Next Two Weeks—Stratospheric Chemistry
READING: Chapter 10 of text
•Mid-latitude Ozone Chemistry (and depletion)
•Polar Ozone Destruction (the Ozone Hole)
Final Projects
See web site for more details
The general idea
To allow you to explore in greater detail a topic area that
we covered (or did not cover) in class. The topic must deal
with atmospheric chemistry or composition.
A manuscript which is between 10 - 15 double spaced pages
This length does not include references or figures.
Typical Formats
Critical Literature Review
NSF-like Proposal
Short Research Project
Ozone and Pop-Culture
Day of the Animals (1977)
A hole in the ozone layer drives forest animals
berserk in Northern California, and a group of
vacationing hikers is caught in the fray…
See clips on You Tube
The Naked Gun II
"A love affair is like the ozone layer," says
Lieut. Frank Drebin. "You only miss it when
it's gone."
The “Ozone Awards”,
Ozone “air purifiers”
Ozone Spa Therapy …
Stratospheric O3: Overview
Most O3 (90%) in
stratosphere.
Remaining 10% in
troposphere.
“Ozone layer” (15-30
km) is 3000 – 5000
ppb of O3.
Surface O3 ~ 40 –
100 ppb.
Stratospheric Ozone: Overview
~3,000 ppb
•sustains terrestrial life by
absorbing UV radiation
•impacts the atmospheric
vertical temperature profile
•important role in Earth’s
overall radiative balance
<100 ppb
Stratospheric Ozone: Motivating Questions
1. What are the natural production and loss
mechanisms for stratospheric O3?
2. What is the effect of anthropogenic emissions on
stratospheric ozone, both past and future?
3. What are the mechanisms responsible for the
drastic ozone decreases in the Antarctic?
4. What role, if any, does the stratospheric aerosol
layer play in the gas-phase chemistry of the
stratosphere?
Stratospheric Ozone—a brief history
1840’s Ozone first discovered and measured in air by Schonbein who
named it based on its smell.
1880-1900’s: Hartley postulates the existence of a layer above the
troposphere, where ozone is responsible for the absorption of solar UV
between 200 and 300 nm.
1913: Fabry and Buisson used UV measurements to estimate that if
brought down to the surface at STP, O3 would form a layer ~ 3 mm thick.
1920-25: Dobson first shows that T(z) in stratosphere not constant but
increases with z and implicates O3 absorption. Makes first extensive set
of O3 column measurements with his spectrophotometer
1930: Chapman proposed that O3 continually produced in a cycle
initiated by O2 photolysis.
Chapman Mechanism
Net O3 formation
Odd Oxygen
Chemical Familly
Ox = O3 + O
O2
hv slow
O
slow
Net O3 loss
O2
hv
Fast
Fast
O3
Mass balance for [O]:
d [O]
 2 jO 2 O2   jO 3 O3   k2 O2 O  M   k4 O3 O  ~ 0
dt
small
small
Odd Oxygen Family: Ox
[Ox] = [O] + [O3]  [O3]
Mass balance for [Ox]:
d [Ox ]
 2 jO 2 O2   2k4 O3 O 
dt
[O3] controlled by slow net production and loss via
O2 + hv (R1) and O + O3 (R4)
NOT by fast production and loss of O3 from O + O2
(R2) and O3 + hv (R3)
Questions
1. The original Chapman mechanism included a fifth reaction:
O + O + M  O2 + M
What would be the effect of this reaction on ozone? Is it
more important in the lower or in the upper stratosphere?
