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