Transcript Free Tropospheric NO 2

Global free tropospheric NO2 Abundances
Derived using a Cloud Slicing Technique
from Aura OMI
S. Choi, J. Joiner, Y. Choi,
B. N. Duncan, and E. J. Bucsela
OMI Science Team Meeting
2014. 3. 13.
Outline
 Background
 Cloud slicing technique
 Evaluation of cloud slicing technique in conjunction
with INTEX-B data
 Global seasonal climatology of free tropospheric NO2
 Profile analysis
 Stratospheric column NO2
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Free Tropospheric NO2
 Atmospheric NO2
– Produced by combustion, lightning, and in soil
– Affects air quality - affects ozone production and criteria
pollutant itself
 Indirect radiative impacts in troposphere
– Ozone has largest warming effect in upper-troposphere
– Impacts methane concentrations
 Tropospheric vertical column densities (VCD) of NO2 are
available from satellites (GOME, OMI, SCIAMACHY, GOME-2)
– No profile information
 A few types of free-tropospheric NO2 measurements available
– Aircraft in situ measurements, MAX-DOAS, NO2 sondes, etc.
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Cloud Slicing Technique
 Utilize above-cloud NO2 column (where CRF > 0.9)
– Good quality column measurements as clouds provide
bright surface
– Usually neglected in the view of surface pollution
 Data obtained
– Free tropospheric NO2 volume mixing ratio (VMR)
– Stratospheric column NO2
 Independent of
– A priori information
– Prescribed stratospheric column estimate
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Cloud Slicing Technique
Cloud scene
pressure
Ground
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Observed Column vs Pressure
0
Cloud scene pressure (hPa)
Measurement at Cloudy Scenes
Top of Atmosphere (TOA)
~1013
NO2 VMR
TOA
dVCD
=
*kB*g/Rair
dP
ΔP
ΔVCD
Ground
NO2 VCD above the clouds
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Cloud Slicing Technique
Cloud scene
pressure
Ground
Observed Column vs Pressure
0
Cloud scene pressure (hPa)
Measurement at Cloudy Scenes
Top of Atmosphere (TOA)
~1013
NO2 VMR
TOA
dVCD
=
*kB*g/Rair
dP
ΔP
ΔVCD
Ground
NO2 VCD above the clouds
The slope between NO2 VCD and cloud pressure is
proportional to NO2 volume mixing ratio (VMR)
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Cloud Slicing Technique
Measurement at Cloudy Scenes
Top of Atmosphere (TOA)
Observed Column vs Pressure
0
TOA
Tropopause
Pressure
Cloud scene
pressure
ΔP
ΔVCD
Ground
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~1013
Strat NO2 VCD
(above-tropopause column)
Extended linear fit to the tropopause gives
an estimate of stratospheric column
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Ground
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Cloud Slicing Technique
 Assumptions
– Uniform NO2 VMR in the sampling pressure ranges
– No stratospheric variation in the sampling
spatial/temporal ranges (6ox 8o, calculated per orbit)
 Limitations
– Represent cloudy conditions only
– Magnitudes only as accurate as above-cloud NO2 VCD
• SCD may be overestimated up to ~30% in current study
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Comparison with INTEX-B Data
 A case near Houston, TX
 Reasonable agreement with positive bias (~35%)
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Comparison with INTEX-B Data
 A case over northeast Pacific
 Reasonable agreement with positive bias (~35%)
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Evaluation of Cloud Slicing NO2 VMR
 All available matchups
 No correlation
 Similar magnitude with positive bias
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Evaluation of Cloud Slicing NO2 VMR
 INTEX-B VMR standard error < 5 pptv
 Weak correlation and positive bias
 Reasonable agreement despite the intrinsic limits:
– Poor collocation, small scale features, clear/cloudy conditions
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Global Seasonal Climatology
 Global seasonal free-tropospheric NO2 climatology
– Take advantage of high spatial/temporal coverage of
satellite measurements
– Concentrate on spatial and seasonal patterns
 Large volume of data required for reasonable results
– 3-year OMI data accumulated (2005-2007)
– Coarse resolution (6o latitude x 8o longitude)
 Distinct patterns in the free tropospheric VMR
– Independent source of data to study free troposphere
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Global Seasonal Climatology
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OMNO2B
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Global Seasonal Climatology
Different spatial patterns
from tropospheric column
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OMNO2B
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Global Seasonal Climatology
Appears to be anthropogenic contributions
(uplifted pollution & outflow)
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OMNO2B
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Global Seasonal Climatology
Appears to be lightning contributions
(dominates during summer months)
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OMNO2B
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Global Seasonal Climatology
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OMNO2B
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Global Seasonal Climatology
Appears to be anthropogenic contributions
(uplifted pollution & outflow)
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OMNO2B
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Global Seasonal Climatology
Appears to be lightning contributions
(dominates during summer months)
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OMNO2B
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Profile Analysis
 Coarse profile analysis (~100 hPa resolution)
 Requires even more data with significant cloud
pressure variability
 Two example cases
– East Asia and its outflow for anthropogenic NO2
– Tropical oceans of NH and SH for lightning NO2
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Profile Analysis
 High NO2 at 600-800 hPa
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Profile Analysis
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Profile Analysis
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Profile Analysis
High NO2 above 500 hPa
in summer
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Estimates of Stratospheric Column NO2
Estimate from cloud slicing agrees very well with independently
derived estimates from OMNO2B – provides further confidence in
derived free-tropopshericOMIVMRs
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Conclusions
 Free tropospheric NO2 VMR derived using cloud slicing
 Global seasonal climatology shows anthropogenic and
natural (lightning) features of free tropospheric NO2,
independent of the tropospheric column
 Profile analysis shows uplifted anthropogenic NO2 in
the middle troposphere as well as lightning NO2 in the
upper troposphere
 Estimated stratospheric column NO2 agrees well with
independent stratospheric NO2 estimates
 Expect collaborations with free tropospheric NO2
measurements including NO2 sondes
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Thank you
S. Choi et al.: Global free tropospheric NO2 Abundances
Derived using a Cloud Slicing Technique from Aura OMI,
Atmos. Chem. Phys. Discuss., 2013
Backup slides
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Detail/Data Screening Criteria
 Use slant column density (OMNO2A) and geometric
AMF
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Cloud radiance fraction > 0.9
Aerosol index < 1.0
No snow/ice surface
Solar zenith angle < 80 degree
Gradient of NO2 VMR < 0.33 pptv / hpa (profiles from
GMI model or INTEX-B measurements)
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Example of Calculated NO2 Climatology
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Free Trop. NO2 VMR (OMI vs GMI)
OMI
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GMI
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Tropospheric Column NO2 (OMI vs GMI)
OMI
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GMI
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