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Stratospheric Chemistry and

Processes

Sophie Godin-Beekmann

LATMOS, OVSQ, IPSL

1 MOSS, Reunion Island, 28 Nov. - 03 Dec. 2016

The Stratosphere


• Layer just above the troposphere

• Altitude depends on latitude and season (higher in the tropics and summer)

• ~10% of atmospheric mass

• Positive gradient of temperature due to solar UV radiation absorption by molecular oxygen and ozone

• Slow mixing in the vertical

• Chemical processes linked to the presence of ozone

The ozone layer

3

Main properties of ozone

Ozone vertical distribution

90%

10%

• Absorbs UVB radiation 280 – 320 nm • Heats the stratosphere • Strong oxidant

MOSS, Reunion Island, 28 Nov. - 03 Dec. 2016

Ozone column abundance

All atmosphere molecules: ~8 km

All ozone molecules compressed to P ~1 atm. and T=0°C: Layer of 3 mm thickness (300 DU)

Ozone formation in the atmosphere ~500 millions years ago allowed life to emerge from the ocean

Ozone equilibrium: chemical processes

4

O2

O2 + hn ® O + O

O + O2 + M ® O3 + M

O3 + hn ® O + O2

O + O3 ® 2 O2

Production

Loss

l < 242 nm

Chapman mechanism proposed in 1930 by Sydney Chapman

R1

R2

l < 310 nm R3

R4

(+DQ)

Chapman mechanism successful in reproducing general shape of ozone layer but: explains only 20% of the loss of Ox in the stratosphere


Ozone loss: catalytic cycles

5

Catalytic cycles

Catalytic agents produced from source gases transported to the stratosphere by dynamic processes

N2O, CH4, H2O, CH3Cl, CH3Br

CFCs, Halons

R4 reaction (O + O3 -> 2O2) too slow in chapman mechanism:

Other mechanisms introduced to explain the loss of Ox via

O3+ X ® XO + O2

O + XO ® X + O2

net: O + O3 ® 2 O2

NOx

CH3Cl & CFCs: CF2Cl2

CF3Cl N2O

CH4

Methyl bromide

& Halons

H2O

HOx ClOx BrOx

Catalytic cycles can occur 1000 times

Overview of chemical processes


Example: NOx cycle

N2O + hn N2 + O1D R1 or N2O + O1D N2 + O2 R2 NO + NO R3

R3: only 5% of N2O loss but only source of NOx radicals

Catalytic cycle

NO + O3 NO2 + O2

NO2 + O NO + O2

Net O + O3 2 O2

Formation of reservoir species limits efficiency of the cycles: HO2 + HO2 H2O2 + O2 ClO + NO2 + M ClONO2 + M NO2 + OH + M HNO3 + M BrO + NO2 + M BrONO2 + M Cl + CH4 HCl + CH3

Rapid exchange of NO-NO2 NOx family

Fragile ozone equilibrium


abundance of radical species 44°N, zenith angle: 60° Reprobus model

Atmospheric abundance of main atmospheric species

Ozone controled by species 1000 time less abundant!!

HOx

NOx

ClOx

BrOx

Efficiency of catalytical science

8

Efficiency of main catalytic cycles


Ozone photochemical lifetime

Lower stratosphere: ozone controlled by dynamical processes

Ozone equilibrium : dynamical processes

9

Brewer-Dobson circulation

Planetary waves

Total ozone climatology


Mainly produced in the tropics, ozone is transported towards the poles by the meridional stratospheric circulation

The perturbed ozone layer

10

Antarctic Ozone hole

October 2015


First alerts on the ozone layer

11

1970s: controversy on the impact on ozone of a fleet of stratospheric supersonic transports

1974: article of Molina and Rowland

The SST project is abandoned (except for the UK-French Concord)

Catalytic destruction of ozone by chlorofluorocarbons (CFC)

1979: aerosol sprays are banned by USA, Sweden, Canada


Substances depleting the ozone layer (SDO)

12

Chlorofluorocarbons (CFC)

• patent in 1928 by DuPont (USA) - Main CFC: CFC-11 (CCl3F), CFC-12 (CCl2F2)

• Long lifetime -> transported in the stratosphere by atmospheric dynamical processes

Halons

• Organic brominated gases • Main halons: halon-1211 and halon-1301 • Applications : dry cleaning, fire extinguishers

Natural sources of chlorine and bromine in the stratosphere CH3Cl (methyl chlorid) et CH3Br (methyl bromid): emitted by terrestrial and oceanic ecosystems.

