STRATOSPHERIC 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 (O 2 + hv) Sink of ozone (O+O 3 )  predicts too much ozone! Other ozone sinks: catalytic loss cycles: 1. HOx 2. NOx 3. ClOx H2OH2O slow fast OH HO 2 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 + Cl ClOO + M  Cl + O 2 2 x [Cl + O 3  ClO + O 2 ] NET: 2O 3  3O 2 Perfect! Key here is high [ClO] Where from?

REVIEWING STRATOSPHERIC CHEMISTRY… THE IMPORTANCE OF PSCs Polar stratospheric clouds: conversion of ClOx reservoirs to Cl 2 ClNO 3 + HCl – PSC  Cl 2 + HNO 3 Cl 2 + hv  2Cl PSC formation Frost point of water Key to polar O 3 depletion is cold T + sunlight Once sun comes up the Cl goes on to react with O 3 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… N 2 O 5 HYDROLYSIS: INCREASING SENSITIVITY TO Cl N 2 O 5 + H 2 O – aerosol  2HNO 3 Effects on catalytic cycles (for ozone loss): 1.NOx cycle: moves from one reservoir (N 2 O 5 ) to a longer-lived reservoir (HNO 3 )  longer for NOx to be recycled ↓ importance of NOx-catalysis 2.ClOx cycle: NOx important for termination of ClOx cycling. A reduction in NOx reduces efficiency of ClO + NO 2 + M  ClNO 3 + M ↑ importance of ClOx-catalysis 3.HOx cycle: When HNO 3 eventually returned to NOx will also produce HOx (HNO 3 + h  NO 2 + 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 O 3 columns are smallest in tropics despite this being the main stratospheric O 3 production region Explanation Rising tropospheric air with low ozone B-D circulation transports O 3 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 O 3 depletion: W/m 2  But note degree of spatial variability (polar vs. mid- latitude) RF from strat water vapour: W/m 2  ONLY from increased methane (not feedbacks) RF from halocarbons: W/m 2  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 CO 2 is expected to cool the stratosphere

TRENDS IN POLAR OZONE Could greenhouse-induced cooling of stratosphere produce an Arctic ozone hole over the next decade? Race between chlorine decrease and climate change

TRENDS IN WATER VAPOUR: COMPLEX AND CHANGING satellite (Arctic, 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 20 th 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? H 2 O 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 CO 2 or UV heating from O 3 ), leads to more PSC formation, O 3 depletion 2.Ozone itself is a GHG 3.Changes in stratospheric T (from either CO 2 or UV heating from O 3 ) 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 O 3 layer) affect the biosphere, biogenic emissions, increase OH production  affecting CH 4 and O 3 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