1. OBSERVATION OF ATMOSPHERIC COMPOSITION FROM SPACE
Colette L. Heald
ATS 737, October 15, 2008
With material from:
Daniel J. Jacob (Harvard), Andreas Richter (Bremen), Cathy Clerbaux (Service d’Aéronomie)

2. WHAT IS THE EFFECT OF ATMOSPHERIC COMPOSITION ON RADIATION?
OBSERVED RADIATION includes :
Reflection (solar, UV-visible)
Emission (Earth/atmosphere, IR)
Absorption (by gases and particles)
Scattering (by gases and particles)
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)
E + hν E hν
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)
Convolution: Voigt lines

3. EXAMPLES OF ABSORPTION SPECTRA
Chappuis band
Huggins
band
Hartley band

4. ALL TOGETHER NOW…

5. STRATOSPHERIC OZONE HAS BEEN MEASURED FROM SPACE SINCE 1979
Method: UV solar backscatter l1 l2
Ozone layer
Scattering by
Earth surface
and atmosphere
Ozone
absorption
spectrum l1 l2

6. SATELLITE OBSERVATIONS REVEAL THE MECHANISM FOR POLAR OZONE LOSS AND HELP US TRACK OZONE RECOVERYDU
Southern hemisphere ozone column seen from TOMS, October
TOMS O3
MLS ClO
Polar ozone depletion driven by halocarbon break-down (source of ClO)
1 Dobson Unit (DU)
= 0.01 mm O3 STP
= 2.69x1016 molecules cm-2

7. ATMOSPHERIC COMPOSITION RESEARCH IS NOW MORE DIRECTED TOWARD THE TROPOSPHERE
Air quality, climate change, ecosystem issues
Tropopause
Stratopause
Stratosphere
Troposphere
Ozone
layer
Mesosphere
…but tropospheric composition measurements from space are difficult:
optical interferences from water vapor, clouds, aerosols, surface, ozone layer
…but tropospheric composition measurements from space are difficult:
optical interferences from water vapor, clouds, aerosols, surface, ozone layer

8. WHY OBSERVE TROPOSPHERIC COMPOSITION FROM SPACE?
Global/continuous measurement capability important for range of issues:
Monitoring and forecasting
of air quality: ozone, aerosols
Long-range transport of pollution
Monitoring of sources:
pollution and greenhouse
gases
Radiative
forcing
solar backscatter
thermal emission
solar occultation
lidar
FOUR
OBSERVATION
METHODS:

9. SOLAR BACKSCATTER MEASUREMENTS (UV to near-IR)
Examples: TOMS, GOME, SCIAMACHY, MODIS, MISR, OMI, OCO
absorption l1 l2 z l1 l2
wavelength
Retrieved column in scattering atmosphere
depends on vertical profile;
need chemical transport
and radiative transfer models
Scattering by
Earth surface
and by atmosphere
concentration
Daytime only
Column only
Interference from stratosphere
sensitivity to lower troposphere
small field of view (nadir)
Pros:
Cons:

10. THERMAL EMISSION MEASUREMENTS (IR, mwave)
Examples: MLS, IMG, MOPITT, MIPAS, TES, HIRDLS, IASI
NADIR
VIEW
LIMB VIEW
elIl(T1) T1
Absorbing gas
versatility (many species)
small field of view (nadir)
vertical profiling
Pros:
Il(To) To
EARTH SURFACE
low S/N in lower troposphere
water vapor interferences
cannot see through clouds
Cons:

11. OCCULTATION MEASUREMENTS (UV to near-IR)
Examples: SAGE, POAM, GOMOS
“satellite
sunrise”
Tangent point; retrieve vertical profile of concentrations
EARTH
sparse data, limited coverage
upper troposphere only
low horizontal resolution
large signal/noise
vertical profiling
Pros:
Cons:

12. LIDAR MEASUREMENTS (UV to near-IR)
Examples: LITE, GLAS, CALIPSO
Pros:
High vertical resolution
Laser pulse
Aerosols only (so far)
Limited coverage
Cons:
Intensity of return vs. time lag
measures vertical profile
backscatter by
atmosphere
EARTH SURFACE

13. 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

14. TROPOSPHERIC COMPOSITION FROM SPACE: platforms, instruments, species
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
2002
2004
2007
2009 O3 X CO
CO2 NO
NO2
HNO3
CH4
HCHO
SO2
BrO
CH3CN
aerosol

15. OBSERVING TROPOSPHERIC OZONE AND ITS SOURCES FROM SPACE
Nitrogen oxide radicals; NOx = NO + NO2
Sources: combustion, soils, lightning
Methane
Sources: wetlands, livestock, natural gas
Nonmethane VOCs (volatile organic compounds)
Sources: vegetation, combustion
CO (carbon monoxide)
Sources: combustion, VOC oxidation
Tropospheric
ozone
precursors

