Transcript Ozone in the Troposphere: Air Quality, Chemical Weather and Climate Oliver Wild Centre for Atmospheric Science, Cambridge Dept.

Ozone in the Troposphere: Air Quality,
Chemical Weather and Climate
Oliver Wild
Centre for Atmospheric Science,
Cambridge
Dept. of Environmental Science, University of Lancaster, 5th June 2007
Why are we Interested in Tropospheric Ozone?
Pollution:
O3 is an important component of
photochemical smog
Tropospheric oxidation:
O3 regulates oxidation through
control of OH and controls
removal of CH4, VOCs, etc.
Climate:
Direct: O3 is a greenhouse gas
Indirect: O3 governs lifetime of
other GHGs via OH
Anthropogenic Influence:
Surface and Tropospheric O3 is
increasing due to human activity
• Environmental impacts on local, regional and global scales
• Secondary pollutant: sensitive to many variables
– Chemical production can be fast in polluted conditions
– Lifetime is sufficiently long for global-scale transport
Ozone in the Troposphere
• Intercontinental transport of O3 from industrial sources
– Very long-range transport and the global O3 background
• Regional meteorology and its impacts on O3
– How do physical processes govern chemistry and transport?
• Characterising the uncertainty in current chemistry models
– Can we explain the observed trends in O3 and CH4?
– What processes affecting O3 are least well understood?
Underlying themes:
1. Development and evaluation of tropospheric chemistry models
2. Thorough testing of models against atmospheric measurements
3. Application to air quality and climate issues (O3 and CH4)
Processes Controlling Tropospheric O3
O3
O3
Strat.-Trop.
Exchange
NMHCs,
CH4, CO
OH
HO2, RO2
NO
CO, O3
NO2
O3
hν
OH
HO2
H2O
hν
Emissions
O3
Deposition
O3
Processes Controlling Tropospheric O3
O3
O3
Strat.-Trop.
STE: Governed by meteorological
Exchange
systems, filamentation and mixing
NMHCs,
CH4, CO
OH
HO2, RO2
NO
NO2
CO, O3
hν production is nonChemistry: O
3
O3
OH
HO
linear; strongly location-dependent2
H2O
hν
Emissions
O3
O3
Deposition: dependent on highly
Deposition
variable surface environment
FRSGC/UCI Global CTM
Wild and Prather [2000]
Wild and Akimoto [2001]
Wild et al., [2003]
2
50
100
Pressure /hPa
200
400
600
Strat-Trop Exchange
Strat. Chemistry: Linoz
Cloud Formation
Lightning NOx source
Convection: Tiedke
Photolysis: Fast-J
Advection: 2nd oM
Tropospheric Chemistry
PBL Turbulence
800
1000
Surface Processes
ASAD, 37 species
Emissions
Deposition
37 Levels
T42 resolution (2.8°x2.8°); driven with ECMWF-IFS forecast fields
1. Intercontinental Transport of Ozone
Current Industrial/Fossil Fuel NOx Emissions
• Industrial emission regions located at similar latitudes
– Transport times about 1 week; chemical lifetime 3-4 weeks
• How much do major emission regions affect each other?
– How much do they contribute to background O3?
– Could this affect attainment of air quality standards?
• Explore O3 production and transport with 3-D global CTM
– Single-region anthropogenic emission perturbation experiments
• Photochemistry active in summer
• Transport most efficient in spring
Largest O3 impacts in late spring
Wild and Akimoto [2001]
East Asian
Emissions
SourceReceptor
Matrix
US
Emissions
European
Emissions
• Major emission regions directly affect each other
– Upwind sources contribute 1-2 ppbv to surface background O3
– This is sufficient to affect attainment of air quality standards
– Study now being repeated with many models (HTAP) to inform policy
2. Regional Meteorology and Chemical Weather
Key Questions and Challenges
– How are regional and global impacts influenced by meteorology?
• What is the variability in O3 production from a given source?
– How does meteorology govern climate impacts of sources?
• How will future changes in meteorology affect climate impacts?
– How well can models simulate the time scales for O3 formation?
Model Approach
– Perturb fossil fuel NOx/CO/NMHC emissions over one region for one day
• Follow atmospheric perturbation for 1 month
– Repeat for each day of March 2001 (TRACE-P measurement campaign)
– Look at variability in magnitude and location of O3 production
Ozone Responses
Look at regional and global O3
from a single day’s emissions over
Shanghai
March 12
– Sunny, high pressure
– Strong regional P(O3)
March 16
– Heavily overcast
– Little regional P(O3)
Regional production different,
Global production similar
– Evolution quite different
– Location of P(O3) different
Meteorological Setting on March 12 and 16, 2001
H
L
H
L
Column- and latitude-integrated gross O3 production over the first 3
days following 1 day of emissions over Shanghai
Ozone Response to Shanghai Emissions
Global Ozone Increase
Regional Ozone Increase
Regional Boundary Layer
Distant Boundary Layer
Free Troposphere
• Effects on O3 burden
– Days with high regional O3 (smog)
have a reduced effect on global O3
– Regional meteorology strongly
influences climate impacts
• P(O3) vs. NOx loss for each day
– O3 production efficiency (OPE)
strongly dependent on location
– Good representation of lifting
processes is required!
