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Formation of the Hydroxyl Radical in the
Troposphere

• Primary formation mechanism is complex and is
  driven by sunlight
• NO2 ? NO O (3P) (hn, ? lt 400 nm)
• (2) O O2 ? O3 (M is a third body)
• (3) O3 ? O2 O (1D) (hn, ? lt 320 nm)
• (4) O H2O ? 2OH?

M
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Formation of the Hydroxyl Radical in the
Troposphere

• NO2 is the product of several oxidation reactions
• 2NO O2 ? 2NO2 R kO2NO2
• Since NO is very low (lt 1 ppmV), NO is not
  oxidized at an appreciable rate as the reaction
  is 3rd order overall and 2nd order with respect
  to NO

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Formation of the Hydroxyl Radical in the
Troposphere

• But, oxidation can take other routes
• NO O3 ? NO2 O2
• NO HO2? (or RO2 ?) ?
• NO2 OH ? (or OR ?)

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Formation of the Hydroxyl Radical in the
Troposphere

• (2),(3) O2 O ? O3
• Ozone formation is faster in the troposphere than
  the stratosphere since both O2 and M are present
  in higher concentrations.
• Note that O is generally lower since the
  wavelengths responsible for the production of O
  are attenuated.
• 1O3 ? 3O2 O (3P) hn, ? lt 320 nm
• 1O3 ? 1O2 O(1D) hn, ? lt 320 nm
• (spin conservation)

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Tropospheric Concentration of OH?

• OH is key to tropospheric chemistry (initiates
  oxidation of oxidizable material)
• Globally, seasonally, daily averaged
  concentration of OH is OH 8 x 10-5
  molecules/cm3 3 x 10-5 ppbV
• production of OH depends on time of day and
  location (photochemical, relative humidity,
  temperature, )
• average concentration is useful for species with
  long residence times but OH is so reactive (t
  short), not useful for modeling tropospheric
  processes
• ? active research in developing new methods for
  detecting
• temporal OHs with lower detection limits

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Tropospheric Concentration of OH?

• Measurements in unpolluted air indicate that OH
  parallels the intensity of sunlight. Knowing
  reactions of ?OH with substrates (e.g. CO, CH4)
  are second order kinetically
• rate of formation of ?OH rate of disappearance
• rate of disappearance S k OHsubstrates
• Can get OH from knowledge of light intensity,
  CO, CH4

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Hydroxyl Radical Sinks

• Two primary sinks for the hydroxyl radical
• (i) reaction with CO
• (ii) reaction with CH4
• In polluted air of the troposphere,
• (i) accounts for 70 and
• (ii) for most of the rest

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The sources, interconversions and sinks for HOx
(and ROx) in the troposphere.
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Table 1 Calculated fractional contribution of
various photolysis rates to radical production
(OH) with altitude for clean conditions
Altitude
j(O(1D)) H2O
j(HCHO)
j(O(1D)) CH4
j(Acetone)
j(H2O2)
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Fig. 2 Typical diurnal variation of OH and
j(O1D) in the clean marine atmosphere (the time
is in Australian Eastern Standard Time).
O3 hn  (l lt 320 nm) ?    O(1D) O2(1 D g)
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Peroxy radical concentrations, though relatively
small, are substantially larger (ca. 50100
times) than the concomitant OH concentration (for
comparison see Fig. 2). HO2, the major peroxy
radical has a lifetime of about a minute in clean
air and much less than a minute in dirty air.
Fig. 3 Typical diurnal variation of HO2
 RO2(in parts per trillion by volume, pptv) and
j(O1D)in the clean marine atmosphere.
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Fig. 7 Schematic representation of the reaction
cycle for HOx. The numbers in the ovals represent
the model calculated radical concentrations (in
106 molecule cm3) and the numbers along the
arrows represent the conversion rates of HOx (in
106 molecule cm3 s1). The model run is for a
typical rural background set of atmospheric
conditions. Only the major pathways are
represented.
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Carbon Monoxide

• CO produced biologically, by incomplete
  combustion and as an intermediate in the
  tropospheric oxidation of hydrocarbons
• as natural constituent 0.1 ppmV
• city concentration can reach 20-50 ppmV
• (health risks present at 100 ppmV)

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Carbon Monoxide

• CO sinks are
• uptake in soils and atmospheric oxidation
• OH? CO ? H CO2
• (only known tropospheric sink of CO)
• perhaps addition then elimination
• OH? CO ? HOCO
• HOCO ? H CO2
• also a source of hydroperoxyl radical
• H O2 ? HO2?

