drives reactions between \(\text{NO}_x\) and VOCs. \(\text{SO}_2\) is associated with industrial smog, not photochemical smog.
Question 2 of 5
During a typical smog episode, at what time of day does ground-level ozone concentration peak?
Ozone peaks in the early afternoon (around 2-3 PM) because it takes several hours of strong sunlight to drive the photochemical reactions that produce it. \(\text{NO}\) peaks during rush hour, but ozone lags behind because it is a secondary product.
Question 3 of 5
VOCs contribute to photochemical smog primarily by:
VOCs scavenge NO by reacting with it, removing the molecule that would normally break down ozone. Without NO to destroy it, \(\text{O}_3\) accumulates. VOCs do not directly produce ozone or block UV radiation.
Question 4 of 5
A temperature inversion worsens photochemical smog because:
A temperature inversion creates a warm air layer above cooler surface air, which acts as a lid. Pollutants cannot rise and disperse, so they accumulate near ground level. The inversion does not change UV intensity or produce additional pollutants: it simply prevents their dispersal.
Question 5 of 5
Which statement correctly distinguishes photochemical smog from industrial smog?
Photochemical smog requires sunlight and is worst in warm, sunny summer conditions. Industrial smog is driven by coal burning and \(\text{SO}_2\) and peaks in cold, humid winter conditions. The pollutant sources and appearances are reversed in choices B and C.