Tropospheric ozone: past, present (and future)

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Tropospheric ozonepast, present (and future)

• David Stevenson
• Institute of Atmospheric and Environmental
  Sciences,
• University of EdinburghThanks to all the ACCENT
  Photocomp Royal Society modellers

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Motivations

• IPCC (2007) Tropospheric O3 is the third largest
  greenhouse gas contributor to radiative forcing
  of climate change 0.35 Wm-2 (CO2 1.66 Wm-2
  CH4 0.48 Wm-2)

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Radiative forcing from tropospheric O3
Forster et al. (2007) IPCC-AR4 WG1 Chapter 2
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Radiative forcing based on emissions not
concentrations
Ozone is a secondarypollutant emissions
ofCH4 CO NOx NMVOChave driven up its
concentration
Forster et al. (2007) IPCC-AR4 WG1 Chapter 2
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Motivations

• IPCC (2007) Tropospheric O3 is the third largest
  greenhouse gas contributor to radiative forcing
  of climate change 0.35 Wm-2 (CO2 1.66 Wm-2
  CH4 0.48 Wm-2)
• Ground level O3 is also a serious air pollutant
  (it is a reactive oxidant), affecting human
  health, damaging crops natural vegetation.

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The biosphere-atmosphere boundary
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Lung function improves at higher O3
Lung function reduces at higher O3
Ozone reducesthe lung functionof
healthychildren
Courtesy of Ross Anderson
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High levels of ozone increase your chances of
dying
Meta-analysis basedon 98 US cities
Bell et al. (2006, EnvironmentalHealth
Perspectives)
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Motivations

• IPCC (2007) Tropospheric O3 is the third largest
  greenhouse gas contributor to radiative forcing
  of climate change 0.35 Wm-2 (CO2 1.66 Wm-2
  CH4 0.48 Wm-2)
• Ground level O3 is a serious air pollutant (it is
  a reactive oxidant), affecting human health and
  damaging crops and natural vegetation.

Or is it even more important for climate?
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AspenFACE Exposure of tree stands to elevated
CO2 and O3
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Indirect and direct radiative forcings from
tropospheric ozone
Symbols are directforcings (IPCC, 2001) Blue
and red curvesare indirect ozoneforcing, due to
ozoneimpacts on vegetation (high ozone
sensitivity) (low ozone sensitivity)
Suggests that the indirect forcing maybe similar
in magnitude to the direct forcing.
Sitch et al. (Nature, 2007)
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Radiative forcing from tropospheric O3
If we believe this O3indirect effect,
then Tropospheric O3approaches CO2 as the No.1
GHG!
Forster et al. (2007) IPCC-AR4 WG1 Chapter 2
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How has ozone changed since 1750?

• No ice-core data (O3 is too reactive)
• Very sparse/poor quality observations

I.e. some evidence that P-I surface O3 in Europe
was lt10 ppb
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Observed trends in surface O3 since the 1970s at
various relatively remote sites
Oltmans et al., 2006
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NH mid-lats, mid-troposphere
Ozonesonde observations since the 1970s
Regionallydifferent trendsregionallydifferent
AQmeasures
Logan et al., 1999 O3 sonde data
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Summary of observed ozone

• A very few measurements before 1900 suggest
  surface O3 in Europe was lt10 ppb before
  industrialisation
• Before 1970s, a few observations show increases
  in surface O3
• Since 1970, surface/sonde monitoring networks
  have expanded
• Most sites show increases in ozone, some show
  strong increases, but significant levels of
  variability (time and space)
• Models needed to produce a global picture

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Modelling tropospheric ozone

• Dynamical core GCM or NWP analyses
• Stratosphere-troposphere exchange
• Tracer transport
• Convection
• BL-free troposphere exchange
• Chemical mechanisms
• Reducing complex schemes
• Photolysis rates (clouds/aerosols)
• Surface exchange
• Biosphere emissions/deposition (stomatal uptake)

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What can models tell us?

• Give a global view of the spatial and temporal
  distribution of ozone and its precursors (more
  detail than observations alone)
• Allow us to diagnose when and where ozone
  chemical production and destruction is taking
  place
• If we have faith in the models, we can use them
  for hindcasts/forecasts, and sensitivity
  experiments (e.g., what happens to ozone if
  emissions and/or climate change?)

