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Tropospheric Chemistry

• Tropospheric chemistry is dominated by the
  oxidation of trace atmospheric species
• organic compounds ? CO2, H2O
• When the concentrations of oxidizable material is
  high, some intermediates in the oxidation process
  can build up and result in pollution problems
• e.g. photochemical smog
• ( NOxs, hydrocarbons, aldehydes...)

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Oxidation Chemistry

• Most atmospheric oxidation processes involve
  either O2 or the hydroxyl radical, OH (at night
  the nitrate radical becomes important)
• Electronic Structure of O2
• highest occupied molecular orbitals are a pair of
  degenerate p orbitals each with an unpaired
  electron
• O2 is a triplet

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These p orbitals are low enough in energy to
attract electrons from donors (O2 is
electronegative) but 3O2 donors with paired
electrons are unreactive (spin forbidden) Donors
with paired electrons do react with 1O2 (two
valence electrons in the same p orbital giving a
two electron vacancy in the other p orbital)
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Require 1O2 for reaction with donor having paired
electrons Fast reaction of 3O2 with a free
radical (having an unpaired electron) Under
normal conditions there is no 1O2
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Oxidation Chemistry

• slow (little) reaction of oxygen with donors with
  paired electrons but
• fast reaction with donors with unpaired electrons
  (i.e. free radicals) can occur.
• reaction of triplet O2 with a free radical (e.g.
  RCH2 alkyl radical) results in one p orbital
  filled (formation of a (RCH2) (O2) bond) and
  one half filled p orbital (production of a
  radical)
• RCH2 3O2 ? RCH2O2 (alkylperoxyl radical)
• since one of the O2 p orbitals is fully
  occupied, the O O bond is weakened (p is
  antibonding)

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Oxidation Chemistry

• alkylperoxyl radicals are good O atom donors
• RCH2O2 X ? XO RCH2O (alkoxyl radical)
• X can be anything wanting O, including NO, SO2 or
  organics with CC double bonds
• (reaction with peroxyl radicals is the primary
  mechanism for oxidation of CC organics).
• hydroperoxyl radical
• RCH2O O2 ? RCHO HO2
• aldehyde
• HO2 is also an O atom donor
• HO2 X ? XO HO

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Atmospheric Chemistry of the Hydroxyl Radical

• The hydroxyl radical is (arguably) the most
  important species affecting the chemistry of the
  troposphere
• Constant production and removal (highly reactive)
• High reactivity means that OH (and other
  radicals) are found in low concentrations
• e.g. typical daytime concentrations 3 x 106
  molecules/cm3
• Very short residence times t OH 1 sec

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Characteristic Reactions with Carbon Compounds

• Abstraction of a hydrogen atom
• OH CH4 ? CH3 H2O
• preferred reaction for most species containing
  hydrogen since the O-H bond in H2O is stronger
  than saturated (sp3) C-H bonds

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Characteristic Reactions with Carbon Compounds

• (ii) Addition to an unsaturated centre
• OH NO2 ? HNO3
• (also the preferred reaction for most unsaturated
  organic compounds
• e.g. benzene derivatives here, the (sp2) C-H
  bonds are stronger)
• note careful not to over-generalize. Some
  molecules (e.g. 2-butene, toluene) can undergo
  addition and abstaction

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Characteristic Reactions with Carbon Compounds

• note OH reacts very differently than the
  hydroxide ion. OH- generally reacts as a base
• Since OH is a free radical (i.e. has an unpaired
  electron) when it reacts with singlet species
  (paired electrons) it gives another free radical
• OH CH4 ? CH3 H2O
• this product can then react very efficiently with
  O2
• CH3 O2 ? CH3OO
• Which can then undergo further oxidation
  processes by similar reactions
• Net result reduced carbon ? CO2 oxidized carbon

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In the majority of cases, the atmospheric
oxidation of emitted VOCs is initiated by
reaction with the hydroxyl radical, .OH.  In the
case of saturated VOCs, the first reaction step
is the abstraction of a hydrogen atom to yield a
molecule of water and a carbon-centred alkyl
radical (R.). 
www.chem.rdg.ac.uk/dept/staff/phys/dj.html
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This carbon-centred alkyl radical (R.) rapidly
adds molecular oxygen to yield a peroxyl radical
(RO2.) which may react with nitric oxide (NO
released into the atmosphere in significant
quantities by combustion activities) to yield a
molecule of nitrogen dioxide (NO2) and an oxyl
radical (RO.). 
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In many cases, RO. reacts with molecular oxygen
to produce a carbonyl molecule and a another
peroxyl radical (HO2., hydroperoxyl).  As with
RO2., HO2. can react with NO to yield NO2. and,
concomitantly, .OH ?thus the initial oxidant
radical is regenerated.  Each time NO2 is formed
a molecule of ozone may be produced NO2 h? (?lt
410 nm) ? NO O. O. O2 (M) ? O3 (M)
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Reactions with Nitrogen Compounds

• OH can also react with nitrogen containing
  species
• e.g. Abstraction
• NH3 OH ? NH2 H2O
• with subsequent steps
• NH2 NO2 ? N2O H2O
• (little NH3 removal since most NH3 dissolves in
  liquid water)
• e.g. Addition
• NO2, produced in many oxidation reactions, can
  also react with OH
• OH NO2 ? HNO3
• (soluble in water rain-out)

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Reactions with Sulfur Compounds

• Recall that many biologically produced sulfur
  compounds are emitted as sulfides
• H2S OH ? HS H2O
• HS is then oxidized through to SO2 (not well
  understood)
• SO2 from this and other sources can then be
  oxidized
• SO2 OH ? HSO3
• HSO3 O2 ? HO2 SO3
• SO3 H2O ? H2SO4

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Reactions with Sulfur Compounds

• Major biological emission, DMS, can react by
  abstraction
• CH3SCH3 OH ? CH3SCH2 H2O
• CH3SCH2 O2 ? CH3SCH2O2
• CH3SCH2O2 NO ? CH3S NO2 HCHO

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Reactions with Sulfur Compounds

• or DMS can react by addition
• CH3SCH3 OH ? CH3SOH CH3
• CH3SOH O2 ? CH3SO3H
• (methanesulphonic acid)
• soluble in water (raindrops) and resistant to
  further oxidation
• used as a marker for marine air mases that once
  contained DMS

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www.physicalgeography.net/fundamentals/8h.html
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Reactions with Itself

• OH can react with itself (directly) to produce
  hydrogen peroxide
• OH OH ? H2O2
• or indirectly through the hydroperoxide radical
• HO2 HO2 ? H2O2 O2
• H2O2 is very soluble in water and an extremely
  strong oxidizing agent
• important in oxidation processes within water
  droplets.

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