7.2

• photochemical smog precursors:

  • NO2: broken by sunlight into NO + O (free O + O2 ---> O3)
  • VOC: volatile organic compounds (hydrocarbons) that bind with NO and form photochemical oxidants
  • carbon-based compounds that volatilize (evaporate) easily and emits a smell
  • Sources: gasoline, formaldehyde, cleaning fluids, oil-based paints, coniferous trees
  • O3: forms when NO2 is broken by sunlight and free O binds to O2
  • respiratory irritant in troposphere (@ earth's surface)
  • damaging to plant stomata, limiting growth
• photochemical smog conditions:

  • SUNLIGHT: drives O3 formation by breaking down NO2 --> NO + O; then free oxygen atom binds with O2
  • WARMTH: hotter atmospheric temperatures speeds O3 formation, evaporation of VOCs and thus smog formation
• Normal O3 formation:

  • Sun breaks NO2 into NO + O
  • Morning commute leads to high NO2 levels from car exhaust
  • O bonds with O2 to form O3
  • O3 formation typically peaks in afternoon when sunlight is most direct and NO2 emissions from morning traffic have peaked
  • At night, O3 reacts with NO to form NO2 and O2 once again; O3 levels drop overnight
• Photochemical smog formation:

  • Sunlight breaks NO2 into NO + O
  • O bonds with O2 to form O3
  • VOCs bond with NO to form photochemical oxidants
  • Without NO to react with, O3 builds up instead of returning to O2 and NO2 overnight
  • O3 combines with photochemical oxidants (NO and VOCs) to form photochemical smog
• Factors that increase smog formation:

  • more sunlight (summer,afternoon) = increased O3
  • higher VOC emissions (gas stations, laundermats, petrochemicals, and plastic factories)
  • increased vehicle traffic, increases NO2 emissions and therefore O3 formation
  • warmer temperatures, speeds evaporation of VOCs and reaction that leads to O3

  ** Urban areas have more smog due all these factors

  • more traffic -> more NO2, hotter temps due to low albedo, more VOCs due to more gas stations and factories, more electricity demand; more NOx emissions from power plants
• Impacts of smog:

  • Environmental: reduces sunlight/photosynthesis, O3 damages plant stomata and irritates animals respiratory tracts
  • Humans: respiratory irritant; worsens asthma, bronchitis, COPD, irritates eyes
  • Economic: increases health care costs to treat asthma, COPD, bronchitis, lost productivity due to sick workers, decreased agricultural yields due to less sunlight
• Reducing smog:

  • Decreasing number of vehicles on the road to decrease NO2 emissions
  • Carpooling, public transport, biking walking, working from home
  • Increased electricity production from renewable sources that don't emit NOx (solar, wind, hydro)
  • natural gas power plants release far less NOx than coal