Chapter 4 Tropospheric chemistry smog

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Chapter 4Tropospheric chemistry - smog

• CH350/EV350
• Spring 2008

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Smog

• Smoke Fog
• Classical
• Unburned carbon soot from coal
• SO2
• Reducing and acidic

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Smog

• Photochemical Made famous in Los Angeles
• Elevated levels of oxidants and carbon rxn
  products
• Thermal reactions favored in warm weather
• Photochemical reactions favored during the day
• Atmospheric inversion is necessary to keep the
  pollution in place where it reacts

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Chemical Compositions of smog events

• NO and hydrocarbons start to build around 600 AM
• NO drops leading to NO2
• NO2 and hydrocarbons drop around 900 AM and lead
  to aldehydes and oxidants
• Everything returns to normal at 800 PM

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Smog Chemistry NOx chemistry

• Internal combustion engines generate NO
• NO is converted to NO2
• NO2 generates ground state O atoms
• O atoms react to form O3

N2 O2 heat ? 2NO 2NO O2 ? 2NO2 NO O3 ?
NO2 O2 ROO. NO ? RO. NO2 NO2 hn
(llt400nm) ? NO O O O2 M ? O3
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Smog Chemistry - .OH production

• O3 then undergoes photolysis generating excited
  atomic oxygen (O)
• O reacts with water to form hydroxyl radicals
  (.OH)
• Higher humidity causes more .OH
• HONO chemistry is important in heavily polluted
  atmospheres

O3 hn (llt315nm) ? O2 O O H2O ? 2.OH NO
NO2 H2O ? HONO 2HONO hn (llt400nm) ? 2NO
2.OH
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Smog Chemistry Oxidation of hydrocarbons

• .OH reacts with hydrocarbons to form
• Carbon centered radicals
• Alkyl peroxides
• Aldehydes (RCHO)
• They hydroxyl radical is regenerated after
  reaction with NO

.OH RCH3 ? .RCH2 H2O .RCH2 O2 M ? RCH2OO.
M RCH2OO. NO ? NO2 RCH2O. RCH2O. O2 ?
RCHO .OOH .OOH NO ? NO2 .OH NET RCH3 2O2
2NO ? RCHO 2NO2 H2O
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Smog Chemistry Radical termination

• .OH reacts NO2 forming nitric acid
• Oxygen radicals terminate at hydrogen peroxide or
  water

.OH .NO2 ? HNO3 2HOO. ? H2O2 .OH HOO. ? H2O
O2
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Smog Chemistry PAN formation

• .OH reacts with aldehydes to form acetylperoxy
  (CH3C(O)OO.) radicals and peroxyacetic nitric
  anhydride (PAN)
• PAN are major eye irritants
• PAN is a nitrogen reservoir species

.OH CH3CHO ? CH3CO. H2O CH3CO. O2 M ?
CH3C(O)OO. (acetylperoxy) CH3C(O)OO. .NO2 ?
CH3C(O)OONO2 (PAN)
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Concentrations of pollutants in a smog event

• Hydrocarbons in cities often come from fuel
  sources, either unburned fuel or vaporizaiton of
  fuel
• Hydrocarbons, like terpenes, in rural areas come
  from trees.
• Air pollution comes from trees Ronald Regan
  early 1980s

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Comparison of emissions from internal combustion
engines

• Four-stroke engines most car and pickup truck
  engines, riding lawn mowers
• Two-stroke engines many push lawn mowers, chain
  saws
• Diesel engines large equipment, most tractors,
  some cars and pickup trucks

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Four-stroke engine

• http//auto.howstuffworks.com/engine1.htm

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Four-stroke engine

• Fuel/air ratio important
• Lean fuel
• Low VOC emissions
• High NO emissions due to higher operating
  temperature
• Rich fuel
• High VOC emissions due to unburned fuel
  exhausting to atmosphere
• Lower NO emissions due to lower operating
  temperature

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Reduce VOC and NOx emissions from four-stroke
engines

• Add oxygenates to fuel causes cleaner burning
  less unburned hydrocarbons in exhaust
• Ethanol common now
• MTBE was common
• Use catalytic converters

2NO 2CO Rh catalyst ? N2 2CO2 RH O2 Pd
or Pt catalyst ? CO2 H2O 2CO O2 Pd or Pt
catalyst ? CO2
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Two-stroke engine

• http//science.howstuffworks.com/two-stroke2.htm

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Two-stroke engine

• Unburned hydrocarbons leaving exhaust
• Some unburned fuel pushes out exhaust
• Oil used to lubricate doesnt burn very well
• Lower operating temperature than four-stroke
  engines causes less NOx emissions

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Two-stroke and four-stroke exhaust gas output
(10-8 g/J)

• Two-stroke
• More VOC
• Less NOx
• Four-stroke
• Very low VOC
• Somewhat more NOx

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Diesel engine

• http//auto.howstuffworks.com/diesel1.htm
• Higher compression ratios than four-stroke
  gasoline engines
• Causes larger pressures and temperatures
• Spontaneous ignition of fuel
• Not always good mixing of air and fuel
• Soot production
• High combustion temperature causes NOx emissions

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Controlling O3

• Ozone production based on NOx and VOC
• Which is limiting reagent?
• Assume polluted city with high VOC (point A)
• NOx is limiting reagent
• Lowering VOC to point B will not lower O3 very
  much
• Lowering NOx to point C could dramatically lower
  O3