16. A NEEDLE IN A HAYSTACK: DERIVING TROPOSPHERIC OZONE
Issues:
high uncertainty
seasonal averages only
does not extend to high latitudes
Fishman and Larson, 1987; Fishman et al., 2008

17. FIRST REMOTE MEASUREMENTS OF CO: ABOARD THE SPACE SHUTTLE
Gas-correlation radiometer (IR: 4.7 m): flew 4 times between 1981 and 1994
APR 1994
OCT 1994
Connors et al., 1999; Reichle et al., 1999

18. RETRIEVALS IN THE IR: THE STANDARD INVERSE PROBLEM
Characteristic absorption features in the IR.
Use a known T profile to estimate the constituents
INVERSE PROBLEM: solution is not unique!
SOLUTION: maximum a posteriori
Averaging kernel (A): describes the relative weighting of the ‘true’ mixing ratio (x) at each level to the retrieved value ( )
Typical MOPITT
Averaging Kernel

19. MOPITT: FIRST SATELLITE INSTRUMENT TARGETTING TROPOSPHERIC POLLUTION
Spring 2001
MOPITT CO Column
CO Column over the NE Pacific in Spring 2001
MOPITT: solid
Model: dotted
MOPITT – Model
Correlation radiometer: 4 thermal channels (4.6 um) and 2 solar reflected (2.3 um)
Observations used to track transpacific transport of pollution
Comparison indicates that emission inventories may be inaccurate
Heald et al., 2004

20. POLLUTION AND BIOMASS BURNING OUTFLOW DURING ICARTT AIRCRAFT MISSION (Jul-Aug 2004)
NEAR-REAL-TIME DATA FOR CO COLUMNS ON JULY 18
AIRS
GEOS-Chem Model
Alaskan fires
U.S. pollution
Asian
pollution
Wallace McMillan (UMBC)
Turquety et al., 2006

21. USING MODIS TO MAP FIRES AND MOPITT CO TO OBSERVE EMISSIONS
Bottom-up emission inventory (Tg CO) for North American fires in Jul-Aug 2004
From above-ground vegetation From peat
18 Tg CO
9 Tg CO
MOPITT CO Summer 2004
GEOS-Chem CO x MOPITT AK
without
peat
burning
with
peat
burning
MOPITT data support large peat burning source,
pyro-convective injection to upper troposphere
Turquety et al., 2006

22. MOPITT daily CO columns Correction to model sources of CO
USING ADJOINTS OF CHEMICAL TRANSPORT MODELS TO INVERT FOR EMISSIONS WITH HIGH RESOLUTION
MOPITT daily CO columns
(Mar-Apr 2001)
Correction to model sources of CO
Inverse of
atmospheric
model
A priori emissions from
Streets et al. [2003] and
Heald et al. [2003]
Kopacz et al., 2008

23. CONSTRAINING NOx AND REACTIVE VOC EMISSIONS USING SOLAR BACKSCATTER MEASUREMENTS OF TROPOSPHERIC NO2 AND FORMALDEHYDE (HCHO)
GOME: 320x40 km2
SCIAMACHY: 60x30 km2
OMI: 24x13 km2
Tropospheric NO2 column ~ ENOx
Tropospheric HCHO column ~ EVOC
~ 2 km
hn (420 nm)
BOUNDARY
LAYER
hn (340 nm)
NO2 NO
HCHO CO OH
hours
O3, RO2
hours
VOC
1 day
HNO3
Emission
Deposition
Emission
NITROGEN OXIDES (NOx)
VOLATILE ORGANIC COMPOUNDS (VOC)

24. DIFFERENTIAL OPTICAL ABSORPTION SPECTROSCOPY
Pioneered for stratospheric ozone, used for detection in UV-visible
Use multiple wavelengths to characterize optical absorption of a species.
 determine the amount of absorber along the light path (slant column, s)
Scattering by
Earth surface
and by atmosphere
Vertical column:
Air mass factor (AMF) depends on the viewing geometry, the scattering properties of the atmosphere, and the vertical distribution of the absorber
GOME backscattered spectrum in nm HCHO band
Requires an RT model and a CTM
Chance et al. [2000]

25. AMF FORMULATION FOR A SCATTERING ATMOSPHERE
w(z): GOME sensitivity (“scattering weight”), determined from LIDORT radiative transfer model including clouds and aerosols
S(z): normalized mixing ratio (“shape factor”) from GEOS-Chem CTM
AMFG: geometric air mass factor (no scatter)
GOME
sensitivity
w(z)
HCHO mixing ratio
profile S(z) (GEOS-Chem)
what
sees
AMFG = 2.08
actual AMF = 0.71
Palmer et al., 2001

