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METO 637
Lesson 14
Photochemical chain initiation
• In the troposphere several species are present that
absorb solar ultraviolet radiation and can initiate radicalchain reactions.
• Ozone is phololyzed at wavelengths less than 310 nm
and the following reactions occur:
O3 + hν → O1(D) + O2(1Δg)
O1(D) + H2O → OH + OH
• Since H2O is a minor constituent in the atmosphere, and
O1(D) is quenched by collisions with O2 and N2 the OH
production rate is not high.
• Note that the O3P that results from the quenching will
quickly be converted back to ozone.
Photochemical chain initiation
• Note that the rate of production of OH depends uniquely
on the quantum yield of O1(D) as a function of
wavelength.
• Another source of atomic oxygen in the troposphere
comes from the dissociation of nitrogen dioxide at
wavelengths less than 400 nm:
NO2 + hν → NO + O
• The NO molecule can be oxidized back to NO2, and so
the NO2/NO cycle can be catalytic. However, in contrast
to the stratosphere, the oxidant is not O.
Oxidation steps
• The OH radical reacts mainly with CO and CH4:
OH + CO → H + CO2
OH + CH4 → CH3 + H2O
• In the unpolluted atmosphere about 70% of the OH
reacts with CO, and 30% with CH4.
• These reactions are followed by:
H + O2 + M → HO2 + M
CH3 + O2 + M → CH3O2 + M
• Both products are peroxy radicals (they oxidize)
• Where the NO concentrations are low we get
HO2 + HO2 → H2O2 + O2
CH3O2 + HO2 → CH3OOH + O2
Oxidation steps – high NO
• If NO is high then the hyperoxides react rapidly:
HO2 + NO → OH + NO2
CH3O2 + NO → CH3O + NO2
• The methoxy radical, CH3O, can then react with
O2 :
CH3O + O2 → HCHO + HO2
• HCHO, formaldehyde, is dissociated ijn the
atmosphere to produce H and CHO
• Followed by HCO + O2 → CO + HO2
• Hence CH4 has been oxidized to CO and HOX
Chemistry of the troposphere
Higher hydrocarbons
• Higher hydrocarbons have similar oxidation steps to
methane.
• In this case we represent the hydrocarbon as RH, (for
methane R = CH3).
• For ethane and propane R is formed by subtraction of
the hydrogen atom. Next RO2 is formed which oxidizes
NO to NO2
RO2 + NO → RO + NO2
RO → R’ + R”CHO
RO + O2 → R’R”CO + HO2
• Followed by
NO2 + hν → NO + O
O + O2 + M→ O3 + M
Tropospheric ozone production
• Note that what the complete cycle has done is to
dissociate molecular oxygen using two photons. One at
about 310 nm (4 eV) and one at 400 nm (3 eV).
NO2 + hν → NO + O
O + O2 + M→ O3 + M
OH + CO → H + CO2
H + O2 + M → HO2 + M
HO2 + NO → OH + NO2
CO + 2O2 + hν → CO2 +O3
• Similar chain reactions can be written for RO2
Tropospheric ozone production
• In the free troposphere, and with a small amount of NO,
the loss mechanism for ozone is:
HO2 + O3 → OH + 2O2
OH + CO → H + CO2
H + O2 + M → HO2 + M
CO + O3 → CO2 + O2
• This loss mechanism is large for low NO concentrations
– provides a photochemical sink for ozone.
Tropospheric ozone oxidation
Importance of NOX
• As we noted before NO is crucial to the production of
ozone, especially in polluted atmospheres.
• Two reservoirs for NOX have been identified – nitric acid
and peroxyacetylnitrate (PAN). Both of these gases
dissociate in the daytime, but are quite significant at
night.
• There are, in fact, a whole range of peroxynitrates that
exist in the atmosphere.
Oxidation of a VOC - daytime
The nitrate radical
• The nitrate radical was first observed in 1881. It is
formed by the reaction:
NO2 + O3 → NO3 + O2
• It can be stored as Nitrogen Pentoxide N2O5
• During the day the NO3 radical is rapidly photolyzed, the
product being either NO or NO2.
• Although the OH radical is usually the main agent of
attack on the hydrocarbons in daylight, NO3 can be the
most important agent at night:
NO3 + RH → HNO3 + R
Concentrations of NO3 and peroxy
radicals
Oxidation of VOC’s at night