Topic 3 - Tropospheric Chemistry Flashcards

1
Q

What are the features of ozone photochemistry in the troposphere?

A

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.

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2
Q

What are two reactions that show ozone making an O atom?

A

O3 –<411 nm→ O( 1D) + O2( 3Σg-).

O3 –<310 nm→ O( 1D) + O2( 1Δg).

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3
Q

What are the features of OH radical production?

A

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

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4
Q

How does the OH radical react in the pristine troposphere?

A

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

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5
Q

How can the OH radical react with volatile organic compounds in the pristine environment?

A

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.

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6
Q

How is O3 produced in the troposphere?

A

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

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7
Q

What are the features of NOx photochemistry.

A

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.

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8
Q

What is smog?

A

Smog = Smoke + Fog.

It applies to the dense winter fogs that occurred in heavily industrialised cities.

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9
Q

How was smog formed in the past?

A

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.

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10
Q

What were the features of the Great Smog of London?

A

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.

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11
Q

What is photochemical smog?

A

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.

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12
Q

How do vehicle emissions contribute to the time of smog events?

A

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.

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13
Q

What are two examples of primary pollutants?

A

Hydrocarbons from vehicle and plant emissions.

NO from vehicle emissions.

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14
Q

How is NO formed from vehicle emissions?

A

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.

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15
Q

What are the reactions of NOx photochemistry?

A

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.

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16
Q

How is ozone produced from NO2?

A

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

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17
Q

What are the features of NOx and O3 mixing ratios?

A

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.

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18
Q

What are the differences between ozone in the troposphere vs. the stratosphere?

A

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.

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19
Q

What is the primary chain carrier in the tropospheric oxidation of VOCs?

A

The OH radical.

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20
Q

What are the features, including equations, for OH radical VOC Oxidation?

A

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

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21
Q

What are the features of carbonyl photochemistry?

A

Carbonyl compounds can absorb photons at wavelengths < 330 nm and photo-dissociate.

CH3CHO –λ<330nm → CH3 + HCO

HCO + O2 → HO2 + CO

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22
Q

What are the features of the OH reaction of carbonyls?

A

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

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23
Q

What are the features of peroxyacyl nitrate (PAN)?

A

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

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24
Q

What are the features of the nitrate radical?

A

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

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25
Q

How is N2O5 formed and what are its features?

A

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.

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26
Q

What are the equations for NO3 radical VOC oxidation?

A

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

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27
Q

How does nighttime VOC oxidation differ to daytime VOC oxidation?

A

Unlike OH oxidation, there is no regeneration of NO3.

The nighttime VOC oxidation produces RO2 and HO2.

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28
Q

What are aerosols?

A

Aerosols are microscopic particles suspended in air and have solid, liquid or multiphase behaviour.

29
Q

What are the features of aerosols?

A

Aerosols directly affect climate by absorbing or scattering solar and terrestrial radiations.

Aerosols can indirectly affect climate as they can initiate cloud and ice formation.

Aerosols provide a medium for heterogenous chemical reactions in the atmosphere.

30
Q

What is the range of aerosol size?

A

1 nm to 10 μm.

31
Q

What are the three regions of aerosol size?

A

Nucleation mode (d < 100 nm).

Accumulation mode (d = 100 nm - 2 μm).

Coarse mode (d > 2 μm).

32
Q

What is the Junge distribution?

A

The slope for aerosols with d > 2 μm is relatively consistent for size distribution across the globe.

33
Q

Where are aerosols found?

A

Most aerosols are limited to the troposphere and only a very small amount exists in the stratosphere.

34
Q

What percentage of aerosols are primary aerosols and what percentage are secondary aerosols?

A

75% aerosol mass is primary aerosol and 25% is secondary aerosol.

Secondary aerosols arise from condensation of organic vapour onto primary aerosols.

35
Q

What are the five main primary aerosols?

A

Sea salt particles.

Sulfates.

Mineral dust.

Soot.

Organics.

36
Q

What are the features of sea salt, sulphate and some organic aerosol particles?

A

They are hygroscopic and take up water. This increases their size and causes them to act as cloud condensation nuclei.

