Atmospheric Chemistry I

Question 1

What are 6 environmental problems that antropogenic emissions have created?

Answer 1

1. ozone depletion: catalytic destruction of stratospheric O3 by radicals
2. photochemical smog: production of ground level O3 and other irritants
3. aersol polluition: particles that are harmful when inhaled, climate impacts
4. acid precipitation: release of SO2 and NOx that are converted to acids
5. climate change: increasing concentration of GHGs
6. long-range transport: organic pollutants and metals can be transported to remote regions, and/or be increasing persistant in the atmosphere

Question 2

How are the regions of the atmosphere defined? What are the regions and boundaries? What defines a boundary/pause?

Answer 2

regions of the atmosphere are distinguised by temperature gradients

in increasing altitude from the earth’s surface:
1. troposphere
2. tropopause
2. stratosphere
3. stratopause
3. mesosphere
4. mesopause
4. thermosphere/ionosphere

boundaries btwn regions are marked by a temperature gradient of zero

Question 3

Troposphere

How does altitude affect temperature? Why?

Why does almost all water vapour exist here?

What is the consequence of the direction of temperature gradient?

Answer 3

Temperature decreases with altitude due to expansion cooling - as air moves upwards, it expands due to drop in pressure –> cooling

Due to cooling, air rapidly reaches water saturation point –> almost all water vapour exists in the troposphere

direction of temperature gradient results in strong vertical mixing –> warm air rises, cools, and falls –> constantly stirring the air in the troposphere

Question 4

What is the planetrary boundry layer? Which atmospheric layer is it located? Why is this layer important? What is an effect of this?

Answer 4

planetary boundry layer: 0-1km (troposphere)

here air is influenced by the planet surface, so highly turbulent

this layer is important with respect to transport of chemicals in the atmosphere, including pollutants

effect: Grasshopper Effect (Gobal Distillation) - if something is released in a hot area –> volitizes –> transported –> condenses in cooler area

Question 5

Describe how the temperature gradient of each atmospheric layer behaves as altitude increased, and describe why this is for each layer

Answer 5

1. the troposphere cools because of expansive cooling
2. the stratosphere heats because of exothermic photochemical reactions (photochemical formation and destruction of ozone)
3. mesosphere cools because of expansive cooling
4. thermosphere heats because of exothermic photochemical reactions (photochemical dissociation of molecular oxygen and molecular nitrogen)

Question 6

What are consequences of UV exposure?

Answer 6

• eye damage
• sunburn/tan and cancer
• damages plants and surface algae

Question 7

Describe the categorization of UV light (largest to smallest wavelength), which UV category is the most damaging to DNA?

Answer 7

wavelenth: UV-A > UV-B > UV-C

UV-C has the shortest wavelength and is the most damaging

Question 8

Describe the absorbance region of molecular oxygen (O2)

Answer 8

• weakly absorbs in the UV-C region
• filters out high energy light from the sun
• both O2 and N2 absorb light lamda < 200nm at the top of the atmosphere (thermosphere), so those wavelength don’t make it any closer towards the surface

Question 9

Describe the absorbance region of ozone (O3)

Answer 9

• strongly absorbing and filters out light in UV-C (and some UV-B)

Question 10

Name 4 reaction pathways of excited molecules: M + hv –> M*

Answer 10

1. reaction
2. fluorescence* (M* –> M + hv)
3. radiative decay (M + heat)
4. energy transfer (M* –> M + X*)

Question 11

Why do molecules differ enormously in their ability to absorb light at a given wavelength?

Answer 11

Due to differences in energy levels (electronic, vibrational, and rotational)

Question 12

When does absorption of radiation occur?

Answer 12

only if the energy of the photo equals the energy difference between two energy levels resulting in an electronically or vibrationally excited species

Question 13

What does Beer-lambert law correlate?

Answer 13

• relationship between concentration and absorption/transmission
• allows us to determine the concentration of a compound based on the change in the amount of transmitted light
• the amound of light transmitted decreases exponetially with the concentration of the absrober

Question 14

What is the implication of when the number density is small, transmitted light is sensitive to small changes in number density?

Answer 14

adding more of a compound that was previously not present in high concentrations results in more energy being absorbed

Question 15

What are the possible reactions of excited molecular oxygen (O2*)?

