observation of atmospheric composition from space with material from: daniel j. jacob (harvard),...
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OBSERVATION OF ATMOSPHERIC COMPOSITION FROM SPACE
With material from: Daniel J. Jacob (Harvard), Andreas Richter (Bremen), Cathy Clerbaux (Service d’Aéronomie)
Colette L. Heald ATS 737, October 15, 2008
Absorption and emission spectra provide a means of identifying and measuring the composition of the atmosphere. Radiation interacts with gases via:
(1) Ionization-dissociation (UV-visible)
(2) Electronic transitions (UV-visible)
(3) Vibrational transitions (IR)
(4) Rotational transitions (far IR and microwave) IR spectra of many molecules is a combination of (3) and (4)
WHAT IS THE EFFECT OF ATMOSPHERIC COMPOSITION ON RADIATION?
Instead of discrete lines, transitions are observed in a whole wavelength region.
• natural line broadening (upper stratosphere, mesosphere)
• Doppler broadening (upper atmosphere: > 40 km)
• pressure broadening (lower atmosphere: < 40 km)
E + hν
E
hν hν
OBSERVED RADIATION includes :•Reflection (solar, UV-visible)•Emission (Earth/atmosphere, IR)•Absorption (by gases and particles)•Scattering (by gases and particles)
Convolution: Voigt lines
- 3 -
EXAMPLES OF ABSORPTION SPECTRA
Chappuis band
Hugginsband
Hartley band
ALL TOGETHER NOW…
STRATOSPHERIC OZONE HAS BEEN MEASURED FROM SPACE SINCE 1979
Method: UV solar backscatter
Scattering by Earth surface and atmosphere
Ozone layer
Ozoneabsorptionspectrum
SATELLITE OBSERVATIONS REVEAL THE MECHANISM FOR POLAR OZONE LOSS AND HELP US TRACK OZONE RECOVERY
DU Southern hemisphere ozone column seen from TOMS, October
1 Dobson Unit (DU) = 0.01 mm O3 STP = 2.69x1016 molecules cm-2
MLS ClOTOMS O3
Polar ozone depletion driven by halocarbon break-
down (source of ClO)
ATMOSPHERIC COMPOSITION RESEARCH IS NOW MORE DIRECTED TOWARD THE TROPOSPHERE
…but tropospheric composition measurements from space are difficult:optical interferences from water vapor, clouds, aerosols, surface, ozone layer
Tropopause
Stratopause
Stratosphere
Troposphere
Ozonelayer
Mesosphere
…but tropospheric composition measurements from space are difficult:optical interferences from water vapor, clouds, aerosols, surface, ozone layer
Air quality, climate change, ecosystem issues
WHY OBSERVE TROPOSPHERIC COMPOSITION FROM SPACE?
Monitoring and forecastingof air quality: ozone, aerosols
Long-range transport of pollution
Monitoring of sources:pollution and greenhousegases
• solar backscatter• thermal emission• solar occultation• lidar
FOUR OBSERVATIONMETHODS:
Global/continuous measurement capability important for range of issues:
Radiative forcing
SOLAR BACKSCATTER MEASUREMENTS (UV to near-IR)
absorption
wavelength
Scattering by Earth surface and by atmosphere
Examples: TOMS, GOME, SCIAMACHY, MODIS, MISR, OMI, OCO
Pros:• sensitivity to lower troposphere• small field of view (nadir) Cons:
• Daytime only• Column only• Interference from stratosphere
concentration
Retrieved column in scattering atmospheredepends on vertical profile; need chemical transportand radiative transfer models
z
THERMAL EMISSION MEASUREMENTS (IR, wave)
EARTH SURFACE
I(To)
Absorbing gas
To
T1
I(T1)LIMB VIEW
NADIRVIEW
Examples: MLS, IMG, MOPITT, MIPAS, TES, HIRDLS, IASI
Pros:• versatility (many species)• small field of view (nadir)• vertical profiling
Cons:• low S/N in lower troposphere• water vapor interferences• cannot see through clouds
OCCULTATION MEASUREMENTS (UV to near-IR)
“satellite sunrise”
Tangent point; retrieve vertical profile of concentrations
Examples: SAGE, POAM, GOMOS
Pros:• large signal/noise• vertical profiling Cons:
• sparse data, limited coverage• upper troposphere only• low horizontal resolution
EARTH
LIDAR MEASUREMENTS (UV to near-IR)
EARTH SURFACE
backscatter by atmosphere
Laser pulse
Examples: LITE, GLAS, CALIPSO
Intensity of return vs. time lag measures vertical profile
Pros: • High vertical resolution
Cons:• Aerosols only (so far)• Limited coverage
ALL ATMOSPHERIC COMPOSITION DATA SO FAR HAVE BEEN FROM LOW-ELEVATION, SUN-SYNCHRONOUS POLAR ORBITERS
• Altitude ~ 1,000 km
• Observation at same time of day everywhere
• Period ~ 90 min.
