Atmospheric Chemistry II Flashcards
Describe mechanism I (major route) for the catalytic destruction of ozone: state the general reactions, the net rxn, and what the catalysts are/can be
rxns:
a) X + O3 –> XO + O2
b) XO + O –> X + O2
net) O3 + O –> 2O2
X = NO, Cl, Br, OH
Why are chlorofluorocarbons (CFCs) bad for the ozone layer?
- CFCs have long atmospheric lifetimes - resistant to common oxidants like OH
- once they reach the stratosphere, energy of light is stronger: CF2Cl2 + hv –> Cl + CF2Cl
Describe mechanism II (minor route) for the catalytic destruction of ozone: state the general rxsns, net rxn, and what the catalysts are/can be
rxns
a) 2(X + O3 –> XO + O2)
b) XO + X’O –> XOOX’ (X and X’ can be the same or different)
c) XOOX’ + hv –> X + X’ + O2
net) 2O3 + hv –> 3O2
X = Cl, Br, OH (not NO)
Is Br or Cl a better radical for destroying ozone, why?
Br - less sinks
In the lower stratosphere which is/are the dominant cycles, why?
In the middle stratosphere which/is are the dominant cycles, why?
lower: OH/OOH, Cl/ClO, and Br/BrO – lower [O]
middle: NO/NO2 – higher [O]
Why is N2O a ozone depleting substance? How is it produced (anthroprogenically)?
fertilizer production: N2 –> fertilizer –> organic nitrogen –> N2O
ozone depleting: N2O + O –> 2NO (NO depletes ozone)
What are halons? What do they contain and what is their function? How do they react?
halons = fire suppressants containing Br
CBr3F + hv –> Br + CBr2F
What substance accounts for most Br in the stratosphere? Where is it produced? How does it produce Br radicals?
methyl bromide (CH3Br):
- soil fumigant
- from combustion of leaded gasoline which contains Br additives
- natural emmissions from the ocean
rxns:
a) CH3Br + hv –> Br + CH3
b) CH3Br + OH –> HCHO + Br
What are four sources of Cl radical?
- chlorofluorcarbons (CFCs)
- hydrochlorofluorocarbons (HCFCs)
- halons (e.g. CF2BrCl)
- methyl chlorine (CH3Cl)
Why did we ever develope CFCs? What were the advantages?
develop: original refridgerants used NH3 and SO2 which are toxic. so looked for a new substance - CFCs
advantages:
- excellent refridgerants
- propellants for aersol applications
- flame retardants
- non-toxic and highly stable
What are the traps for the Cl, Br, NO cycles?
Cl:
a) Cl + CH4 –> HCl + CH3
b) ClO + NO2 –> ClONO2
Br:
a) BrO + NO2 –> BrONO2
NO:
a) ClO + NO2 –> ClONO2
b) NO2 + OH –> HNO3
c) NO2 + NO3 –> N2O5
How do polar stratospheric clouds (PSCs) form/conditions?
low temperatures –> crystals of water to form –> PSCs
Why isn’t there an ozone hole over the Arctic?
flow of air in troposphere over mountain ranges in N. hemisphere –> waves of air that mix with polar air –> warming of stratosphere –> fewer PSCs + vortex of cold air breaks up before spring (i.e. before light shows up)
Describe how Cl sinks can turn into a source? Why does the ozone hole form in early spring (september, remember: S. hemisphere spring)?
a) ClONO2 + HCl –> Cl2 + HNO3
b) ClONO2 + H2O (heterogenous) –> HOCl + HNO3
spring = sunlight time:
a) Cl2 + hv –> 2Cl
b) HOCl + hv –> OH + Cl
Why does the ozone hole disappear in late summer?
late spring = warm temperatures –> PSCs melt –> generation of Cl2 is shut down