3. Exploring the Uncertainty in Current CTMs
O3 Burden vs. O3 Lifetime
Diagonals in grey show
O3 loss rate (Tg/year)
(τO3 = Burden/Loss)
• ACCENT studies
• CTM with NMHC
• CTM without NMHC
• CTM studies show large differences in O3 burden and lifetime
– Where do these differences originate?
• Perform sensitivity study on key processes in a single CTM
– Identify processes contributing to this uncertainty
3. Exploring the Uncertainty in Current CTMs
O3 Burden vs. O3 Lifetime
330 Tg/yr
Diagonals in grey show
O3 loss rate (Tg/year)
(τO3 = Burden/Loss)
• ACCENT studies
• CTM with NMHC
• CTM without NMHC
22.4 days
Best estimates from
recent model studies
• CTM studies show large differences in O3 burden and lifetime
– Where do these differences originate?
• Perform sensitivity study on key processes in a single CTM
– Identify processes contributing to this uncertainty
3. Exploring the Uncertainty in Current CTMs
O3 Burden vs. O3 Lifetime
Diagonals in grey show
O3 loss rate (Tg/year)
800 Tg STE
60 Tg NOx
650 Tg Isop
7.5 Tg NOx
lightning
460 Tg dep
−20%
T−5°C
T+5°C
Sensitivity to key
variables explains
much of the scatter
975 Tg dep
+20%
30 Tg NOx
+20% H2O
0 Tg
250 Tg STE
0 Tg
−20% H2O
3. Exploring the Uncertainty in Current CTMs
• ACCENT studies
• CTM with NMHC
• CTM without NMHC
• Summary of key sensitivities
–
–
–
–
Account for 2/3 of
NOx emissions:
more O3, P(O3), more OH
model variability
Isoprene emissions: more O3, P(O3), less OH
Lightning NOx:
poorly constrained, large impact on O3 and OH
Meteorology:
effects of humidity and STE
• Implications
– Current models are not good enough to model trends in O3 and CH4!
Future Studies
• Modelling atmosphere-vegetation interactions
– Important feedbacks between O3, VOC, N-species and plants
– Interaction of anthropogenic and vegetation emissions is very
poorly understood and requires spatial disaggregation
– Currently lead the ‘biogenic fluxes’ theme in JULES
Climate
aerosol
NOx, CO
VOC
O3
VOC
NOy
NO
Soils
Requires improved treatment
of biogenic emissions and
deposition.
Crops
Involves collaboration with
land use and vegetation
community and a full
Earth System approach
Future Studies
• Improved treatment of urban emissions in climate models
–
–
–
–
Improved simulation of O3 production in coarse-resolution models
Reduced bias in regional/global O3 important for climate
Allows better testing against surface observations
Important for assessing environmental impacts of Megacities
Background
These processes function on a
range of scales, but their
impacts on climate have not
been assessed.
Plume
Mixing zone
Wind Direction
Involve strong collaboration
with the UK and EU urban &
local modelling community
Future Studies
• Modelling the evolution of tropospheric oxidation
–
–
–
–
Reproducing the observed trends in CH4 and O3
Important for climate and air quality communities
Requires improved understanding of tropospheric chemistry
Need a better characterization of variability in CH4 sources
Need more thorough testing of
models vs. observations
Contributes to goals of new
international Atmospheric
Chemistry and Climate project
Annual Mean Impacts on O3
Wild and Akimoto [2001]
Daily O3 from Source Regions in Springtime
Global Impact
Receptor Region
r2=0.92
OPE=35
Evolution of O3 profile over Cheju, Korea in CTM
TRACE-P Ozonesondes
– Very different profiles
Pressure /hPa
• Stratospheric intrusion at Cheju,
Korea, March 1–2, 2001
• Intercepted by sondes on
successive days
Tropopause
• CTM captures evolution of features
well
–
–
–
–
Two layers on March 1
Background strat. enhancement
One high layer on March 2
Residual strat air mixed in
• Suggests mechanisms for STE can
be captured, but demonstrates high
degree of variability in ozone
Sonde data: Sam Oltmans, NOAA/CMDL
March 1, 2001
March 2, 2001
Net O3 Production Rate
• Instantaneous O3 production in
CTM vs. box model constrained by
observations
• Mean latitude-altitude profile over
all DC8/P3B flights
• Net destruction in tropical marine
boundary layer
• Strong production over Japan
• Strong plume activity in outflow
region, 23º–32ºN
• Net production in upper trop
(underestimated in CTM)
(Box model: Jim Crawford, NASA
Langley, Doug Davis, Georgia Tech.)