• Coverage is global but sparse
TROPOSPHERIC COMPOSITION FROM SPACE:platforms, instruments, species
Platform multiple ERS-2
ADEOS Terra Envisat Aqua Space station
Aura MetOp-A
Sensor TOMS AVHRR/SeaWIFS
GOME IMG MOPITT MODIS/MISR
SCIAMACHY
MIPAS AIRS SAGE-3 TES OMI MLS HIRDLS CALIPSO IASI OCO
Launch 1979 1995 1996 1999 1999 2002 2002 2002 2004 2004 2004 2004 2004 2004 2007 2009
O3 X X X X X X X X X
CO X X X X X X X
CO2 X X X
NO X
NO2 X X X X
HNO3 X X X
CH4 X X X
HCHO X X X
SO2 X X X X
BrO X X X
CH3CN X
aerosol X X X X X X X
OBSERVING TROPOSPHERIC OZONE AND ITS SOURCES FROM SPACE
Nitrogen oxide radicals; NOx = NO + NO2
Sources: combustion, soils, lightningMethaneSources: wetlands, livestock, natural gasNonmethane VOCs (volatile organic compounds)Sources: vegetation, combustionCO (carbon monoxide)Sources: combustion, VOC oxidation
Troposphericozone
precursors
A NEEDLE IN A HAYSTACK: DERIVING TROPOSPHERIC
OZONE
Fishman and Larson, 1987; Fishman et al., 2008
Issues:• high uncertainty• seasonal averages only• does not extend to high latitudes
FIRST REMOTE MEASUREMENTS OF CO: MAPS ABOARD THE SPACE SHUTTLE
Gas-correlation radiometer (IR: 4.7 m): flew 4 times between 1981 and 1994
Connors et al., 1999; Reichle et al., 1999
APR 1994
OCT 1994
RETRIEVALS IN THE IR: THE STANDARD INVERSE PROBLEM
Typical MOPITTAveraging Kernel
GεAxxAIx a )(ˆ
Averaging kernel (A): describes the relative weighting of the ‘true’ mixing ratio (x) at each level to the retrieved value ( )
INVERSE PROBLEM: solution is not unique!
SOLUTION: maximum a posteriori
Characteristic absorption features in the IR.
