atmospheric chemistry lecture 4: stratospheric ozone chemistry dr. david glowacki university of...
TRANSCRIPT
Atmospheric chemistry
Lecture 4:
Stratospheric Ozone Chemistry
Dr. David GlowackiUniversity of Bristol,UK
Yesterday…
• We discussed tropospheric chemistry• The troposphere is a massive chemical reactor that
depends on pressure, temperature, sunlight, and ground level chemical emissions
Today…
• We will discuss some of the chemistry in the stratosphere• Stratospheric chemistry is a little bit simpler than
tropospheric chemistry because there’s less pollutants• Also, the molecules involved are smaller so there’s fewer
branching reactions
Integrated column - Dobson unit
Atmospheric O3 profiles
• In the 1920s, observations of the solar UV spectrum suggested a significant atmospheric [O3]
• At the ground: [O3] ~ 10-100 ppb
• In the stratosphere: [O3] ~ 5-10 ppm
O3 altitude profile measured from satellite
The Chapman Cycle
O2 + hv O + O (1)O + O2 + M O3 + M (2)O3 + hv O2 + O (3) O3 + O O2 + O2
(4)
O2 O(3P) + O(1D) - Threshold < 176 nm
Chapman Cycle Step 1: O2 + hv O + O
O2 O(3P) + O(3P) - Threshold < 242 nm
Chapman Cycle Step 2: O + O2 + M O3 + M
O + O2 reaction coordinate
O OO
M
M = O2 or N2
O3
UV absorption spectrum of O3 at 298 K
Hartley bands
Very strong absorption
Photolysis mainly yields O(1D) + O2, but as the stratosphere is very dry (H2O ~ 5 ppm), almost all of the O(1D) is collisionally relaxed to O(3P)
Chapman Cycle Step 3: O3 + hv O2 + O
Small but significant absorption out to 350 nm (Huggins
bands)
λ < 336 nm
UV absorption spectrum of O3 at 298 K Chapman Cycle Step 4
O3 + O O2 + O2
Occurs via an abstraction mechanism
The Chapman Cycle
O2 + hv O + O (1)O + O2 + M O3 + M (2)O3 + hv O2 + O (3) O3 + O O2 + O2
(4)
Rate coefficients for each reaction have been measured in the lab
Solving for [O3] using the Chapman Mech
(1)
(2)
(3)
(4)
€
[O] =k1[O2]
k4[O3]
€
[O3] =k1k2
k3k4
⎛
⎝ ⎜
⎞
⎠ ⎟
1/ 2
CO2na
3 / 2
€
[M] = na[O2] =CO2
na€
[O]
[O3]=
k3
k2[M][O2]
(A1)
(A2)
(B1)
(B2)
(na is the atmospheric number density)
(CO2 is the O2 mixing ratio)
Substitute (A2) into (B2)
How good is the Chapman mechanism?
€
k1 = j1 = σ A (λ ,T)φA (λ ,T)∫ I(λ )dλ
€
[O3] =k1k2
k3k4
⎛
⎝ ⎜
⎞
⎠ ⎟
1/ 2
[O2]na3 / 2
Beer Lambert Law
Atmospheric optical depth
k1 & k3 are photolysis rates
• Determining stratospheric [O3] using the above Chapman equation isn’t entirely straightforward because k1 and k3 are photolysis rates!
where
and
How good is the Chapman mechanism?
Increasing photolysis with altitude
Chapman overpredicts by a factor of 2
The maximum reflects k1, which is affected by:(1)Decreasing [O2] with altitude following the barometric law(2)Increasing hv with altitude
€
[O3] =k1k2
k3k4
⎛
⎝ ⎜
⎞
⎠ ⎟
1/ 2
[O2]na3 / 2
A
ltit
ude
Q: Why does Chapman overpredict?
A: Catalytic Ozone loss cycles
Catalytic ozone destructionThe loss of odd oxygen can be accelerated through catalytic cycles whose net result is the same as the (slow) 4th step in the Chapman cycle
Uncatalysed: O + O3 O2 + O2 k4
Catalysed: X + O3 XO + O2 k5
XO + O X + O2 k6
Net rxn: O + O3 O2 + O2
X is a catalyst and is reformed
X = OH, Cl, NO, Br (and H at higher altitudes)Reaction (4) has a significant barrier and so is slow at stratospheric temperatures
Reactions (5) and (6) are fast, and hence the conversion of O and O3 to 2 molecules of O2 is much faster, and more ozone is destroyed.
Using the steady-state approximation for XO, R5=R6 and hence k5[X][O3] = k6[XO][O]
Rate (catalysed) / Rate (uncatalysed) = R5/R4 = k5[X][O3]/k4[O][O3]= k5[X]/k4[O]
Or Rate (catalysed) / Rate (uncatalysed) = R6/R4 = k6[XO][O]/k4[O][O3]=k6[XO]/k4[O3]
• X+O3 (k5) and XO+O (k6) are up to a factor of ~104 faster than O + O3 (k4)!
• A little bit of XO makes a big difference!
k5 (220K) k4
k6 (220K)
Catalytic ozone loss kinetics
Catalytic O3 loss via HOx
• OH is an even more efficient catalyst because the intermediate HO2 also destroys O3
• OH in the stratosphere is generated in the same way it is generated in the troposphere
Predominant fate of stratospheric NO
(null cycle, no net change)
A small fraction of NO2 reacts with O
Catalytic O3 loss via NOx
Catalytic Loss Cycle
Loss of stratospheric NOx
• Primarily via formation of HNO3, transport to troposphere, & deposition
• HNO3 & N2O5 are NOx ‘reservoirs’
• Very stable & have a long lifetime
daytime
nighttime
N2O: another source of stratospheric NOx
• Because the N2O lifetime is very long, it may be transported to the stratosphere, where it undergoes the following:
• Consideration of N2O brings the Chapman model into much better agreement with observations
• Ice Core data show increase of atmospheric [N2O] of ~0.3% year since 18th century
Some complications to stratospheric O3 chemistry
• Catalytic Loss cycles are coupled to each other• Aerosols