Skip to main content

Photochemical smog and its reactions

Short answer

Photochemical smog is a brownish oxidising haze from sunlight reacting with NOx and hydrocarbons. NO2 photolysis forms NO and O, producing ozone; VOCs yield peroxy radicals that oxidise NO to NO2 without consuming O3, leading to O3 and PAN build-up. Favourable conditions are heavy traffic, strong sun and inversion. Ozone peaks afternoon, causing eye irritation, plant damage and reduced visibility.

What is photochemical smog? Explain the photochemical reactions involved in its formation, with the favourable conditions, the typical daily variation of pollutants and its effects.

Photochemical smog is a brownish haze formed when sunlight acts on nitrogen oxides and volatile organic compounds (VOCs) from vehicle exhaust. It was first observed in Los Angeles, so it is also called Los Angeles smog. It contains ozone, PAN and aldehydes (an oxidising mixture).

Favourable conditions

  • Heavy traffic (source of NOx\text{NO}_x and hydrocarbons)
  • Strong sunlight and warm weather
  • Low wind speed and a temperature inversion that traps pollutants

Reactions

  1. NO is formed in engines: N2+O2→2NO\text{N}_2 + \text{O}_2 \rightarrow 2\text{NO}, and some oxidises to NO2\text{NO}_2:
2NO+O2→2NO22\text{NO} + \text{O}_2 \rightarrow 2\text{NO}_2
  1. Photolysis of NO2\text{NO}_2 by sunlight (λ<400\lambda < 400 nm):
NO2+hν→NO+O\text{NO}_2 + h\nu \rightarrow \text{NO} + \text{O}
  1. Atomic oxygen reacts with oxygen to form ozone:
O+O2+M→O3+M\text{O} + \text{O}_2 + M \rightarrow \text{O}_3 + M
  1. Ozone reacts back with NO:
O3+NO→NO2+O2\text{O}_3 + \text{NO} \rightarrow \text{NO}_2 + \text{O}_2

Steps 2-4 alone give a steady state with no net ozone build-up. 5. Hydrocarbons (RH) react with OH and O radicals to form peroxy radicals (RO2\text{RO}_2):

RH+OH→R+H2O,R+O2→RO2\text{RH} + \text{OH} \rightarrow \text{R} + \text{H}_2\text{O}, \qquad \text{R} + \text{O}_2 \rightarrow \text{RO}_2
  1. Peroxy radicals oxidise NO to NO2\text{NO}_2 without destroying ozone, so ozone accumulates:
RO2+NO→RO+NO2\text{RO}_2 + \text{NO} \rightarrow \text{RO} + \text{NO}_2
  1. Organic radicals combine with NO2\text{NO}_2 to give peroxyacyl nitrates (PAN) and aldehydes (HCHO).

Daily variation

conc.
 |  NO        NO2          O3
 |  /\      /    \       /  \
 | /  \____/      \____/    \___
 +--------------------------------> time
   6am     10am      1pm     6pm

NO peaks in the morning rush hour, then NO2\text{NO}_2 rises and ozone peaks around noon to early afternoon.

Effects

  • Eye irritation (PAN, aldehydes), cough, chest pain, and reduced lung function.
  • Plant damage (leaf spotting) and crop loss from ozone and PAN.
  • Cracking of rubber, and damage to paints and fabrics.
  • Reduced visibility.

✓ Answer checked by Claude Opus 5.5 (high effort)

Related questions

All Environment and Pollution Control chapter 1 questions →