Use a known T profile to estimate the constituents
Fy (x) + ε Kx + ε
x
1 T -1 -1
ε aS K S K +S
1aI SS A
MOPITT: FIRST SATELLITE INSTRUMENT TARGETTING TROPOSPHERIC POLLUTION
Comparison indicates that emission inventories may be inaccurate
MOPITT CO Column
MOPITT – Model
Heald et al., 2004
MOPITT: solidModel: dotted
Observations used to track transpacific transport of pollution
CO Column over the NE Pacific in Spring 2001
Spring 2001
AIRS GEOS-Chem Model
POLLUTION AND BIOMASS BURNING OUTFLOW DURING ICARTT AIRCRAFT MISSION (Jul-Aug 2004)
Asianpollution
U.S. pollution
Alaskan fires
Wallace McMillan (UMBC) Turquety et al., 2006
NEAR-REAL-TIME DATA FOR CO COLUMNS ON JULY 18
USING MODIS TO MAP FIRESAND MOPITT CO TO OBSERVE EMISSIONS
MOPITT CO Summer 2004 GEOS-Chem CO x MOPITT AK
Bottom-up emission inventory (Tg CO) for North American fires in Jul-Aug 2004
withoutpeat burning
withpeat burning
MOPITT data support large peat burning source, pyro-convective injection to upper troposphere
Turquety et al., 2006
18 Tg CO 9 Tg CO
From above-ground vegetation From peat
USING ADJOINTS OF CHEMICAL TRANSPORT MODELS TO INVERT FOR EMISSIONS WITH HIGH RESOLUTION
MOPITT daily CO columns(Mar-Apr 2001)
A priori emissions fromStreets et al. [2003] andHeald et al. [2003]
Kopacz et al., 2008
Inverse ofatmospheric
model
Correction to model sources of CO
CONSTRAINING NOx AND REACTIVE VOC EMISSIONS USING SOLAR BACKSCATTER MEASUREMENTS
OF TROPOSPHERIC NO2 AND FORMALDEHYDE (HCHO)
Emission
NOh (420 nm)
O3, RO2
NO2
HNO3
1 day
NITROGEN OXIDES (NOx) VOLATILE ORGANIC COMPOUNDS (VOC)
Emission
VOC
OHHCHOh (340 nm)
hoursCO
hours
BOUNDARYLAYER
~ 2 km
Tropospheric NO2 column ~ ENOx
Tropospheric HCHO column ~ EVOC
Deposition
GOME: 320x40 km2
SCIAMACHY: 60x30 km2 OMI: 24x13 km2
DIFFERENTIAL OPTICAL ABSORPTION SPECTROSCOPY
Use multiple wavelengths to characterize optical absorption of a species.
determine the amount of absorber along the light path (slant column, s)
Pioneered for stratospheric ozone, used for detection in UV-visible
Scattering by Earth surface and by atmosphere
/S AMF Vertical column:
Air mass factor (AMF) depends on the viewing geometry, the scattering properties of the atmosphere, and the vertical distribution of the absorber
Requires an RT model and a CTM
Or alternate of DOAS: direct fit of GOME backscattered spectrum in 338-
356 nm HCHO bandChance et al. [2000]
GOME sensitivityw(z)
HCHO mixing ratioprofile S(z) (GEOS-Chem)
what GOMEsees
AMFG = 2.08actual AMF = 0.71
AMF FORMULATION FOR A SCATTERING ATMOSPHERE
0
= ( ) ( )GAMF AMF S z w z dz
Palmer et al., 2001
w(z): GOME sensitivity (“scattering weight”), determined from LIDORT radiative transfer model including clouds and aerosolsS(z): normalized mixing ratio (“shape factor”) from GEOS-Chem CTMAMFG: geometric air mass factor (no scatter)
GOME CONSTRAINTS ON NOx EMISSIONS
1015 molecules cm-2
r = 0.75 bias=5%
JJA 1997
Tropospheric NO2 ColumnsGOMEGEOS-CHEM model
(GEIA)
Errorweighting
A priori emissions (GEIA) A posteriori emissions Difference
Martin et al. [2003]
HIGHER SPATIAL RESOLUTION FROM SCIAMACHY
Launched in March 2002 aboard Envisat
Potential for finer resolution of sources, but need to account for transport will complicate the inversion
320x40 km2 60x30 km2
K. Folkert Boersma (KNMI)
TROPOSPHERIC NO2 FROM OMI: CONSTRAINT ON NOx SOURCES
October 2004
NOX MEASUREMENTS REVEAL TRENDS IN DOMESTIC EMISSIONS
East-Central China
NO2 emissions in US, EU and Japan decline …
while emissions growing in China
Importance of long-term record!
