Source Apportionment and Air Pollution Chemistry

Question 1

What are the main 3 atmopsheric constituents?

Answer 1

Nitrogen (0.78)
Oxygen (0.21)
Argon (0.9 x 10¯²)

Question 2

What are the 4 atmospheric oxidants which can process organic and inorganic pollutants

Answer 2

• Hydroxyl radical (OH)
• Nitrate radical (NO₃)
• Ozone
• Chlorine

Question 3

Substances like Led will not react with any of the atmospheric oxidants
How will this affect its lifetime?

Answer 3

The lifetime of a species will depend on its reactivity with atmospheric oxidants
Hence led will remain in the atmosphere for longer

Question 4

Are are two key features of the hydroxyl radical in the atmosphere?

Answer 4

• Initiates the removal of most volatile organic compounds (VOC)
• Highly reactive and very short lifetime

Question 5

What is the main way formation of the hydroxyl driven by

Answer 5

Ozone and sunlight (hence is the main atmospheric daytime cleaner)
Hence the profile for ozone and hydroxyl radicals will look very similar

Question 6

What is the second biggest way (30%) formation of hydroxyl radical is driven

Answer 6

Alkenes + ozone → OH + products

Question 7

What is the smallest contributor to the source of OH radicals in the atmosphere

Answer 7

Sunlight will degrade HONO to form OH
(many unknown sources of HONO)

Question 8

The majority of OH radicals come from the breakdown of ozone forming O¹D
What do OH and O¹D measurments look like when plotted over 24hr period

Answer 8

• In the summer the OH and O¹D follow another (apart from during the evening due to alkene-ozone reaction)
• In the winter, they do not mirror another very well due to non-photo driven dominating, which do not involve ozone
  (However both compounds oxidise quickly so are hard to measure)

Question 9

When considering OH sources, what issues arise?

Answer 9

• The ozone reactions is most of the time the biggest contributor but HONO photolysis is demonstrated to be a large contributor in some places
• Contributions will depend on your location (and time e.g. winter vs summer)

Question 10

In the hydrocarbon oxidation cycle:
Once OH radical is formed from ozone, what two things can occur?

Answer 10

It can either react with methane to for a methyl radical or react with nitrogen dioxide to form nitric acid (termination)

Question 11

Once a methyl radical is form, what can then occur?

Answer 11

• React with O₂ to form ¯O-CH₂-OH
• Which could then react with hydrogen dioxide to form OH-CH₂-OH (termination)

Question 12

The ¯O-CH₂-OH ion formed from methyl ion, could then go on to react how?

Answer 12

react with NO to form CH₃-O¯ and NO₂
(NO₂ can the react to form more ozone)

Question 13

The CH₃-O¯ ion can then go on to react …

Answer 13

with oxygen to form methanal (H₂CO) and hydrogen dioxide (and the further react to form hydrogen peroxide)

Question 14

In the last step of the hydrocarbon oxidation cycle, what does the hydrogen dioxide react with to form hydroxyl radical again

Answer 14

Reacts with NO to form NO₂ and OH

Question 15

True or false:
The hydrocarbon oxidation cycle can occur during both daytime and night-time

Answer 15

True
(except for sunlight driven reactions)

Question 16

How many molecules of ozone can be formed each time OH react with methane in the presence of NO

Answer 16

2 molecules of ozone
BUT OH is also recycles

Question 17

Each VOC has a different structure, hence …

Answer 17

A different reactivity with OH

Question 18

A VOC with the potential to form photochemical ozone is called

Answer 18

POCP
POCP can provide useful information for a particular VOC pontential to form O₃

Question 19

Which types POCP will form more ozone?

Answer 19

Alkenes due to high electron density of carbon double bond

Question 20

In the absence of sunlight, which compound is the dominat oxiding agent in the atmosphere

Answer 20

The nitrate radical
(because OH is sunlight dependent)

Question 21

How is the nitrate radical formed (NO₃)

Answer 21

Nitrogen dioxide will react with ozone to form nitrate and oxygen
However, nitrate will then be photolysed by light (red)

Question 22

What are the conetrations of nitrate like relative to hydroxyl

Answer 22

Nitrate concentrations are 3 orders of magnitude larger than OH
But nitrate is not as reactive
(hence the only reason nitrate chemistry would occur during the daytime is if NO₂ pollution levels are really high

Question 23

When considering the lifetime for OH, nitrate and ozone, what factor must be taken into consideration

Answer 23

For each oxidiser the lifetime of each atmospheric cleaner will be different