37
Q

How are sea-salt aerosols mainly formed?

A

Through the bursting of droplets at the ocean surface that contain NaCl.

38
Q

What are the features of mineral dust aerosol?

A

They are insoluble but can function as effective ice nuclei.

39
Q

What are the features of soot or carbon aerosols?

A

They are insoluble but efficiently absorb solar radiation and provide a net warming effect.

40
Q

What are the features of organic aerosols?

A

They result from condensation of gas phase molecules with low vapour pressure (main oxygenated and polar) emitted by the biosphere. These can have both a warming and a cooling effect and provide a feedback link between the atmosphere and the biosphere.

41
Q

What are the features of sulphur emissions?

A

There are various natural sources and human activities that emit SO2 and other reduced sulfur species such as DMS.

Reduced sulphur species are oxidised to SO2 and SO3 and are eventually converted into H2SO4.

OH + SO2 –M→ HSO3

HSO3 + O2 → SO3 + HO2

SO3 + H2O →H2SO4

42
Q

What are the features of sulphate aerosols?

A

Sulfuric acid easily condenses with water and is neutralised by NH3 to form sulfate aerosol.

H2SO4 (g) + NH3 (g) ↔ 2NH4+ (aq) + SO42- (aq)

Sulphate aerosols are very hygroscopic and efficiently produce clouds which can perturb global albedo and stratospheric ozone chemistry.

Nitrate aerosols are formed in a similar manner, depending on excess NH3 and NOx emissions.

43
Q

How does vapour pressure change over a surface?

A

Kelvin or curvature effect: vapour pressure over a surface increases with a decrease in droplet diameter.

Raoult’s law: vapour pressure over a surface decreases with an increase in solute concentration.

44
Q

What is a Kohler curve?

A

A combination of curvature and solute effects that describes the particle growth as a function of the saturation ratio.

45
Q

What is the saturation ratio?

A

The ratio of vapour pressure above the particle (radius, r) against saturated vapour pressure of water.

46
Q

What are the features of particle activation?

A

If the ambient saturation ratio is above the critical saturation ratio, the droplets grow spontaneously by condensation and is called activated.

Accumulation mode aerosols are relatively large and are activated at relatively low supersaturations to form cloud condensation nuclei.

47
Q

What is the direct aerosol effect?

A

Aerosol particles particularly in accumulation mode can scatter solar radiation which results in a decrease in surface temperature.

Some particles can absorb solar radiation resulting in a warming atmosphere but decreasing surface temperature.

48
Q

What is the semi-direct aerosol effect?

A

Absorption by aerosol particles can result in decreased vertical temperature gradient and lower vertical transport.

Absorption by aerosol particles can result in increased droplet evaporation and less cloud cover which allows more solar radiation to reach the surface.

Surface warming can lead to positive feedback and therefore the semi-direct effect can be larger than the direct effect.

49
Q

What is the indirect aerosol effect?

A

Higher concentrations of CCNs in a polluted area result in clouds with smaller droplets and larger surface area which can increase albedo and create ‘haze’ events.

This also leads to lower conversion of CCNs to rain droplets which causes less frequent rainfall, slower wet scavenging rate and the atmosphere remains polluted for longer.

50
Q

What is Henry’s law?

A

Oxidation chemistry produces various gas molecules that are soluble and can be taken up into particle droplets.

Henry’s Law quantifies the amount of gas molecules partitioning into the liquid phase.

51
Q

What are the features of CO2 solubility?

A

CO2 is the most common acidic gas in the atmosphere that can partition into rain droplets to form carbonic acid.

Carbonic acid in water dissociates which results in natural lowering of rainwater pH.

52
Q

How does solubility depend on pH?

A

Typical aqueous phase acid concentration increases with increasing pH.

53
Q

What are the features of heterogenous reactions on droplets?

A

For droplets, surface processes such as gas diffusion and mass accommodation and bulk processes such as solubility and aqueous reactions needs to be considered.

All processes can be combined to give an effective heterogenous reaction rate expression.

54
Q

What is the resistance model for gas uptake?

A

The net uptake coefficient is taken as a combination of all the fundamental processes, each with its own resistance either in series or parallel.