Richter et al., 2005; Fishman et al., 2008
FORMALDEHYDE COLUMNS MEASURED BY GOME (JULY 1996)
High HCHO regions reflect VOC emissions from fires, biosphere, human activity
-0.5
0
0.5
1
1.5
2
2.5x1016
moleculescm-2
SouthAtlanticAnomaly(disregard)
detectionlimit
SEASONAL VARIATION OF GOME FORMALDEHYDE COLUMNS reflects seasonal variation of biogenic isoprene emissions
SEP
AUG
JUL
OCT
MAR
JUN
MAY
APR
GOME GEOS-Chem (GEIA) GOME GEOS-Chem (GEIA)
Abbot et al., 2003
AEROSOLS FROM SPACE
To retrieve aerosol optical depth need aerosol properties (size distribution, index of refraction). Can use wavelength dependence to get idea of composition/size
ISSUE: Need to characterize Rayleigh scattering and surface reflectance (including sun glint) thus easier over oceans (dark surfaces)
MIE SCATTERING• scattering on „large“ particles (aerosols, droplets, suspended matter in liquids)• explained by coherent scattering from many individual particles• for spherical particles, Mie scattering can be computed from the refractive index using
the Maxwell equations • wavelength of incoming radiation is not changed• angular distribution is changed• depending on , forward scattering
is strongly favoured• effectiveness of Mie scattering
is proportional to s
Mie () -1 ... -1.5
• in general, Mie scattering is not polarising
Extinction = Scattering + AbsorptionUsually in visible
MODIS
MULTI-SPECTRAL: 7 bands from 0.4 – 2.1 µm
MISR
MULTI-ANGLE: 9 cameras (visible)
TRANSPACIFIC TRANSPORT OF ASIAN AEROSOL POLLUTION AS SEEN BY MODIS
Heald et al., 2006
Detectable sulfate pollution signal correlated with MOPITT CO
MAPPING SURFACE PM2.5 USING MISR (2001 data)
MISR PM2.5
MISR AOD (annual mean)
EPA (FRM+STN) PM2.5
Evaluate against EPA station data: R = 0.78, Slope = 0.91
Liu et al.,2004
Validation withAERONET:R2=0.80Slope=0.88
Convert AOD to surface PM2.5 using GEOS-CHEM +GOCART scaling factors
NASA AURA SATELLITE (launched July 2004)
AuraAuraMLS
TES nadirTES nadirOMIOMI
HIRDLS Direction of motion
TES limbTES limb
Polar orbit; four passive instruments observing same air mass within 14 minutes
•OMI: UV/Vis solar backscatter• NO2, HCHO. ozone, BrO columns
• TES: high spectral resolution thermal IR emission• nadir ozone, CO• limb ozone, CO, HNO3
•MLS: microwave emission• limb ozone, CO (upper troposphere)
• HIRDLS: high vertical resolution thermal IR emission• ozone in upper troposphere/lower stratosphere
Tropospheric measurement capabilities:
GOME JJA 1997 tropospheric columns (Dobson Units)
TROPOSPHERIC OZONE OBSERVED FROM SPACE
IR emission measurement from TES UV backscatter measurement from GOME
Liu et al., 2006 Zhang et al., 2006
Coincident CO measurements from TES
Coincidental observations of COand O3 with TES allows us to look at ozone production
(sensitivity)
OBSERVING CO2 FROM SPACE:Orbiting Carbon Observatory (OCO) to be launched in 2009
Averaging kernel
Pre
ss
ure
(h
Pa
)OCO will provide powerful constraints on regional carbon fluxes
Polar-orbiting solar backscatter instrument, measures CO2 absorption at 1.61 and 2.06 m, O2 absorption (surface pressure) at 0.76 m: global mapping of CO2 column mixing ratio with 0.3% precision
UV-IR sensors would provide continuous high-resolution mapping (~1 km)
on continental scale: boon for air quality monitoring and forecasting
LOOKING TOWARD THE FUTURE: GEOSTATIONARY ORBIT
NRC Decadal Survey Recommendation: GEO-CAPE in 2013-2016, with Aura-like GACM in 2016-2020
(also ACE for aerosols 2013-2016) NRC, 2007