Week 2

Question 1

What is photolysis?

Answer 1

When chemical bonds in a molecule are broken down by an incident photon

Question 2

Equation for photolysis reaction?

Answer 2

X+hv –> Y+Z

Question 3

What does AB* represent?

Answer 3

Excited activated complex that reacts rapidly with A&B or C&D

Question 4

Why is a third body (M) needed ?

Answer 4

To stabilise the excited complex of AB*

Question 5

Common three body reaction equation?

Answer 5

A+B+M –> AB+M

Question 6

What can M be made up of?

Answer 6

Inert molecules (O2 or N2)

Question 7

Rate constants of 3 body reaction?

Answer 7

k1[A][B] = k2[AB] + k3[AB][M]

Question 8

Definition of a radical?

Answer 8

A chemical species with an unpaired electron in the outer (valence) shell

Question 9

What is needed to form radicals from non radicals?

Answer 9

An external energy source

Question 10

Initiation step- radical formation

Answer 10

nonradical + hv –> radical + radical

Question 11

What does hv represent?

Answer 11

Light energy

Question 12

Propagation step- radicals

Answer 12

Radical + Non radical –> radical + non radical

Question 13

What is a branching reaction?

Answer 13

When a new non radical may be photolysed

Question 14

Termination step equation - radicals

Answer 14

Radical + Radical (+M) –> nonradical +nonradical

Question 15

What is the hydroxyl radical (OH) responsible for?

Answer 15

The removal of many pollutants

Question 16

Hydroxyl radical - equation 1- Initiation step

Answer 16

O3+hv → O2 + O(1D)

Question 17

Hydroxyl radical- equation 2- propagation 2

Answer 17

O(1D)+M → O + M

Question 18

Hydroxyl radical - equation 3- propagation step

Answer 18

O(1D) + H2O → 2OH

Question 19

What is actinic flux?

Answer 19

Photons entering slab of unit area atmosphere per unit time from any direction (photons cm-2s-1)

Question 20

What do we assume in a steady state of AB*?

Answer 20

Production=Loss

Question 21

The only reactions occuring at appreciable rates in the atmosphere are those involving at least what?

Answer 21

One radical species

Question 22

Role of the sun in chemical kinetics and the atmosphere?

Answer 22

• Photon energies= used to split apart chemical bonds
• Many environmental species undergo photolysis
• Aids Biogenic Volatile Organic Compound emission (BVOC)

Question 23

Why is OH used when looking into air pollution?

Answer 23

It is responsible for the removal of many pollutants

Question 24

Assumptions made about OH previously?

Answer 24

• Incoming radiation was insifficiently energetic to produce O(1D)
• Photochemical destruction of CH4 and CO occurred in stratosphere
• CH4 and CO had long lifetimes

Question 25

Photolysis of Ozone equations

Answer 25

1. O3+hv --> O2 +O(1D) 2. O(1D) +M --> O+M 3. O(1D) + H2O --> 2OH

Question 26

Tropospheric production of O(1D) in stratosphere vs troposphere

Answer 26

Slower production of O(1D) in troposphere

Question 27

Features of OH

Answer 27

- Responds rapidly to changes in sources and sinks - Difficult to measure directly - Highly variable - Main sinks : CO and CH4

Question 28

How to measure [OH]

Answer 28

Need a trace gas whose only sink is [OH]

Question 29

What does the rate of (OH) production depend on providing O(1D) is in a steady state?

Answer 29

Amount of water vapour present

Question 30

What is the rate of Oh production (POH) in terms of (O1D), H2O and rate constants?

Answer 30

POH= 2k3[O(1D)][H2O]

Question 31

How is [O(1D] expressed using steady state approximation?

Answer 31

[O(1D)] = k1[O3]/k2[M]+k3[H2O]

Question 32

What does the OH production rate depend on after substitution?

Answer 32

POH=2k1k3[O3][H2O]/k2[M]+k3[H2O

Question 33

Why is OH production in the troposphere slower in the troposphere than the stratosphere?

Answer 33

Less O(1D) is made in the troposphere due to weaker UV, but more water vapour compensates to some extent

Question 34

What is the expression for Oh lifetime in the atmosphere?

Answer 34

TOH= 1/Sum ki ni Where ni= number densities of species reacting with OH

Question 35

Why is Methyl Chloroform (CH3CCl3) useful?

Answer 35

An industrial solvent that can be used to estimate global mean [OH]

Question 36

Sinks of CH3CCl3?

Answer 36

Main sink = [OH] Small additional sinks = oxidation and photolysis in stratosphere - uptake by oceans

Question 37

Main sources of CO

Answer 37

* Fossil fuel combustion * Oxidation of methane * Biomass burning * Vegetation * Oxidation of methane * Oxidation of NMCHCs * Oceans

Question 38

Main sinks of CO

Answer 38

* Troposphere oxidation by OH * Stratosphere * Soil Uptake

Question 39

Ratio of sources and sinks of CO

Answer 39

Sources and sinks= balanced

Question 40

Main sources of CH4

Answer 40

* Wetlands * Fossil fuels *Ruminants * Landfills/waste *Rice *Biomass burning

Question 41

Main sinks of CH4

Answer 41

* Troposhperic OH * Stratopsheric loss *Tropospheric Cl

Question 42

Ratio of CH4 sources and sinks

Answer 42

Sources > sinks therefore steady atmopsheric growth

Question 43

What is CH4 the primary source of?

Answer 43

Atmospheric CO