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Year 2000 Ensemble meanof 26 models AnnualZonal
Mean Annual TroposphericColumn
ACCENT Photocomp Stevenson et al.,2006, JGR
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Comparison of ensemble mean model with O3 sonde
measurements
ACCENT Photocomp Stevenson et al.,2006, JGR
Individualmodels in grey
UT250 hPa
Model 1SD
Observed 1SD
J F M A M J J A S O N D
MT 500 hPa
LT 750 hPa
30S-Eq
30N-Eq
90-30N
90-30S
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Year 2000 Inter-model standard deviation
() AnnualZonalMean Annual
TroposphericColumn
ACCENT Photocomp Stevenson et al.,2006, JGR
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Seasonal variation of surface ozone
Ensemble mean of 26 ACCENT Photocomp models
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Remote NHSpring peak
Polluted NHsummer peak
Tropics biomassburning (dry) season
Remote SHwinter peak
Broadly in agreement with observations
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Multi-model ensemble mean ozone P, L, NCP
Ship NOx
? 0.997
Surface
ACCENT Photocomp
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Multi-model ensemble mean ozone P, L, NCP
? 0.975
ACCENT Photocomp
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Multi-model ensemble mean ozone P, L, NCP
? 0.930
ACCENT Photocomp
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Multi-model ensemble mean ozone P, L, NCP
? 0.870
ACCENT Photocomp
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Multi-model ensemble mean ozone P, L, NCP
Mid-tropnet destruction
? 0.792
ACCENT Photocomp
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Multi-model ensemble mean ozone P, L, NCP
Mid-tropnet destruction
? 0.700
ACCENT Photocomp
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Multi-model ensemble mean ozone P, L, NCP
Mid-tropnet destruction
? 0.600
ACCENT Photocomp
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Multi-model ensemble mean ozone P, L, NCP
? 0.505
ACCENT Photocomp
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Multi-model ensemble mean ozone P, L, NCP
? 0.422
ACCENT Photocomp
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Multi-model ensemble mean ozone P, L, NCP
Upper-tropnet productionlightning
? 0.355
ACCENT Photocomp
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Multi-model ensemble mean ozone P, L, NCP
Upper-tropnet productionlightning
? 0.300
ACCENT Photocomp
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Multi-model ensemble mean ozone P, L, NCP
Upper-tropnet productionlightning
? 0.250
ACCENT Photocomp
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Multi-model ensemble mean ozone P, L, NCP
Upper-tropnet productionlightning
? 0.200
ACCENT Photocomp
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Multi-model ensemble mean ozone P, L, NCP
Upper-tropnet productionlightning
? 0.150
ACCENT Photocomp
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Multi-model ensemble mean ozone P, L, NCP
Upper-tropnet productionlightning
? 0.099
ACCENT Photocomp
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Assumes no change in biomass burning or soil NOx
between P-I and present
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O3 radiative forcing since 1750
Preindustrial conditions Typically
anthropogenicemissions set to zero,biomass
burning to 10present-day
-0.1
0.35 W m-2
0.3
Lower panel all ACCENT models Gauss et al.,
2006, ACP
Forster et al. (2007) IPCC-AR4 WG1 Chapter 2
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Conclusions (past present O3)

• The direct radiative forcing from tropospheric
  ozone is 0.35 W m-2 (range 0.25 to 0.65 W m-2)
• An indirect effect of O3, via reduced growth of
  vegetation, may add a further 0.2 to 0.4 W m-2,
  suggesting O3 may approach CO2 in terms of
  radiative forcing
• O3 affects human (as well as plant) health, and
  legislation exists in most countries to limit
  emissions of O3 precursors
• Ozone precursor emissions from ships and aircraft
  are not currently regulated, and are growing fast
• Strong legislation to reduce all emissions will
  bring important benefits for both air quality and
  climate, and may be an important short- to
  medium-term tool to reduce radiative forcing,
  especially if O3 does have a larger forcing than
  currently thought
• Models appear to realistically simulate
  present-day ozone, but a more stringent test will
  be for them to reproduce longer term regional
  trends

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Ozone at Mace Head, Ireland 1987-2007
Courtesy Dick Derwent, Peter Simmonds et al.
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GOME NO2 Tropospheric Column 2000
Mean of 3 retrieval methods
Std. Dev. of 3 retrieval methods
Mean of 17 models
Std. Dev. of 17 models
ACCENT Photocomp van Noije et al., 2006, ACP
E. Asian NOx emissions too low Biomass burning
emissions too high
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Models CO underestimates observations in
Northern Hemisphere- Asian CO emissions too low
ACCENT Photocomp Shindell et al., 2006, JGR
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Ozone in the future

• Will depend strongly on the trajectory of
  anthropogenic emissions, in particular NOx, but
  also CH4, CO and VOCs.
• IPCC SRES probably too pessimistic new
  projections from IIASA expect air quality
  legislation to significantly reduce NOx emissions
  by 2050
• Climate change is likely to impact ozone

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Under current legislation, NOx emissions should
reduce in most places
SRES B2
IIASAcurrentlegislation
Courtesy Markus Amann, IIASA
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Ship and aircraft emissions

• Both have essentially no legislation to regulate
  them
• Likely to keep increasing if nothing is done
• Ships a particularly large NOx source
• In the 2050 scenario used here, optimistically
  assumed that ship emissions will be controlled

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NOx emissions 2050-2000
Reduce almost everywhere global total down 40
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Methane emissions 2000-2100
Courtesy Markus Amann, IIASA
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Tropospheric O3 responds approximately linearly
to anthropogenic CH4 emission changes across
models
Courtesy ofArlene Fiore
Anthropogenic CH4 contributes 50 Tg (15) to
tropospheric O3 burden 5 ppbv to
surface O3