Atmospheric chemistry

Lecture 4:

Stratospheric Ozone Chemistry

Dr. David GlowackiUniversity of Bristol,UK

david.r.glowacki@bristol.ac.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