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)
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)
E + hν
hν
hν
(2) Electronic transitions (UV-visible)
(3) Vibrational transitions (IR)
E
(4) Rotational transitions (far IR and microwave)
 IR spectra of many molecules is a combination of (3) and (4)
Instead of discrete lines, transitions are observed in a whole wavelength region.
• natural line broadening (upper stratosphere, mesosphere)
• Doppler broadening (upper atmosphere: > 40 km)
Convolution: Voigt lines
• pressure broadening (lower atmosphere: < 40 km)
EXAMPLES OF ABSORPTION SPECTRA
Hartley
band
Chappuis band
Huggins
band
-3-
ALL TOGETHER NOW…
STRATOSPHERIC OZONE HAS BEEN MEASURED FROM
SPACE SINCE 1979
Method: UV solar backscatter
l1
Ozone layer
Scattering by
Earth surface
and atmosphere
Ozone
absorption
spectrum
l1 l2
l2
SATELLITE OBSERVATIONS REVEAL THE MECHANISM FOR
POLAR OZONE LOSS AND HELP US TRACK OZONE RECOVERY
DU
Southern
hemisphere ozone
column seen from
TOMS, October
TOMS O3
Polar ozone depletion
driven by halocarbon breakdown (source of ClO)
1 Dobson Unit (DU)
= 0.01 mm O3 STP
= 2.69x1016 molecules cm-2
MLS ClO
ATMOSPHERIC COMPOSITION RESEARCH IS NOW
MORE DIRECTED TOWARD THE TROPOSPHERE
Air quality, climate change, ecosystem issues
Mesosphere
Stratopause
Ozone
layer
Stratosphere
Tropopause
Troposphere
…but tropospheric composition measurements from space are difficult:
optical interferences from water vapor, clouds, aerosols, surface, ozone layer
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
FOUR
OBSERVATION
METHODS:
• solar backscatter
• thermal emission
• solar occultation
• lidar
SOLAR BACKSCATTER MEASUREMENTS (UV to near-IR)
Examples: TOMS, GOME, SCIAMACHY, MODIS, MISR, OMI, OCO
absorption
l1
Scattering by
Earth surface
and by atmosphere
l2
l1 l2
z
wavelength
Retrieved column in scattering atmosphere
depends on vertical profile;
need chemical transport
and radiative transfer models
concentration
Pros:
• sensitivity to lower troposphere
• small field of view (nadir)
• Daytime only
Cons: • Column only
• Interference from stratosphere
THERMAL EMISSION MEASUREMENTS (IR, mwave)
Examples: MLS, IMG, MOPITT, MIPAS, TES, HIRDLS, IASI
NADIR
VIEW
elIl(T1)
T1
LIMB VIEW
Absorbing gas
Pros:
• versatility (many species)
• small field of view (nadir)
• vertical profiling
Il(To)
To
EARTH SURFACE
Cons:
• low S/N in lower troposphere
• water vapor interferences
• cannot see through clouds
OCCULTATION MEASUREMENTS (UV to near-IR)
Examples: SAGE, POAM, GOMOS
“satellite
sunrise”
Tangent point; retrieve vertical
profile of concentrations
EARTH
• large signal/noise
Pros:
• vertical profiling
• sparse data, limited coverage
Cons: • upper troposphere only
• low horizontal resolution
LIDAR MEASUREMENTS (UV to near-IR)
Examples: LITE, GLAS, CALIPSO
Pros:
Laser
pulse
Cons:
• High vertical resolution
• Aerosols only (so far)
• Limited coverage
Intensity of return vs. time lag
measures vertical profile
backscatter by
atmosphere
EARTH SURFACE
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- ADEOS
2
Sensor
TOMS
AVHRR/ GOME
SeaWIFS
Launch
1979
1995
O3
IMG
1996
Terra
Envisat
MOPITT MODIS/ SCIAMA MIPAS AIRS
CHY
MISR
1999
1999
X
CO
X
Aqua
X
CO2
2002
2002 2002
X
X
X
X
Space
station
SAGE-3
2004
X
X
Aura
TES
OMI
2004 2004 2004
X
X
X
HIRDLS
2004
CALIPSO
IASI
2004
2007
X
X
X
X
HNO3
CH4
X
X
X
X
X
X
X
X
HCHO
X
X
X
SO2
X
X
X
BrO
X
X
X
CH3CN
X
X
X
X
X
X
2009
X
X
NO2
OCO
X
X
X
NO
aerosol
MLS
MetOp
-A
X
X
X
X
OBSERVING TROPOSPHERIC OZONE AND ITS SOURCES FROM SPACE
Nitrogen oxide radicals; NOx = NO + NO2
Sources: combustion, soils, lightning
Tropospheric
ozone
precursors
Methane
Sources: wetlands, livestock, natural gas
Nonmethane VOCs (volatile organic compounds)
Sources: vegetation, combustion
CO (carbon monoxide)
Sources: combustion, VOC oxidation
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
FIRST REMOTE MEASUREMENTS OF CO:
MAPS ABOARD THE SPACE SHUTTLE
Gas-correlation radiometer (IR: 4.7 mm): flew 4 times between 1981 and 1994
APR 1994
OCT 1994
Connors et al., 1999; Reichle et al., 1999
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!
