Stratospheric Chemistry HS 2016
Solution to Homework Problem Set 4
For questions:
[email protected] (CHN O15.2)
Problem 1: Age of air
a) The "age" of an air parcel is defined as the time elapsed since it crossed the tropical
tropopause (i.e. since it entered the stratosphere). Since air parcels then get mixed in the
stratosphere, age of air is always a "mean" of different contributions
b) For being suitable as an "age of air tracer" a chemical specie needs to be not too
short-lived (lifetime > 2 yrs approx., minimum time for poleward transport in BrewerDobson) and not too long-lived (with uniform spread over the global stratosphere
information about transport is lost) unless past emissions are known (e.g. CO2, lifetime
30-95 yr). Species with very long lifetimes can also be used, if their emissions started
only recently (e.g. SF6, lifetime = 3200 yr)
Brewer-Dobson circulation
(from lecture notes)
Problem 1: Age of air
c)
Age = 4.8 yr
3.75 pptv
Age = 7.8 yr
3.0 pptv
35 km
0.75 pptv / 0.24 pptv yr-1 = 3 yr
1.15 pptv / 0.24 pptv yr-1 = 4.8 yr
Tropopause
Red = Young air
Blue = Old air
90° S
15 km
4.9 pptv
Age = 0
Equator
90° N
Problem 2: Tracer correlation method
a) A dynamics tracer is a chemical specie that can be used to obtain information about
the air circulation based on its distribution in the atmosphere
A dynamics tracer needs to be long-lived enough to participate in dynamical processes,
but measurably change concentration during transport (e.g. undergo known chemistry).
CH4, N2O and HF are suitable to be used as dynamics tracers
Example: Methane in the stratosphere
CH4 + OH → CH3 + H2O
Lifetime ≈ 10 yrs
Time for equator-to-pole transport
in Brewer-Dobson circulation ≈ 2 yrs
HALOE satellite
Problem 2: Tracer correlation method
b) To reach the high-latitude lower stratosphere air masses follow the Brewer-Dobson
circulation (see figure)
Due to: CH4 + OH → CH3 + H2O
The more the air parcels travel, the more methane
will be oxidized → CH4 depletion
By rising into the tropical stratosphere the air masses
pass through regions of highest ozone production and
concentration → O3 enrichment
Therefore CH4 and O3 are anti-correlated in the high-latitude lower stratosphere
HF is produced by photolysis of CFCs in the stratsphere (> 20 km) → HF enrichment
during transport in the Brewer-Dobson circulation
Therefore HF and O3 are correlated in the high-latitude lower stratosphere
Problem 2: Tracer correlation method
c) From the reaction of methane with OH:
CH4 + OH → CH3 + H2O
It follows
[CH4](t) = [CH4]t=0 exp(–kt)
CH4 concentration is maximum at the time of entrance in the stratosphere (Age=0) and
then decreases exponentially with time
→ CH4 concentration is inversely proportional to the age of air
Problem 2: Tracer correlation method
d) Müller et al. measured the CH4–O3
correlation inside the polar vortex, before
the onset of chemical depletion (Nov-Jan)
and at the end of polar winter, before the
break-down of the vortex (Mar-Apr).
The difference between the two curves is
the chemical loss of O3 during this time
(i.e. the ozone hole)
The compactness of the distributions is
due to the strenght of the polar vortex,
which chemically isolates the air inside it.
This means that all the air in the polar
vortex come from the upper-middle
stratosphere through the descending
branch of the Brewer-Dobson circulation,
and no perturbations are allowed to enter
the vortex from the sides.
Müller et al., 1997
Chemical
O3 loss
Black, orange = Early vortex (Nov, Jan)
Red, purple = Late vortex (Mar, Apr)
Problem 2: Tracer correlation method
e) HF is sufficiently long-lived and its concentration changes with altitude (it is produced
by photolysis of CFCs above 20-25 km) therefore it can be used as a tracer.
HCl cannot be used as a dynamics tracer because its lifetime is too short and it is
strongly perturbed by the formation/evaporation of PSCs (in the figure: the HCl–CH4
correlation is very loose due to PSCs formation events)
Müller et al., 1997
Problem 2: Tracer correlation method
Löwenstein et al., 1990
f) N2O is emitted at the surface and
transported to the polar lower stratosphere
by the Brewer-Dobson circulation
During the transport in the stratosphere it is
photolyzed by UV radiation
The air parcels that descent inside the polar
vortex are those that spent the longest time
in the stratosphere, meaning those being
most depleted in N2O
Air parcels that descent outside the polar
vortex are likely to have spent less time in
the stratosphere, therefore will have larger
N2O content
→ N2O concentration is lower inside the
polar vortex than outside
Outside
vortex
Inside
vortex