• Applications : refrigeration, air conditioning, blowing agents ... wonder chemicals!


Early 1980s

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Evolution of knowledge on impact of CFCs on ozone

CFC emissions grow again...

But signature of the Vienna Convention for the protection of the ozone layer in 1985

Effect of CFC on ozone

First report on the state of the ozone layer


Ozone hole discovery in Antarctica

Farman et al., Large Losses of total ozone in Antarctica reveal seasonal ClOx/NOx interaction, Nature, 1985

Syowa

At Syowa, total ozone and sonde measurements show also large decreases in Spring

Chubachi et al., QOS, 1984

14

TOMS Total Ozone monthly average

Stolarski et al., Nature, 1986


Elucidation of ozone destruction mechanisms

15

NASA campaign: Airborne Antarctic Ozone expedition – 1987

ER-2 stratospheric plane

August September Before 1985 : Ozone theory based on chemical processes in gaseous phase only

Antarctica : ozone loss rate : ~5 % per day, not explainable by theory

Hypothesis :

Chemical reactions at the surface of polar stratospheric clouds:

• Activation of chlorine compounds(Solomon, 1986)

• Very fast catalytical cycles (Molina & Molina, 1987)

Scientific proof

pôle

ClO

O3

Anderson et al., 1991

Polar ozone destruction mechanism

16

Ingredients for the formation of the ozone hole: 1. Increase chlorine and bromine content in the stratosphere (Cly x 5)

2. Isolated polar air masses in winter (polar vortex)

3. Very low temperatures(< - 80°C)

Complete total O3 destruction between 14 and 20 km

Cl2 + hn 2Cl

HOCl + hn OH + Cl

Cl + O3 ClO + O2

2060:12.8b:1/96:blm

In the light of therising spring sun

I n darkness

1

2

3

4

Formation of polarstratospheric clouds:

Formation of ClOx:

Activation of Clx: Catalyticozone depletion:

cold, isolatedpolar vortex

-80°C

Antarctica

H2O

HNO3 H2O

ClO + ClO Cl2O2

Cl2O2 + hn 2Cl + O2

Cl + O3 ClO + O2

ClONO2

HOCl

N2O5

Cl2,HOCl

HNO3

HCl

Stratosphere

Troposphere

9 km

Polar stratospheric clouds


Main PSC particles: Supercool ternary solution (STS), Nitric Acid trihydrate (NAT) and ice

CALIOP

Main heterogeneous reactions:

- Convert chlorine and bromine reservoir species into more reactive forms

- HCl, HNO3 and H2O remain in the particles

Peter, 2013

backscatter

depolarisation

Polar chemistry


Antarctic winter-spring 2006

Denitrification and dehydration observed by MLS

Main catalytic cycles ClO + ClO + M → ClOOCl +M BrO + ClO → Br + Cl + O

Cycles need ClO > 1 ppbv to be efficient

Destruction rate: ~5 % /day

The Montreal Protocol

Signed in 1987 – entered into force in 1989

• Regulation of CFC and brominated halons emissions -> Substitutes less toxic for the ozone layer (First HCFC then second HFC that don’t contain chlorine)

• Technology transfer towards developing countries

• Regular reports on the state of the ozone layer and substitutes productions: the evolution of the protocol depending on scientific results

• Multilateral fund for technological transfers : ~ 4 billions dollars in 2014

• Reference for the creation of IPCC in 1988 related to climate change


Montreal Protocol Amendments

« Science driven » protocol :

1996 2010

CFC

HCFC

2020 2040


Ozone Destruction in the Arctic?

Formation of stratospheric clouds but polar vortex in the Arctic less isolated and warmer: polar ozone loss weaker and more variable

Polar Stratospheric Clouds


Arctic ozone loss

22

Various international campaigns to quantify ozone loss : AASE, EASOE, SESAME, THESEO-SOLVE, RECONCILE Various measurements and methods (more difficult to distinguish chemical loss from dynamical loss): Match, passive ozone tracer

