GROUNDLEVEL OZONE

1
GROUND-LEVEL OZONE

• In the absence of VOCs

2
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

3
Radicals in tropospheric chemistry

• R - alkyl
• OH - hydroxyl
• NO3 - nitrate
• RO - alkoxy
• RO2 - alkyl peroxy
• HO2 - hydroperoxy
• RCO - acyl
• RCOO2 - acyl peroxy

4
Reactions of radicals

• Initiation

5
Reactions of radicals

• Propagation

6
Reactions of radicals

• termination

7
OH-VOC reactions

• Reaction with VOCs goes through propagation
  reactions
• radical population maintained or enhanced through
  photolysis of intermediate products
• NO oxidized to NO2 by radicals
• O3 levels increase because there is less NO to
  react with

8
OH-NO2 reaction

• Reaction with NO2 is a termination reaction
• removes OH radicals from the reaction pool

9
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

10
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

11
Importance of VOC/NOx ratio

• Increasing VOC concentrations means more ozone
• increasing NOx may lead to either less or more
  ozone depending on prevailing VOC/NOx ratio
• At a given VOC, there is an optimum NOx
  (from an ozone production perspective)
• below the optimum VOC/NOx increase in NOx
  leads to a decrease in ozone

12
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

13
Organic reactivity for ozone formation

• The rate of ozone production from a given VOC
  depends on
• its atmospheric concentration, VOC
• its rate of reaction with OH, NO3 and O3
• number of O3 molecules produced each time the
  species is oxidized

14
OH reactivity

• Propene-equivalent concentration
• Accounts for the combined effect of OH reactivity
  and concentration

15
Maximum Incremental reactivity (MIR)

• Carters MIR a scenario derived by adjusting the
  NOx emissions in a base case scenario to yield
  the highest incremental reactivity of the base
  ROG (VOC) mixture

16
Maximum Incremental reactivity (MIR)

• Methane 0.015
• isobutane 1.21
• Ethylene 7.4
• Benzene 0.42
• Toluene 2.7
• 1,3,5 -trimethylbenzene 10.1
• Formaldehyde 7.2

17
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

18
EMISSION FACTORS

• Amount of pollutant emitted per unit activity
• (mg HC) / (mile driven)
• (g NOx) / (kg coal fired)
• (g NOx) / (kJ heat released)
• Empirical, applicable for the specific activity
  i.e. heat released by burning a specific fuel
  with specific type of burners in a specific type
  of furnace.
• AP-42 has comprehensive compilation

19
SPECIATION OF VOC EMISSIONS

• VOC profiles of emissions from specific sources
  (activities).
• Mass fraction of each VOC in the total emissions
  from that source
• SPECIATE database has a compilation of profiles
• Useful in quantifying total ozone reactivities,
  human health hazards