Atmospheric Chemistry I Flashcards
What are 6 environmental problems that antropogenic emissions have created?
- ozone depletion: catalytic destruction of stratospheric O3 by radicals
- photochemical smog: production of ground level O3 and other irritants
- aersol polluition: particles that are harmful when inhaled, climate impacts
- acid precipitation: release of SO2 and NOx that are converted to acids
- climate change: increasing concentration of GHGs
- long-range transport: organic pollutants and metals can be transported to remote regions, and/or be increasing persistant in the atmosphere
How are the regions of the atmosphere defined? What are the regions and boundaries? What defines a boundary/pause?
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
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?
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
What is the planetrary boundry layer? Which atmospheric layer is it located? Why is this layer important? What is an effect of this?
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
Describe how the temperature gradient of each atmospheric layer behaves as altitude increased, and describe why this is for each layer
- the troposphere cools because of expansive cooling
- the stratosphere heats because of exothermic photochemical reactions (photochemical formation and destruction of ozone)
- mesosphere cools because of expansive cooling
- thermosphere heats because of exothermic photochemical reactions (photochemical dissociation of molecular oxygen and molecular nitrogen)
What are consequences of UV exposure?
- eye damage
- sunburn/tan and cancer
- damages plants and surface algae
Describe the categorization of UV light (largest to smallest wavelength), which UV category is the most damaging to DNA?
wavelenth: UV-A > UV-B > UV-C
UV-C has the shortest wavelength and is the most damaging
Describe the absorbance region of molecular oxygen (O2)
- 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
Describe the absorbance region of ozone (O3)
- strongly absorbing and filters out light in UV-C (and some UV-B)
Name 4 reaction pathways of excited molecules: M + hv –> M*
- reaction
- fluorescence* (M* –> M + hv)
- radiative decay (M + heat)
- energy transfer (M* –> M + X*)
Why do molecules differ enormously in their ability to absorb light at a given wavelength?
Due to differences in energy levels (electronic, vibrational, and rotational)
When does absorption of radiation occur?
only if the energy of the photo equals the energy difference between two energy levels resulting in an electronically or vibrationally excited species
What does Beer-lambert law correlate?
- 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
What is the implication of when the number density is small, transmitted light is sensitive to small changes in number density?
adding more of a compound that was previously not present in high concentrations results in more energy being absorbed
What are the possible reactions of excited molecular oxygen (O2*)?
- dissociation: O2* –> O + O (radicals)
- direct rxn: O2* + O3 –> 2O2 + O
- photoionization: O2* –> [O2]+ + e-
When does photodissociation occur?
The energy of the photon must exceed the reations activation energy
Where are peroxides reactive?
HO-OH bonds can break in the visible light region; therefore, peroxides are reactive in the troposphere
k = ΣΦ(λ)α(λ)F(λ)
What is Φ(λ)?
Quantum yield:
- efficieny of the rxn
- fraction of rxns occuring with respect to photons absorbed
k = ΣΦ(λ)α(λ)F(λ)
What is α(λ)?
Absorption cross section
k = ΣΦ(λ)α(λ)F(λ)
What is F(λ)?
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
What is direct vs indirect photochemistry?
- 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
Is our atmosphere reducing or oxidizing?
Oxidizing
What is the most important atmospheric oxidant? What is it also referred to as? What is also an importnat oxidant?
OH radical, called the “cleaning agent” of the atmosphere
ozone is also important – OH and ozone are interrelated
What are the two sources of OH radical? Describe each reaction
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)
What are two ways in which OH reacts? Which is faster? Is faster good or bad, why?
- addition to double bonds - faster because double bonds are more reactive (smaller resident time of pollutant –> better for the environment)
- abstraction of hydrogen
How does ozone react?
ozone adds to double bonds to create primary ozonides
What is 6PPD? What is it added to? What is an environmental issue with this?
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
Which atmopheric layer contains the ozone layer? What is the importance of the ozone layer?
ozone layer in the stratosphere
ozone layer is our shield against harmful UV (specifically UV-C)
What are the risks associated with ozone depletion?
- increase skin cancer, eye cataracts
- decrease photosynthesis (harmful to phytoplankton)
decrease temperature in the stratosphere…trickledown effects on other reactions
How was the ozone layer discovered?
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
Describe Chapman’s 4 coupled reactions
- O2 + hv –> 2O (λ < 240nm)
- O + O2 + M –> O3 + M*
- O3 + hv –> O2 + O (λ < 325)
- O + O3 –> 2O2
What are Dobson Units (DU)?
measure of the total ozone from Earth’s surface upwards
1DU = 0.01mm ozone at 0C and 1atm
Using the chapman equations, and assuming ss, what is the formual to determine [O3]? Both addition and subtraction.
[O3] = (k2[O][O2][M])/(k3) = [O2]sqrt((k1k2M)/(k3k4))
Using the chapman equations, and assuming ss, what is the formual to determine [O]?
[O] = (k3[O3])/(k2[O2][M])
Where in the stratosphere is [O] greatest?
middle part, compared with the lower part
Do the Chapman Equations accurately predict the amount of ozone?
no - too large, indicated that the rxns do not account for all ozone sinks