WMO, 2006

Arctic ozone loss in 1999/2000 compared to typical Antarctic ozone loss


Chlorine activation in cold arctic winters

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2005 2004/2005

AURA MLS measurements of HCl, ClO and O3 Santee et al., 2008

HCl ClO O3


Ozone depletion at global scale

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WMO, 1998

• No trends in the tropics • Mid-latitude ozone trends due to dilution of polar ozone loss, chemical in situ

processes and change in meteorology

Total ozone trends 1979 – 2000 Trend vertical profile 1980 – 1996

Fioletov et al., 2002

Ground-based Satellite


Effect of volcanic stratospheric aerosols


Increases sensitivity of ozone to ClOx and decreases sensitivity to NOx • High chlorine levels: decrease of O3

• Low chlorine: increase of O3

25

Pinatubo aerosol effect on ozone A few % temporary decrease

Mt Pinatubo eruption, June 1991

Significant injection of ~20 MT of SO2 that converted into H2SO4 aerosol droplets

El Chichon Pinatubo Lidar (Garmisch) Sage II

Main heterogeneous reactions on strat aerosols

Aerosol integrated backscatter 694.3 nm

Present state of the ozone layer 27 years after enforcement of Montreal Protocol

26

Evolution of CFC and halons content

27

EESC: Evolution of stratospheric halogen content weighted by the toxicity of species

towards ozone

Evolution of Antarctic ozone hole

28

Ozone hole: Recurrent seasonal feature in Southern Hemisphere since 1980

Ozone hole area

Minimum d’ozone


Ozone evolution at global scale

29

Annual ozone anomalies (with respect to 1998 – 2008 means)

WMO, 2014


Evolution of polar ozone

30

Ozone minimum values in March (Arctic) and October (Antarctic)

Record Arctic ozone loss in 2011


2002: major warming in Antarctica

Record Arctic ozone depletion 2011

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IASI total ozone

Manney et al., Nature 2011

also Sinnüber et al., 2012; Kuttipurath et al., 2012

25 % additional ozone loss due to denitrification Low ozone also explained partly by low ozone transport to the pole

Looking for ozone recovery due to decrease of ODS


Quantification of ozone variability

Multiple regression models

TOZ(t)=TOZ° + aEESCEESC(t)+Sai Proxyi(t) +

residual(t)

Main proxies used

- QBO (30 & 10 hPa)

- NAO or ENSO index

- Solar flux

- Eddy heat flux averaged over 45-75°N or 45-75°S (BDC)

- aerosol optical depth

- EESC or PWLT (piecewise linear trend with turning point around ~1996)


Influence of proxies (polar regions)

34

Linear relationship between Spring/Fall ozone ratio and eddy heat flux Weber et al., 2011; Weber et al., 2012

± 4 DU

± 5 DU

± 10 DU


Ozone recovery in Antarctica

35

Identification of second stage recovery claimed by Salby et al., GRL, 2011; JGR, 2011; Kuttipurath et al., 2013, and Knibbe et al., 2014

De Laat (2015) use « big data approach » to trace recovery, e.g. MC on period, proxy, etc..: 30–60% of the regressions result in statistically significant positive springtime ozone trend over Antarctica


1st stage recovery 2nd stage recovery

Solomon et al., 2016 Significant recovery in September • increases in ozone

column amounts and vertical profile

• decreases in the area of the ozone hole

Yang et al., JGR, 2008

Ozone recovery in the upper stratosphere?

Past changes in the vertical distribution of ozone : Analysis and interpretation of trends

Harris et al., ACP, 2015

error bars: black: std of weighted average red: Joint distribution

light blue: drifts taken into account

no significant upward trends different conclusion from WMO, 2014

NDACC Alpine station (OHP, Hohenpeissenberg, Bern) more than 2 decades of measurements

temporal instrumental artefacts vs geophysical signals: 3%/decade range in trends

Godin-Beekmann et al., 2016

Global ozone trends

ODS decrease

Climate change effect:

Acceleration of Brewer-Dobson circulation

-> decrease of ozone in tropics

-> increase of ozone in extratropics

WMO, 2014

Evidence for an increase of the lower branch of the BDC :

Decrease of ozone in the LS in the 1980s (e.g. Randel and Thompson, 2011; Sioris et al, 2014, Shepherd et al., 2014) but:

• Hiatus in upwelling since 2002 from SAGEII + SCIAMACHY + SHADOZ ozone observations (Aschmann et al., 2014)

• Increase of ozone in the troposphere?

