Atmospheric Flashcards
▪ Photolysis rate constant (k1
) depends on…
on the UV actinic flux
– Increases with altitude
Actinic flux
the
radiation flux at height z,
integrated over all
angles
– Captures flux from
direct solar radiation
minus absorption, plus
scattering from clouds,
aerosols, etc.
▪ Rowland-Molina hypothesis
▪ CFCs had long enough
lifetimes that they could be
transported to the
stratosphere
▪ Photodissociation of CFCs
could be a source of
stratospheric Cl
▪ Cl much more reactive with O3
than NO
2
The Molina-Rowland hypothesis was
eventually confirmed by…..
observations
of O3 and ClO
They had expected a
relatively
uniform decrease in [O3
what had causes a high amount of increase in the o3 in Antarctic regions
– Polar vortex: No sunlight and isolated air in the Antarctic stratosphere during
the winter. Cold!
– PSCs form: surfaces for heterogeneous Chemistry
– Reservoir species react and release Cl2
– Sun comes up in October, Cl2 photolyses
– Cl reacts with O3 and begins catalytic cycle involving ClO dimer
The Montreal Protocol
Phase out schedule
of compounds
depending on ozone
depletion potential:
CFCs > HCFCs >
HFCs
what effects the ozone depeletion
lifetime
The Kigali Amendment
to the Protocol controls
HFCs due to their impact on climate, not because
of their impact on ozone: major change for the
Protocol
Reserves and depletion
– HF is extremely stable. Therefore, F participates in almost no ozone
depletion
– HBr readily photodissociates. Therefore, Br is 10 – 100 times more effective
at destroying ozone than Cl
depletion depends on…
– Lifetime, particularly in the troposphere: species that are primarily removed
in the troposphere (e.g., by reaction with OH) can be largely destroyed
before Cl/Br is released in the stratosphere
– Br or Cl content: Bromine-containing species tend to have substantially
higher ODPs
ODP
– The ratio of the global loss
of ozone due to a given
substance, to that of the
same mass of CFC-11
Problems with ODP
ODP works well if a compound is
long-lived in the atmosphere,
and can therefore be considered
well-mixed
▪ However, because transport to
the stratosphere is dominated by
the tropics, particularly during
monsoons, short-lived
compounds can destroy very
different amounts of ozone,
depending on where they are
released
▪ Problem for species such as
CH2Cl2
, CHCl3
(6-month lifetime
in troposphere)
Sources of water vapour to the stratosphere:
– Transport from the troposphere (1-2 year mixing time)
– Oxidation of methane (by OH)
Sources of N2O to the stratosphere:
– N2O is produced by microbes in the soil
– N2O is inert in the troposphere (lifetime > 100 years)
– Transported to the stratosphere where:
Ozone depleting compounds
Chlorofluorocarbons (CFCs)
– Long lifetimes (~10 – 100 years)
– Used as refrigerants, foam blowing agents, etc.
– CFC-11 (CFCl3), CFC-12 (CF2Cl2), etc.
▪ Hydrofluorocarbons (HCFCs)
– Short lifetimes compared to CFCs, due to C-H bond (reacts with OH in troposphere)
– Used as replacements for CFCs
– HCFC-22 (CHClF2), HCFC-141b (C2H3Cl2F), etc.
▪ Chloromethanes
– Wide range of lifetimes (~6 months – 30 years)
– Used as chemical feedstocks
– CCl4, CHCl3, CH2Cl2
▪ Halons
– Bromine-containing halocarbons, with lifetimes ~10 – 100 years
– Used as fire retardants
– Halon-1211 (CBrClF2), H-1301 (CBrF3), and H-2402 (CBrF2CBrF2)
why are HFCs being phase out?
▪ Hydrofluorocarbons (HFCs) were introduced as
replacements for HCFCs, but are now also
being phased out under the Montreal Protocol
– HFCs, like CFCs and HCFCs are potent
greenhouse gases
– The Kigali Amendment to the Protocol controls
HFCs due to their impact on climate, not because
of their impact on ozone: major change for the
Protocol
▪ The next generation of refrigerants are using
very short-lived compounds such as
hydrofluoroolefins (double bond), alkanes, etc