Reactions can take place either on the surface or in the bulk. ‘Heterogenous’ reaction generally refers to surface reactions.

55
Q

What are the features of the bulk reaction of S(IV)?

A

For SO2, various bulk reactions with other dissolved species inside the droplet can oxidise dissolved S(IV) species to S(VI) further increasing solubility of SO2.

H2O2 reacts directly with HSO3- whereas other trace gases undergo radical assisted reactions.

HSO3- (aq) + H2O2 (aq) → HSO4- (aq) + H2O (l)

HSO4- (aq) + H+ (aq) → H2SO4 (aq)

56
Q

What does a typical photochemical smog contain?

A

Carbon monoxide, nitrogen dioxide, ozone, hydrocarbons, nitrate species.

57
Q

How can photochemical smog be hazardous to health?

A

Individually, the components are hazardous to vegetation, materials and human health. However, a combination of pollutants can act synergistically.

58
Q

What are the health effects of carbon monoxide?

A

CO binds to haemoglobin in the blood to form the strongly bound carboxyhaemoglobin adduct.

At concentrations of a few ppm, up to 10% of haemoglobin will be tied up results in lethargy and headaches.

Chronic CO exposure can lead to psychological problems.

59
Q

What are the health effects of ozone and NOx?

A

Ozone and NO2 are strong oxidising agents and cause inflammation of linings in the lungs which aggravates lung diseases such as asthma and bronchitis.

Peroxyacetyl nitrate (PAN) is one of the secondary products of VOC oxidation and is also a strong oxidising and lachrymatory (tear forming).

60
Q

What are the health effects of particulate matter?

A

Particulate matter smaller than 2.5 microns can reach deep into the lungs and aggravate lung diseases.

PMs have enhanced toxicity and may cause cancer and premature death in people with heart and lung disease.

The WHO estimate around 800,000 premature deaths every year.

61
Q

What are the effects of pollution on the urban environment?

A

Ozone is damaging to polymers, textiles and paints.

Increased aerosol and particle loading in urban areas results in reduced visibility (haze) and precipitation. Reduced precipitation means air stays toxic for longer episodes.

Photochemical smog leads to acid rain which damages buildings and other structures.

62
Q

What are the effects of air pollution on the rural environment?

A

Ozone and NOx are damaging to young and juvenile plants.

Acid rain with sulphate is toxic to plant leaf and root systems which decreases forest cover and crop yields.

Acid rain with nitrate results in eutrophication of lakes and rivers and loss of aquatic life.

Acidification of water bodies makes it difficult for aquatic calcifying organisms such as shellfish and corals to grow their shells. This is also a significant CO2 sink.

63
Q

How has photochemical smog been controlled?

A

There has been a steady drop in emissions despite rising vehicle usage. This is due to the introduction of compulsory fitting of catalytic converters to all new cars in the EU starting from 1995.

64
Q

What are the features of catalytic converters?

A

The three-way automotive exhaust catalyst has been adopted.

NO is reduced to N2.
CO is oxidised to CO2.
Unburned fuel is oxidised to CO2 and H2O.

65
Q

What are the features of greener city planning?

A

Encouraging greener behaviour through policies and better city planning.

For example, congestion charges and low emission zones in inner cities, safe cycling lanes and walking paths and efficient, reliable and low polluting public transportation.

66
Q

What are the features of cleaner energy sources?

A

Energy generation is a major source of acid pollution and lots of effort has gone into reducing acid emissions.

Coal power plants have been retrofitted with a scrubber method such as flue gas desulfurisation using aerosolised water and dissolved limestone to increase efficiency and reduce SO2 emission.

67
Q

What are the equations for flue gas desulfurisation?

A

CaCO3 –heat→ CaO + CO2

CaO + SO2 + 1/2O2 → CaSO4.

68
Q

What are other sources of cleaner energy?

A

Natural gas has been suggested as a transition fuel as it has lower emissions. However, even minor gas leaks can cause a global warming effect due to their high GWP.

Sustainable and carbon neutral sources are desirable for the future.

69
Q

How has acid rain been reduced in America?

A

Sulfur emissions have been better regulated through government policies. This includes using better quality coal.