Answer 15

1. dissociation: O2* –> O + O (radicals)
2. direct rxn: O2* + O3 –> 2O2 + O
3. photoionization: O2* –> [O2]+ + e-

Question 16

When does photodissociation occur?

Answer 16

The energy of the photon must exceed the reations activation energy

Question 17

Where are peroxides reactive?

Answer 17

HO-OH bonds can break in the visible light region; therefore, peroxides are reactive in the troposphere

Question 18

k = ΣΦ(λ)α(λ)F(λ)

What is Φ(λ)?

Answer 18

Quantum yield:

• efficieny of the rxn
• fraction of rxns occuring with respect to photons absorbed

Question 19

k = ΣΦ(λ)α(λ)F(λ)

What is α(λ)?

Answer 19

Absorption cross section

Question 20

k = ΣΦ(λ)α(λ)F(λ)

What is F(λ)?

Answer 20

Photon flux

• number of photons reaching the earth over time, in a defined area
• in addition to being a function of λ, it is a function of many meterological conditions

Question 20

What is direct vs indirect photochemistry?

Answer 20

• direct photochemistry – the reactant is reacting with light itself: A + hv –> products
• indirect photochemistry – light is initiating a reaction and that reactant is proceeding to react with another reactant (often include radical chemistry): A + hv –> A; A + B –> products

Question 21

Is our atmosphere reducing or oxidizing?

Answer 21

Oxidizing

Question 22

What is the most important atmospheric oxidant? What is it also referred to as? What is also an importnat oxidant?

Answer 22

OH radical, called the “cleaning agent” of the atmosphere

ozone is also important – OH and ozone are interrelated

Question 23

What are the two sources of OH radical? Describe each reaction

Answer 23

Ozone:
O3 + hv –> O2 + O(1D) (UV-B or lower)
O(1D) + H2O –> 2OH

HONO (nitrous acid):
HONO + hv –> OH + NO (UV-A or lower; rapidly photolyzes in morning after buidling up over night)

Question 24

What are two ways in which OH reacts? Which is faster? Is faster good or bad, why?

Answer 24

1. addition to double bonds - faster because double bonds are more reactive (smaller resident time of pollutant –> better for the environment)
2. abstraction of hydrogen

Question 25

How does ozone react?

Answer 25

ozone adds to double bonds to create primary ozonides

Question 26

What is 6PPD? What is it added to? What is an environmental issue with this?

Answer 26

6PPD = antioxidant, more specifically an antiozonant

added to rubber in tires, to prevent the rubber from degrading

reaction with ozone can create 6PPD-quinone which is toxic to some fish species

Question 27

Which atmopheric layer contains the ozone layer? What is the importance of the ozone layer?

Answer 27

ozone layer in the stratosphere

ozone layer is our shield against harmful UV (specifically UV-C)

Question 28

What are the risks associated with ozone depletion?

Answer 28

• increase skin cancer, eye cataracts
• decrease photosynthesis (harmful to phytoplankton)
  decrease temperature in the stratosphere…trickledown effects on other reactions

Question 29

How was the ozone layer discovered?

Answer 29

knew that something was removing UV-C radiation, Fabry and Buisson measured the absorbance spectrum of the missing radiation and matched it to ozone’s absorbance spectrum

Question 30

Describe Chapman’s 4 coupled reactions

Answer 30

1. O2 + hv –> 2O (λ < 240nm)
2. O + O2 + M –> O3 + M*
3. O3 + hv –> O2 + O (λ < 325)
4. O + O3 –> 2O2

Question 31

What are Dobson Units (DU)?

Answer 31

measure of the total ozone from Earth’s surface upwards

1DU = 0.01mm ozone at 0C and 1atm

Question 32

Using the chapman equations, and assuming ss, what is the formual to determine [O3]? Both addition and subtraction.

Answer 32

[O3] = (k2[O][O2][M])/(k3) = [O2]sqrt((k1k2M)/(k3k4))

Question 33

Using the chapman equations, and assuming ss, what is the formual to determine [O]?

Answer 33

[O] = (k3[O3])/(k2[O2][M])

Question 34

Where in the stratosphere is [O] greatest?

Answer 34

middle part, compared with the lower part

Question 35

Do the Chapman Equations accurately predict the amount of ozone?

Answer 35

no - too large, indicated that the rxns do not account for all ozone sinks