Topic 3 - Tropospheric Chemistry Flashcards
What are the features of ozone photochemistry in the troposphere?
Ozone plays a key role in promoting the photo-oxidation of volatile compounds emitted into the troposphere.
This is because ozone is easily photo-dissociated by photons to produce an O atom.
What are two reactions that show ozone making an O atom?
O3 –<411 nm→ O( 1D) + O2( 3Σg-).
O3 –<310 nm→ O( 1D) + O2( 1Δg).
What are the features of OH radical production?
O(1D) is rapidly relaxed to O(3P) at tropospheric pressures through a collision with a third body.
O(1D) –M→ O(3P)
However, enough remains to react with H2O and act as a source of OH radicals in the troposphere.
O(1D) + H2O → 2OH
How does the OH radical react in the pristine troposphere?
OH mainly reacts with CO and CH4. It follows a four reaction scheme.
OH + CO → H + CO2
H + O2 → HO2
HO2 + O3 → OH + 2O2
2HO2 → H2O2
How can the OH radical react with volatile organic compounds in the pristine environment?
The OH radical reacts with VOCs such as CH4 and isoprene to produce organic hydroperoxides.
OH + CH4 → CH3 + H2O
CH3 + O2 –M→ CH3O2
CH3O2 + HO2 → CH3OOH.
As H2O2 and CH3OOH are water soluble they are rained out of the atmosphere.
Overall, in the pristine environment, oxidation processes lead to an overall loss of O3.
How is O3 produced in the troposphere?
It is predominantly produced from NOx photochemistry. NO and NO2 are emitted from soils or produced during lightning events.
There is a three step process involved.
NO2 –hv→ NO + O( 3P).
O( 3P) + O2 –M→ O3
NO + O3 → NO2 + O2
What are the features of NOx photochemistry.
Typically, [NO2] = 2 x [NO] during daytime. However, NOx photochemistry is significantly altered by human activities.
Increased NOx concentrations results in the net production of O3 and photochemical smog events in polluted environments.
What is smog?
Smog = Smoke + Fog.
It applies to the dense winter fogs that occurred in heavily industrialised cities.
How was smog formed in the past?
Sulfur-rich coals were very common which resulted in smoke loaded with acid particulates.
Temperature inversions in early afternoons and mornings trapped the toxic smoke close to the surface.
What were the features of the Great Smog of London?
In December 1952, a week-long smog event caused at least 4000 (and perhaps >10,000) deaths that were directly attributable to respiratory illness arising from the smog.
This led to the 1956 Clean Air Act which led to a cleaner troposphere.
What is photochemical smog?
The mixture of solid and gaseous products arising from the tropospheric photo-oxidation of organic molecules.
It was first characterised in the Los Angeles basin in the 1960s which was associated with high motor vehicle use and temperature inversions.
How do vehicle emissions contribute to the time of smog events?
Smog events generally occur in the early mornings (5-7 am).
This is because there is a burst of hydrocarbon and NO as people use their cars. NO2 is generated from the NO with around a 30 minute delay.
As primary pollutants decay, the oxidants (O3, PAN) are consumed as evening approaches and products appear.
What are two examples of primary pollutants?
Hydrocarbons from vehicle and plant emissions.
NO from vehicle emissions.
How is NO formed from vehicle emissions?
N2 + O2 ↔ 2NO
As the ΔrG is very positive, a high temperature is required to push the equilibrium to the right. This temperature occurs in the combustion engines of vehicles.
What are the reactions of NOx photochemistry?
In air, NO is oxidised to NO2:
2NO + O2 ↔ 2NO2
NO2 efficiently absorbs light at λ < 400 nm and dissociate to produce an O atom.
NO2 –λ<400nm→NO + O.
How is ozone produced from NO2?
The presence of NO2 is a trigger for the formation of photochemical smog.
NO2 photolysis releases O atoms into the troposphere which forms O3, a key species for smog formation.
NO2 –λ < 400 nm→ NO + O
O + O2 –M→ O3
NO + O3 → NO2 + O2
What are the features of NOx and O3 mixing ratios?
High NOx emissions in urban areas can result in the lowering of average daytime O3 levels.
Peak daytime O3 concentrations are still high and toxic.
High NOx concentrations leads to dispersal of O3 producing species such as NO2 and PAN to suburban regions.
What are the differences between ozone in the troposphere vs. the stratosphere?
Ozone in the stratosphere is beneficial as a global sunscreen.
Ozone concentration is significantly increased in areas of high population density and the peripheral areas during and following smog events.
Ozone is toxic and adversely affects air quality and has a high global warming potential in the troposphere.
What is the primary chain carrier in the tropospheric oxidation of VOCs?
The OH radical.
What are the features, including equations, for OH radical VOC Oxidation?
At least two NO2 is produced and OH radical is recycled under high NOx conditions in urban areas.
OH + RH → R + H2O
R + O2 → RO2 (peroxy radical).
RO2 + NO → RO + NO2 (alkoxy radical).
RO + O2 → HO2 + R’COR’’
HO2 + NO → OH + NO2
OH + NO2 → HNO3
What are the features of carbonyl photochemistry?
Carbonyl compounds can absorb photons at wavelengths < 330 nm and photo-dissociate.
CH3CHO –λ<330nm → CH3 + HCO
HCO + O2 → HO2 + CO
What are the features of the OH reaction of carbonyls?
Carbonyl compounds can also react with OH radicals to form acyl radicals e.g.
OH + CH3CHO → H2O + CH3C(O).
Acyl radicals can react with O2 to form acylperoxy radicals, RC(O)O2 e.g.
CH3C(O) + O2 –M→ CH3C(O)O2
What are the features of peroxyacyl nitrate (PAN)?
Acylperoxy radicals can react with NO2 to form peroxyacyl nitrates such as peroxyacetyl nitrate (PAN) or decompose to CO2.
CH3C(O)O2 + NO2 –M→ CH3C(O)O2NO2.
CH3C(O)O2 + NO → CH3 + CO2 + NO2
PAN is analogous to holding species and can decompose back to NO2 which results in the transport of NOx.
CH3C(O)O2NO2 → CH3C(O)O2 + NO2
What are the features of the nitrate radical?
During the daytime, NO3 is photolysed efficiently which means it is not very important. However, at night, NO3 is important.
As NO2 photolysis is absent, the bimolecular reaction with O3 becomes dominant:
NO2 + O3 → NO3 + O2
How is N2O5 formed and what are its features?
NO3 can react with NO2 to form N2O5. N2O5 is thermally unstable and can decompose depending on temperature. N2O5 can be hydrolysed on aerosol surfaces to form HNO3 which is washed out of the atmosphere leading to a loss of NOx.
NO2 + NO3 ↔N2O5.
What are the equations for NO3 radical VOC oxidation?
NO3 + RH → R + HNO3
R + O2 → RO2
RO2 + NO3 →RO + NO2 + O2
RO + O2 → HO2 + R’COR’’
HO2 + NO3 → OH + NO2 + O2
OH + NO2 → HNO3
How does nighttime VOC oxidation differ to daytime VOC oxidation?
Unlike OH oxidation, there is no regeneration of NO3.
The nighttime VOC oxidation produces RO2 and HO2.