26. GOME CONSTRAINTS ON NOx EMISSIONS
GEOS-CHEM model
(GEIA)
Tropospheric NO2 Columns
GOME
JJA 1997
r = 0.75 bias=5%
1015 molecules cm-2
Martin et al. [2003]
Error
weighting
A priori emissions (GEIA)
A posteriori emissions
Difference

27. HIGHER SPATIAL RESOLUTION FROM SCIAMACHY
Launched in March 2002 aboard Envisat
320x40 km2
60x30 km2
Also has additional IR channels for CO/CH4 retrievals
Potential for finer resolution of sources, but need to account for transport will complicate the inversion

28. TROPOSPHERIC NO2 FROM OMI: CONSTRAINT ON NOx SOURCES
October 2004
K. Folkert Boersma (KNMI)

29. NOX MEASUREMENTS REVEAL TRENDS IN DOMESTIC EMISSIONS
NO2 emissions in US, EU and Japan decline …
while emissions growing in China
East-Central China
Importance of long-term record!
Richter et al., 2005;
Fishman et al., 2008

30. FORMALDEHYDE COLUMNS MEASURED BY GOME (JULY 1996)
2.5x1016
molecules
cm-2 2
1.5 1
detection
limit
0.5
South
Atlantic
Anomaly
(disregard)
-0.5
High HCHO regions reflect VOC emissions from fires, biosphere, human activity

31. SEASONAL VARIATION OF GOME FORMALDEHYDE COLUMNS reflects seasonal variation of biogenic isoprene emissions
GOME GEOS-Chem (GEIA) GOME GEOS-Chem (GEIA)
MAR
JUL
APR
AUG
SEP
MAY
JUN
OCT
Abbot et al., 2003

32. AEROSOLS FROM SPACE 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 sMie ()   -1.5
in general, Mie scattering is not polarising
Usually in visible
Extinction = Scattering + Absorption
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)
MODIS
MISR
Depending on the ratio of the size of the scattering particle (r) to the wavelength () of the light:
Mie parameter  = 2 r / ,
different regimes of atmospheric scattering can be distinguished.
MULTI-SPECTRAL:
7 bands from 0.4 – 2.1 µm
MULTI-ANGLE:
9 cameras (visible)

33. TRANSPACIFIC TRANSPORT OF ASIAN AEROSOL POLLUTION AS SEEN BY MODIS
Detectable sulfate pollution signal correlated with MOPITT CO
Heald et al., 2006

34. MAPPING SURFACE PM2.5 USING MISR (2001 data)
MISR AOD (annual mean)
Validation with
AERONET:
R2=0.80
Slope=0.88
Convert AOD to surface PM2.5 using GEOS-CHEM +GOCART scaling factors
MISR PM2.5
EPA (FRM+STN) PM2.5
Evaluate against EPA station data:
R = 0.78, Slope = 0.91
Liu et al.,2004

35. NASA AURA SATELLITE (launched July 2004)
Polar orbit; four passive instruments observing same air mass within 14 minutes
Tropospheric measurement capabilities:
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
Aura
MLS
TES nadir
OMI
HIRDLS
Direction of motion
TES limb

36. TROPOSPHERIC OZONE OBSERVED FROM SPACE
IR emission measurement from TES
UV backscatter measurement from GOME
GOME JJA 1997 tropospheric columns (Dobson Units)
Coincident CO measurements from TES
July obs
Coincidental observations of CO
and O3 with TES allows us to look at ozone production
Liu et al., 2006
Zhang et al., 2006

37. OCO will provide powerful constraints on regional carbon fluxes
OBSERVING CO2 FROM SPACE: Orbiting Carbon Observatory (OCO) to be launched in 2009
Polar-orbiting solar backscatter instrument, measures CO2 absorption at 1.61 and 2.06 mm, O2 absorption (surface pressure) at 0.76 mm: global mapping of CO2 column mixing ratio with 0.3% precision
Pressure (hPa)
Averaging kernel
(sensitivity)
OCO will provide powerful constraints on regional carbon fluxes

38. LOOKING TOWARD THE FUTURE: GEOSTATIONARY ORBIT
UV-IR sensors would provide continuous high-resolution mapping (~1 km)
on continental scale: boon for air quality monitoring and forecasting
NRC Decadal Survey Recommendation:
GEO-CAPE in , with Aura-like GACM in (also ACE for aerosols )
NRC, 2007