Figure 1'2 de Nevers

1
Figure 1.2 de Nevers

• NO, NO2, O3, and CO concentrations
• as a function of time of day in Los Angeles

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Chemistry of ground level ozone formation
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GROUND-LEVEL OZONE

• In the absence of VOCs

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GROUND-LEVEL OZONE

• VOCs represent an alternative path for NO
  oxidation
• The OH radical plays an important role in this
  overall reaction
• NO2 can also react directly with OH
• Thus, adding NO2 can interfere with the first
  reaction above

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OXIDATION OF NO BY VOCs
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VOC - NOx competition for reaction with OH

• OH - NO2 generally faster than OH - VOC for an
  average urban air mix
• reaction rates become equal when VOC/NO2
  5.5

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VOC - NOx competition for reaction with OH

• If VOC/NO2 lt 5.5
• reaction with NO2 dominates
• Reaction with NO2 is a termination reaction
• removes OH radicals from the reaction pool,
  retarding O3 production
• Thus, decreasing NO2 leads to higher O3

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Figure D.1 de Nevers

• Calculated maximum afternoon O3 concentrations
  as a function of the morning NOx and VOC
  concentratiuons for the same air mass

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M. Walsh, FQS Workshop
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VOC/NOx ratio

• Low VOC/NOx can occur
• in city centers, e.g.
• O3 Slater street lt O3 Wurtemburg street
• in plumes downstream of strong NO sources
• Rural environments characterized by high
  VOC/NOx ratios, ozone production is NOx
  limited

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GROUND-LEVEL OZONE

• The EKMA results show
• At low NOx, high VOC, increasing NOx increases
  O3
• At low VOC, high NOx, increasing NOx decreases O3
• At constant NOx value, VOC reductions always help
• At constant VOC value, NOx reductions may
  increase or decrease O3