B4.1 (Stratosphere)

Main processes that produce O3 (includes loss reactions)

• Chapman Mechanism (over-predicts O3)

  • O 3P (O) and O2 (+ M)

  • O2 + hv → 2 O

  • O + O3 → 2 O2

  • O3 + hv → O + O2

• catalytic cycles

  • X + O3 → XO + O2

  • XO + O → X + O2

  • net = O + O3 → 2 O2

  • X = NO, HO, H, HO2, Cl

Major source of HOx

H2O, CH4

Major source of ClOx

CFCs + hv

Majour source of NOx

N2O + O(1D)

Major sink of ClOx

HCl

Major sink of HOx + NOx

HONO2

Resevoir for ClOx + NOx

ClONO2

Resevoir of HOx

H2O2

Where does HOx dominate with O3 production

lower stratosphere due to most H2O

where does NOx dominate O3 production

middle stratosphere as needs high levels of O(1D) to react with N2O

where does ClOx dominate O3 production

upper stratosphere, high energy photons needed to dissociate CFCs

What is the altitude range for the stratosphere

20-50 km

how is the mixing rate in the stratosphere

slow

[M]z =

[M]0 exp(-z/H)

average Mr of air

28.9 g mol-1

mixing timescale horizontal on Earth

1-2 weeks

mixing timescale vertically on Earth

6 months (from equator)

why is the stratosphere dry

too cold for water, it freezes out

at what altitude does [O3] peak

around 40 km

how do [CH4] and [N2O] change with altitude

generally constant initially then slopes to lower concentration and plateaus

Equation for column of a mixed gas

[X]z H (all in cm)

J(i) =

φ(i,λ) F(λ) σ(i,λ)

F(λ) =

F(TOA) exp(-OD)

Optical depth, OD, =

Σσ(i,λ) x column(i)

what wavelength penetrates to the troposphere

λ>300 nm

Chapman Cycle and its limit

O2 + hv → O + O

O + O2 + M → O3 + M

O3 + hv → O + O2

O + O3 → O2 + O2

it over-predicts the amount of O3

Ox family

O (3P) + O3