Photochemistry and Ozone
Learning Objectives
Students will be able to:
- Explain the relationship between photon energy and wavelength using E = hv
- Describe photodissociation and its role in atmospheric chemistry
- Analyze the Chapman cycle for stratospheric ozone formation and destruction
- Distinguish between stratospheric ozone (protective) and tropospheric ozone (pollutant)
- Explain how ground-level ozone forms through NOx-VOC photochemistry
The Big Question
"How does sunlight drive chemical reactions in the atmosphere, and why is ozone both essential and harmful?"
Photon Energy and Wavelength
Photochemical reactions are initiated when molecules absorb photons with sufficient energy to break chemical bonds.
E = hv = hc/λ
| UV Region | Wavelength (nm) | Energy (kJ/mol) | Atmospheric Significance |
|---|---|---|---|
| UV-C | 100-280 | 430-1200 | O2 photolysis; absorbed by stratospheric O3 |
| UV-B | 280-315 | 380-430 | O3 photolysis; causes sunburn |
| UV-A | 315-400 | 300-380 | NO2 photolysis; reaches surface |
The Chapman Cycle: Stratospheric Ozone
Sydney Chapman (1930) described the basic chemistry of the stratospheric ozone layer:
Formation Reactions
- O2 + hv (λ < 240 nm) → O + O (photolysis)
- O + O2 + M → O3 + M (termolecular)
Destruction Reactions
- O3 + hv (λ < 320 nm) → O2 + O (photolysis)
- O + O3 → 2O2 (bimolecular)
At steady state, the ozone layer maintains a balance between formation and destruction, protecting life from harmful UV-C and most UV-B radiation.
Tropospheric Ozone: The Photochemical Smog Cycle
At ground level, ozone forms through a different mechanism involving nitrogen oxides (NOx) and volatile organic compounds (VOCs):
NOx Photostationary State
- NO2 + hv → NO + O
- O + O2 + M → O3 + M
- O3 + NO → NO2 + O2
Without VOCs, this cycle produces no net ozone
VOC Perturbation
VOCs produce peroxy radicals (RO2) that convert NO to NO2 without consuming ozone:
RO2 + NO → RO + NO2
This allows NET ozone accumulation
Indoor Ozone: Sources and Sinks
Indoor Ozone Sources
- Infiltration from outdoor air
- Photocopiers and laser printers
- Some "air purifiers" (ionizers)
- UV germicidal lamps (if unshielded)
Indoor Ozone Sinks
- Reaction with surfaces (dominant)
- Reaction with NO (from gas stoves)
- Reaction with VOCs (terpenes)
- Air exchange (ventilation)
Indoor ozone is typically 20-70% of outdoor levels, with the ratio depending on air exchange rate, surface reactivity, and indoor sources.
Activity: Photochemistry Calculations
Problem Set
-
Bond Energy: The O=O bond in O2 has a bond energy of 498 kJ/mol.
- Calculate the maximum wavelength of light that can break this bond
- What region of the UV spectrum is this?
- Use: E = hc/λ where hc = 1.196 x 105 kJ-nm/mol
- Quantum Yield: NO2 photolysis has a quantum yield (fraction of absorbed photons leading to reaction) of 1.0 at 400 nm but only 0.3 at 450 nm. What does this tell us about the photodissociation process?
- Indoor-Outdoor Ratio: If outdoor ozone is 60 ppb and the indoor-outdoor ratio is 0.4, what is the indoor ozone concentration? At what level would you recommend reducing outdoor air intake?
Key Takeaway
Photochemistry is the engine driving atmospheric oxidation. The same molecule, ozone, plays opposite roles at different altitudes: protective in the stratosphere, harmful at ground level. Understanding the photochemical mechanisms that produce and destroy ozone is essential for managing both outdoor air quality and indoor environments where ozone can react with other pollutants to form harmful secondary products.