STRATOSPHERIC CHEMISTRYSTRATOSPHERIC CHEMISTRY

TOPICS FOR TODAY

1. Review of stratospheric chemistry

2. Recent trends in stratospheric ozone and forcing

3. How will stratospheric chemistry be affected by climate

change?

REVIEWING STRATOSPHERIC CHEMISTRY…BASIC MECHANISM

Chapman Mechanism: Source of ozone (O2 + hv)Sink of ozone (O+O3)

predicts too much ozone!

Other ozone sinks: catalytic loss cycles:1. HOx2. NOx3. ClOx

H2Oslow

slow

fastOH HO2

Closes the overall stratospheric ozone budget: sinks balance source

REVIEWING STRATOSPHERIC CHEMISTRY…WHY DOESN’T THIS EXPLAIN THE OZONE HOLE?

Antarctic ozone hole observed in austral SPRING!

Catalytic NOx and ClOx cycles all depend on [O] The source of O is photolysis (peaks in summer not spring!)

Need another mechanism to explain ozone depletion in spring:

ClO + ClO + M ClOOCl + M

ClOOCl + hv ClOO + ClClOO + M Cl + O2

2 x [Cl + O3 ClO + O2]

NET: 2O3 3O2

Perfect!Key here is high [ClO]Where from?

REVIEWING STRATOSPHERIC CHEMISTRY…THE IMPORTANCE OF PSCs

Polar stratospheric clouds: conversion of ClOx reservoirs to Cl2

ClNO3 + HCl –PSCCl2 + HNO3

Cl2 + hv 2Cl

PSC formation

Frost point of water

Key to polar O3 depletion is cold T + sunlight

•Once sun comes up the Cl goes on to react with O3

•At first ClO builds up (remember not enough [O]), eventually ClO + ClO cycle takes over•ClOOCl photolyzed back to Cl more ozone depletion•Then stratosphere warms up, no more PSCs, no more conversion of reservoir species, slows down depletion

REVIEWING STRATOSPHERIC CHEMISTRY…N2O5 HYDROLYSIS: INCREASING SENSITIVITY TO Cl

N2O5 + H2O –aerosol 2HNO3

Effects on catalytic cycles (for ozone loss):

1.NOx cycle: moves from one reservoir (N2O5) to a longer-lived reservoir (HNO3) longer for NOx to be recycled

↓ importance of NOx-catalysis2.ClOx cycle: NOx important for termination of ClOx cycling. A reduction in NOx reduces efficiency of ClO + NO2 + M ClNO3 + M

↑ importance of ClOx-catalysis3.HOx cycle: When HNO3 eventually returned to NOx will also produce HOx (HNO3 + h NO2 + OH)

↑ importance of HOx-catalysis

Overall: generally has little effect on TOTAL ozone loss, but does make ozone loss more sensitive to Cl levels

Also: will enhance ozone loss in the presence of aerosols (eg. volcano)

Observation

O3 columns are smallest in tropics despite this being the main stratospheric O3 production region

Explanation

Rising tropospheric air with low ozone

B-D circulation transports O3 from tropics to mid-high latitudes

BREWER-DOBSON CIRCULATION

TOPICS FOR TODAY

1. Review of stratospheric chemistry

2. Recent trends in stratospheric ozone and forcing

3. How will stratospheric chemistry be affected by climate

change (and vice versa)?

MORE CULPRITS ON THE RADIATIVE FORCING FIGURE….

RF from strat O3 depletion:-0.05 W/m2

But note degree of spatial variability (polar vs. mid-latitude)

RF from strat water vapour:+0.07 W/m2

ONLY from increased methane (not feedbacks)

RF from halocarbons:+0.32 W/m2

forcing here is direct (as LLGHGs)

[IPCC, 2007]

TREND IN HALOCARBONS

Not just a catalyst for stratospheric ozone depletion, also make up 12% of GHG forcing!

[IPCC, 2007]

Halocarbons are regulated by the Montreal Protocol. Long lifetimes means it takes some time for strict emission controls to slow down growth.

TRENDS IN GLOBAL OZONE

Mt. Pinatubo

LONG-TERM COOLING OF THE STRATOSPHERE

Sep 21-30, 25 km, 65-75˚S

Increasing CO2 is expected to cool the stratosphere

TRENDS IN POLAR OZONECould greenhouse-induced cooling of stratosphereproduce an Arctic ozone hole over the next decade?

Race between chlorine decrease and climate change

TRENDS IN WATER VAPOUR: COMPLEX AND CHANGING

satellite(Arctic, 16-18 km)

balloon (16-18 km)

satellite (global)

Recent decreases linked to changes in circulation?

[Randel et al., 2004]

Water vapour trends difficult to interpret, can

differ vertically and may be strong dynamical link.

Increasing trend in late 20th century

TOPICS FOR TODAY

1. Review of stratospheric chemistry

2. Recent trends in stratospheric ozone and forcing

3. How will stratospheric chemistry be affected by

climate change (and vice versa)?

WILL WATER VAPOUR INCREASE IN THE STRATOSPHERE?

H2O mixing ratio

UNCLEAR

If so:(1) modeling studies predict increase in HOx ozone depletion(2) in polar regions this would raise the T threshold for PSC formation (phase diagram), potentially increasing ozone depletion

?

INTERACTIONS BETWEEN STRATOSPHERIC OZONE AND CLIMATE

1. Cooling stratosphere (from either CO2 or UV heating from O3), leads to more PSC formation, O3 depletion

2. Ozone itself is a GHG

3. Changes in stratospheric T (from either CO2 or UV heating from O3) alter the Brewer-Dobson circulation and rate of cross-tropopause transport

4. Increases in B-D circulation increases T in the polar regions and decrease T in the tropics

5. Increases in UV radiation (from depletion of the O3 layer) affect the biosphere, biogenic emissions, increase OH production affecting CH4 and O3 in the troposphere

CCM VALIDATION ACTIVITY FOR STRATOSPHERIC PROCESSES AND THEIR ROLE IN CLIMATE

(SPARC CCMVal)

Models that integrate chemical changes, transport changes and other changes to the climate (chemistry-climate models)

SKIN CANCER EPIDEMIOLOGY PREDICTIONS