Tropospheric photo-oxidation of pollutants

Question 1

What is smog

Answer 1

Smog = smoke + fog

It is a mixture of solid and gaseous products from tropospheric photo-oxidation of organic molecules such as Carbon Monoxide, Ozone, Nitrogen Dioxide, Organic and Inorganic nitrates

Question 2

How does smog arise

Answer 2

At 5-7 a.m.p people commute (drive) to work, the nitrogen dioxide is generated by the nitrogen monoxide with a 20 minute delay. As the primary pollutants decay, oxidants such as ozone are consumed close to the evening therefore producing aldehydes

Question 3

What are the origins of smog

Answer 3

Primary pollutants are:

1. Hydrocarbons (produced from incomplete combustion, emissions of volatile organic compounds)
2. Nitric Oxide (NO) (produced as a byproduct of high temperature combustion of hydrocarbons in air)

Question 4

Show and explain the chemical mechanisms relating to smog

Answer 4

NO in air: 2NO + O2 -> 2NO2 where NO2 is a trigger for producing SMOG as it is a brown gas and its absorption of light (wavelength <400nm results in the photodissociation of NO2).

1. Photolysis of NO2: NO2 -> NO + O (wavelength less than 400nm)
2. O + O2 -> O3 with a third body M
3. Ozone then oxidising NO back to NO2: NO + O3 -> NO2 + O2

Question 5

Give the steady state analysis for the chemical mechanisms relating to smog

Answer 5

ASK LECTURER IF THIS NEEDS TO BE REMEMBERED

Question 6

Explain why is ozone in the troposphere not seen as beneficial

Answer 6

1. Toxic, strong oxidising agent, long tropospheric lifetime, VOC photo-oxidation produces further detrimental compounds
2. O(‘D) -> O(3P) at tropospheric pressures where O(3P) is a relaxed oxygen state, unreactive at low T but still reactive at high T. However, enough O(‘D) remains and reacts with water in troposphere to form OH radicals:

O(‘D) + H2O -> 2OH

Question 7

Explain the chemical reactions carbonyls undergo once they are formed

Answer 7

Photodissociation of molecule: CH3CHO -> CH3 + HCO (lambda < 330nm)

Reactions with OH radicals: OH + CH3CHO -> H2O + CH3CO

Question 8

Does SMOG end at night?

Answer 8

No because as the sun sets: NO2 + O3 -> NO3 + O2

the nitrate then behaves like OH when its night, where:
NO3 + RH -> R + HNO3 (nitrate radical oxidising VOCs)

the following reactions are:

R + O2 -> RO2
RO2 + NO -> RO + NO2
RO + NO2 -> HO2 + R'COR''
HO2 + NO -> OH + NO2 
OH + NO2 -> HNO3

Question 9

Explain how SMOG evolves at night (SMOG Equations)

Answer 9

The nitrate then behaves like OH when its night, where:
NO3 + RH -> R + HNO3 (nitrate radical oxidising VOCs)

the following reactions are:

R + O2 -> RO2
RO2 + NO -> RO + NO2
RO + NO2 -> HO2 + R'COR''
HO2 + NO -> OH + NO2 
OH + NO2 -> HNO3 

The reactivity of NO2 with VOCs < OH’s reactivity but it is higher at night time. This means oxidation of VOCs continue over night

Question 10

What are VOCs?

Answer 10

Volatile Organic Compounds