y  F(x) + ε
Kx + ε
SOLUTION: maximum a posteriori
xˆ  (I  A)xa  Ax  Gε
1
S  K TSε -1K + Sa-1
Averaging kernel (A): describes the relative
weighting of the ‘true’ mixing ratio (x) at each level
to the retrieved value ( x)
A  I  SSa 1
Typical MOPITT
Averaging Kernel
MOPITT: FIRST SATELLITE INSTRUMENT TARGETTING
TROPOSPHERIC POLLUTION
Spring 2001
MOPITT CO Column
CO Column over the NE Pacific in Spring 2001
MOPITT – Model
Comparison indicates that emission
inventories may be inaccurate
MOPITT: solid
Model: dotted
Observations used to track transpacific
transport of pollution
Heald et al., 2004
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
Asian
pollution
U.S.
pollution
Wallace McMillan (UMBC)
Turquety et al., 2006
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
9 Tg CO
18 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
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
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
BOUNDARY
LAYER
hn (420 nm)
hn (340 nm)
HCHO OH
CO
hours
hours
NO2
NO
O3, RO2
1 day
VOC
HNO3
Emission
NITROGEN OXIDES (NOx)
Deposition
Emission
VOLATILE ORGANIC COMPOUNDS (VOC)
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)
Vertical column:
  S / AMF
Scattering by
Earth surface
and by atmosphere
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 338356 nm HCHO band
Chance et al. [2000]
AMF FORMULATION FOR A SCATTERING ATMOSPHERE

AMF = AMFG  S ( z ) w( z )dz
0
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)
AMFG = 2.08
actual AMF = 0.71
what
GOME
sees
GOME
sensitivity
w(z)
HCHO mixing ratio
profile S(z) (GEOS-Chem)
Palmer et al., 2001
GOME CONSTRAINTS ON NOx EMISSIONS
GOME
Tropospheric NO2 Columns
GEOS-CHEM model
(GEIA)
JJA 1997
r = 0.75
bias=5%
1015 molecules cm-2
Martin et al. [2003]
A priori emissions (GEIA)
Error
weighting
A posteriori emissions
Difference
HIGHER SPATIAL RESOLUTION FROM SCIAMACHY
Launched in March 2002 aboard Envisat
320x40 km2
60x30 km2
Potential for finer resolution of sources, but need to account for transport
will complicate the inversion
TROPOSPHERIC NO2 FROM OMI: CONSTRAINT ON NOx SOURCES
October 2004
K. Folkert Boersma (KNMI)
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 longterm record!
Richter et al., 2005;
Fishman et al., 2008
FORMALDEHYDE COLUMNS MEASURED BY
GOME (JULY 1996)
2.5x1016
molecules
cm-2
2
1.5
1
0.5 detection
limit
South
Atlantic
Anomaly
(disregard)
0
-0.5
High HCHO regions reflect VOC emissions from fires, biosphere, human activity
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
MAY
SEP
JUN
OCT
Abbot et al., 2003
AEROSOLS FROM SPACE
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
MULTI-SPECTRAL:
7 bands from 0.4 – 2.1 µm
MULTI-ANGLE:
9 cameras (visible)
TRANSPACIFIC TRANSPORT OF ASIAN AEROSOL
POLLUTION AS SEEN BY MODIS
Detectable sulfate pollution signal correlated with MOPITT CO
Heald et al., 2006
MISR AOD (annual mean)
MAPPING SURFACE PM2.5
USING MISR (2001 data)
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
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
HIRDLS
TES limb
OMI
TES nadir
Direction of motion
TROPOSPHERIC OZONE OBSERVED FROM SPACE
IR emission measurement from TES
UV backscatter
measurement
GOME
GOME
JJA 1997 tropospheric
columns from
(Dobson
Units)
Coincident CO measurements from TES
Coincidental observations of CO
and O3 with TES allows us to look
at ozone production
Liu et al., 2006 Zhang et al., 2006
OBSERVING CO2 FROM SPACE:
Orbiting Carbon Observatory (OCO) to be launched in 2009
Pressure (hPa)
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
Averaging kernel
(sensitivity)
OCO will provide powerful constraints on regional carbon fluxes
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 2013-2016, with Aura-like GACM in 2016-2020
(also ACE for aerosols 2013-2016)
NRC, 2007