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 Meeting2014. 3. 13.
2OMI Science Team Meeting 2014
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
Ground
Top of Atmosphere (TOA)
Clou
d sc
ene
pres
sure
(hPa
)
NO2 VCD above the clouds
Cloud scenepressure
NO2 VMR
= *kB*g/Rair
dVCDdP
ΔP
ΔVCD
Measurement at Cloudy Scenes Observed Column vs Pressure
Ground
TOA
~1013
0
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Cloud Slicing Technique
The slope between NO2 VCD and cloud pressure is proportional to NO2 volume mixing ratio (VMR)
Ground
Top of Atmosphere (TOA)
Clou
d sc
ene
pres
sure
(hPa
)
NO2 VCD above the clouds
Cloud scenepressure
NO2 VMR
= *kB*g/Rair
dVCDdP
ΔP
ΔVCD
Measurement at Cloudy Scenes Observed Column vs Pressure
Ground
TOA
~1013
0
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Cloud Slicing Technique
Extended linear fit to the tropopause gives an estimate of stratospheric column
Ground
Top of Atmosphere (TOA)
Cloud scenepressure
ΔP
ΔVCD
Measurement at Cloudy Scenes Observed Column vs Pressure
Ground
TOA
~1013
0
TropopausePressure
Strat NO2 VCD (above-tropopause column)
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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 bias3/13/2014
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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 conditions3/13/2014
13OMI Science Team Meeting 2014
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
3/13/2014 OMNO2B
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Global Seasonal Climatology
Different spatial patterns from tropospheric column
3/13/2014 OMNO2B
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Global Seasonal Climatology
Appears to be anthropogenic contributions (uplifted pollution & outflow)
3/13/2014 OMNO2B
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Global Seasonal Climatology
Appears to be lightning contributions(dominates during summer months)
3/13/2014 OMNO2B
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Global Seasonal Climatology
3/13/2014 OMNO2B
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Global Seasonal Climatology
Appears to be anthropogenic contributions (uplifted pollution & outflow)
3/13/2014 OMNO2B
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Global Seasonal Climatology
Appears to be lightning contributions(dominates during summer months)
3/13/2014 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-tropopsheric VMRs3/13/2014
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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
29OMI Science Team Meeting 2014
Backup slides
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Detail/Data Screening Criteria
Use slant column density (OMNO2A) and geometric AMF
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 GMI
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Tropospheric Column NO2 (OMI vs GMI)OMI GMI
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