Contrasted trends observed in 2000 – 2013

Main drivers of ozone long term changes


Ozone changes and climate

• Sun drives radiative, dynamical and chemical processes affecting ozone and temperature

• Stratospheric temperature determined by concentrations of radiatively active gases (ozone, long-lived greenhouse gases, H2O) and aerosols via absorption of SW and LW radiation

• Transport determines amounts of stratospheric ozone and related long-lived compounds

• Temperature influences temperature-dependent reaction rates

transport


Forecasted evolution of EESC

40

Equivalent effective stratospheric chlorine

Total chlorine abundance at Jungfraujoch (x1015 mol.cm-2)

• Rate of decline depends on atmospheric lifetime of halogen compounds

• In the Polar regions age of air larger than in mid-latitude regions

• Return date to 1980 : Mid-latitudes ≈ 2040 – Polar regions ≈ 2065

Newman et al., acp, 2007

mid-latitudes Polar


Source gases evolution


IPCC 2007 WMO, GHG report 2015

2015 abundance relative to year 1750 of CH4 and N2O:

• CH4: 256% • N2O: 121% • CFC and HCFC abundances regulated by the Montreal Protocol

Evolution of stratospheric aerosols

42

• Tenfold increase of aerosols due to large volcanic eruptions (e.g. El Chichon, Pinatubo) induces a temporary ozone decrease (in the presence of large ODS levels)

• Since 2000: temporary increase of background aerosol due to small volcanic eruption

Vernier et al., GRL, 2011

SAOD 17-30km

Looking for new stratospheric aerosol background: volcanos vs impact of asian pollution

Khaykin et al., 2016

20°S – 20°N

NH mid-latitudes & OHP (44°N, 6°E)


Radiative forcing (RF) of volcanic eruptions for the years 2008 – 2011 of ~ - 0.11 W m-2 (Solomon et al., 2011)

Non volcanic period: 17% increase

Evolution of stratospheric temperature

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SSU temperature anomalies Global mean temperature anomalies from multiple data sets 10 – 25 km

• Global-mean lower stratosphere cooled by 1–2 K from 1980 to 1995

• Upper stratosphere cooled by 4–6 K from 1980 to about 1995.

• No significant long-term trend since 1995. • Cooling of stratosphere due to stratospheric

ozone depletion and GHG increase


Effect of stratosphere cooling on ozone

44

45°N for equinox conditions (end of March)

40 km 20 km

Homogeneous chemistry

Heterogeneous chemistry

Potential increase of PSC in polar regions Larger effect in the Arctic region (unsaturated ozone loss)

Slows ozone destruction rates, e.g. O + O3 → 2O2 at 40 km -> ozone increase


Stratospheric water vapor

45

• No recent increase in stratospheric water vapor at global scale

• Mechanisms driving long-term changes in stratospheric water vapor not well understood.

Effect on ozone

• Increase in HOx

• Increases temperature threshold for PSC formation in polar regions

Hartman et al., 2013, WMO, 2014


Simulation of ozone recovery by climate models


Future ozone evolution: summary

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ODS O3 destruction CO2 cools the stratosphere + impact on circulation (GHG) N2O O3 destruction (NOx cycle) CH4 O3 destruction + impact on circulation

Fleming et al., 2011, Portmann et al., 2012

Shepherd and Jonsson, acp, 2007


Tropical regions

Eyring et al., JGR, 2013

• Ozone less sensitive to ODS

• Sensitive to BDC

• Decrease in stratosphere?

• Increase in troposphere?

• Observations?

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Polar ozone

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• Antarctic: return of October total column ozone to 1980 projected after mid century

• Arctic: return of March-mean Arctic total column ozone projected in 2020-2035

• Climate change dominates polar ozone evolution after 2050

• Larger sensitivity in NH

RCP: 8.5 6 4.5 2.6

Some take home messages

• Montreal protocol successful in reducing the amount of ozone depleting substances (ODS) in the atmosphere but the return to pre-1980 levels will take decades

• The ozone layer should return to pre-1980 levels by 2020 – 2060 depending on latitudes and climate change effects

• Models predict by 2100 a super-recovery of ozone at mid-latitude and polar regions and an under-recovery in the tropics

• In the polar regions: competition between stratospheric cooling and decrease of ODS in the next decade. Also changes in meteorologic variability

• Future threat to the ozone layer: geoengineering by sulfate aerosols injection into the stratosphere

• Evaluation of future ozone levels needs an adequate ozone monitoring system: issues both in satellite and ground-based observing systems


Polar Stratospheric Cloud observed at Haute-Provence Observatory on February 3, 2016

Thank you !

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