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Methane (and continuity)

• CH4 results primarily from microbial activity
  (soils, guts)
• Simplest hydrocarbon but oxidation mechanism is
  extremely complex

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Continuity

• Continuity transfer of material along various
  reaction pathways
• Methane emitted from Earths surface at a mean
  rate of 2 x 1011 molecules/cm2sec
• For steady state ?
• Formation (emission) rate of destruction

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• Estimate formaldehyde concentration
• rate of production of CH4 rate of destruction
  of CH4
• k1OHCH4 k5OHHCHO J5a HCHO J6HCHO

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For CH4 1.72 ppm 4.47 x 1013 molecules/cm3
and OH 106 molecules/cm3 k1 8 x 10-15
cm3/moleculessec ? destruction rate of CH4
3.6 x 105 molecules/cm3 sec HCHO k1
OHCH4/(k5 OH J5a J6) For k5 1.3 x
10-11 cm3/moleculessec, (J5a J6) 4.5 x 10-5
sec-1 ? HCHO 4.3 x 109 molecules/cm3 close
to mean HCHO 6.2 x 109 molecules/cm3
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Note!

• Oxidation of hydrocarbons occurs by successive
  attack on the C-H bonds and their replacement by
  CO bonds, or the elimination of H2O.
• For higher hydrocarbons, replace CH3 by R.
• For alkenes and aromatics, addition at the
  unsaturated carbons also must be considered as
  well as abstraction.

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Fig. 4 Simplified mechanism for the photochemical
oxidation of CH4 in the troposphere.
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Table 2 Global turnover of tropospheric gases and
fraction removed by reaction with OH
Global emission rate /Tg yr1
Removal by OHa ()
Trace Gas
One of the roles of atmospheric radical
chemistry, as driven by OH, is to  cleanse the
troposphere of a wide-range of the organic
compounds. The OH radical is known to react with
most trace gases and in many instances it is the
first and rate determining step. Thus, OH
controls the removal and, therefore, the
tropospheric concentrations of many trace gases.
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Fig. 6 A schematic representation of the
oxidation of an alkene initiated by reaction with
ozone
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The Nitrate Radical

• OH? radical controls most tropospheric chemistry
  but its importance is limited by its photolytic
  production concentrations are very low at night
• At low light levels, the nitrate radical becomes
  important
• NO2 O3 ? NO3? O2

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The Nitrate Radical

• daytime concentrations are very low since it
  photolyses (even via visible light)
• NO3? ? NO2 O
• and high concentrations of NO (from NO2
  photolysis during day) also reduces its
  concentrations
• NO NO3? ? 2NO2
• At night NO3? can act as a hydrogen atom
  abstractor much like OH?
• CH4 NO3? ? CH3 HNO3
• (reactions with alkanes not very fast but
  reactions with aldehydes, alkenes, terpenes and
  aromatic compounds are faster)

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The Nitrate Radical

• Nitrate radical can also react to give dinitrogen
  pentoxide (N2O5)
• NO3 NO2 M ? N2O5 M
• which can react heterogeneously with water to
  give nitric acid or decompose to give the
  reactants
• Nitrate can also generate HO2
• NO3 HCHO ? HCO HNO3
• HCO O2 ? CO HO2
• ? Nitrate plays an important role in night-time
  chemistry of the troposphere

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Fig. 8 Measurements of OH, HO2, j(O1D), NO, NO2
and CO during a summer urban campaign. The figure
demonstrates the dramatic feedback of NOx on HOx
radicals, the HO2 clearly suppressed at high
NOx while the OH remains largely unaffected
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Fig. 9 Schematic representation of the dependence
of the net ozone (N(O3)) production (or
destruction) on the concentration of NOx. The
magnitudes reflect clean free tropospheric
conditions.
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Fig. 14 A schematic representation of the chain
propagation reactions in the NO3 radical
initiated oxidation of propene.
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Fig. 15 A simplified reaction scheme for
night-time chemistry involving the nitrate
radical.
One important difference between NO3 chemistry
and daytime OH chemistry is that NO3 can
initiate, but not catalyse, the removal of
organic compounds. Therefore its concentration
can be suppressed by the presence of
fast-reacting (with respect to NO3) organic
compounds.
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Fig. 18 a) Scatter plot of the total night-time
rate through the NO3, O3 and alkene reactions
versus HO2 RO2 for Mace Head, Ireland
demonstrating the correlation between peroxy
radical concentrations and night-time alkene
oxidation. b) The estimated percentage
contribution of NO3 reactions and O3 reactions to
peroxy radical formation at night during the same
campaign.
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Summing up Nitrate Radical
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• a) The radicals can control NOy speciation in the
  atmosphere at night.
• b) Nitric acid can be formed, by hydrolysis of
  N2O5, as a product of a hydrogen abstraction
  process or indirectly via the NO3 mediated
  production of OH, which can react with NO2 to
  produce nitric acid.
• c) Primary organic pollutants can be oxidised and
  removed at night.
• d) Radicals (HOx and RO2) produced by NO3
  chemistry can act as initiators for chain
  oxidation chemistry.
• e) Toxic or otherwise noxious compounds such as
  peroxyacylnitrates, other nitrates and partially
  oxidised compounds may be formed.
• f) Nitrate products or NO3 itself may act as
  temporary reservoirs in the presence of NOx.
• g) Under highly polluted conditions NO3 may be a
  significant daytime oxidant for certain compounds.

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