2. [O]~109 molec/cm3 in the upper stratosphere and ~ 106 in
the lower stratosphere, if the lifetime of O3 w.r.t.
transport away from equator is ~ month, is it fair to
assume O3 is in a chemical steady state throughout the
stratosphere?
kO+O3 (US) ~ 4x10-15 cm3/molec/s; kO+O3(LS) ~ 3x10-16 cm3/molec/
Stratospheric OH Profile
Wennberg, et al
Science 1994
Hydrogen oxide (HOx) radical family
HOx = H + OH + HO2
 Initiation:
H2O + O(1D)  2OH
From troposphere
and CH4 oxidation
 Propagation through cycling of HOx radical family
(example):
OH + O3  HO2 + O2
HO2 + O3  OH + 2O2
Net: 2O3  3O2
 Termination (example):
OH + HO2  H2O + O2
HOx is a catalyst for O3 loss but not the only one…
Nitrogen oxide (NOx) radical family
NOx = NO + NO2
• Initiation
• Propagation
N2O + O(1D) 2NO
NO + O3  NO2 + O2
NO2 + h  NO + O
O + O2 + M  O 3 + M
Null cycle
• Termination
NO2 + OH + M  HNO3 + M
NO3 + NO2 + M  N2O5 + M
NO + O3  NO2 + O2
NO2 + O  NO + O2
Net O3 + O  2O2
Recycling
HNO3 + h  NO2 + OH
N2O5 + h NO2 + NO3
Cycling of NOx and NOy
What have we learned about NOy?
Production:
N2O + O(1D) - well understood natural source
Loss: via transport from stratosphere to
troposphere. Residence time for air in stratosphere is
1-2 years. Loss rate well constrained
Cycling: O3 loss related to NOx/NOy ratio. Under
most conditions s.s. between NOy species is a good
approximation
NOx catalytic cycle reconciled Chapman theory with
observations…1995 Nobel Prize
Stratospheric O3 Production and Loss
Constrained
from 1980s
Space Shuttle
Observations
Gas-phase
chemistry
only
•NOx is most important Ox sink throughout middle stratosphere
•Fully accounts for missing O3 sink in Chapman Mechanism
Human Influence on Stratospheric NOx
Year
IPCC SYR Figure 2-1
Questions
1. Of the ozone loss mechanisms we have examined so far, can
any operate at night?
2. There are several variations possible for NOx and HOx
catalyzed O3 destruction cycles. Given that NO3 photolysis
can proceed via one of two channels
a) NO3 + hv NO2 + O
b) NO3 + hv NO + O2
Come up with at least one additional catalytic cycle for NOx.
3. A minor oxidation pathway for NO is
HO2 + NO -> OH + NO2
What is the net effect of this reaction on ozone?
Latitude
Lovelock’s First Measurements of CFCs
CCl3F (pptv)
Roland and Molina
What is the fate of this, non-toxic CFC?
Stratospheric Distribution of CFC-12
CF2Cl2
Chlorine radical family (ClOx=Cl + ClO)
Cl not the only halogen…Br chemistry is important too
• Initiation:
CF2Cl2 + hv  CF2Cl + Cl
• Propagation:
Cl + O3  ClO + O2
ClO + O  Cl + O2
Net: O3 + O  2O2
d [O3 ]
 2[ClO][O]
O3 loss rate: 
dt
• Termination:
Recycling:
Cl + CH4  HCl + CH3
HCl + OH  Cl + H2O
ClO + NO2 + M  ClNO3 + M ClNO3 + hv  ClO + NO2
Cycling of ClOx and Cly
Questions
1.
Can any of the catalytic cycles we’ve studied
explain why severe ozone depletion occurs over
the South Pole, just after the sun rises there?
Stratospheric O3 Production and Loss
Constrained
from 1980s
Space Shuttle
Observations
Gas-phase
chemistry
only
•NOx is most important Ox sink throughout middle stratosphere
•Fully accounts for missing O3 sink in Chapman Mechanism
Early Warning Signs
Atmospheric Trend of CFC-11
Montreal
protocol
CFC production
Is banned
Ozone Column Trend, 60oS-60oN
[WMO, 1998]
Stabilization
of chlorine?
Chlorine
Or dynamics (transport)?
Pinatubo
Recent Rise in HCFC’s (CFC Replacements)
Coupling Between HOx, NOx, and ClOx Cycles
What is the effect of increasing stratospheric NOx
on the rate of ClOx-catalyzed ozone loss?
Give an example of how HOx and NOx are coupled.
How might an increase in OH affect ClOx cycles?
SPADE Mission
Observations of HOx, NOx, and ClOx
Couplings lead to non-linear
responses to increases in NOx
Wennberg, et al Science 1994