UV-VISIBLE MEASUREMENTS

WHAT CAN WE RETRIEVE IN THE UV-VISIBLE?

Most easily retrieved (strongest features): O3 (~300 nm), NO2 (~300-500 nm), H2O(>500 nm)HCHO (250-350 nm)ClO (250-300 nm)BrO (300-380 nm)SO2 (250-320 nm)

[Gottwald et al., The SCIAMACHY Book, 2006]

DIFFERENCES BETWEEN UV-VISIBLE AND IR RETRIEVALS

1. Atmospheric Scattering must now be considered

2. There is no emission from the atmosphere no vertical information from emission at different T, therefore total columns ONLY*

3. Absorption cross-sections generally extend across larger wavelength regions, unlike strong IR features

In general, UV-visible retrievals are “physical retrievals” and do not employ optimal estimation*

*exception = tropospheric ozone

UV-VISIBLE RETRIEVALS ARE A 2-STEP* PROCESS

1. From spectra estimate a slant column(S)Two common approaches employed:A. Direct Spectral FittingB. DOAS (Differential Optical Absorption Spectroscopy)

2. Translate from slant column to vertical column ()Slant column depends on the location of the sun and the satellite and therefore the light path length is variable! Need to translate to some geophysical quantity that’s useful. But it’s not just geometry…

Slant Column

(along light path)

VerticalColumn

/S AMF

AMF=air mass factor

* There can be additional steps (corrections, considerations), depending on the species

DIRECT SPECTRAL FITTING

Example: direct fit of GOME backscattered spectrum in 338-356 nm HCHO band [HCHO]=3.0x1016 molecules/cm2

Chance et al. [2000]

REFERENCE SPECTRA

+ TEMPERATURE DEPENDENCE

MEASUREDSPECTRA

LEAST SQUARES FITTING

SLANT COLUMN OF [X]

DIFFERENTIAL OPTICAL ABSORPTION SPECTROSCOPYUse multiple wavelengths to characterize optical absorption of a species.

i i R M

C ( )Lo

L ( )C ( ) ( )

o

I( ) I ( )e

I( ) I ( )e

C=concentrationL=light-path length=absorption cross-section RRaleigh extinctionMMie extinctionIo’=intensity in absence of differential absorber

Separate slowly moving component (io) from rapidly moving component (i’):

'i i io i R M

io i R M

'i io i

L ( )C L ( )C ( ) ( )

o

L ( )C ( ) ( )'o o

'' 'o

i i

( ) ( ) ( )

I( ) I ( )e e

I ( ) I ( )e

I ( )D log L ( )C

I( )

Differential optical

density:

DOAS requires species with reasonably narrow absorption features

AIR MASS FACTORS (AMF)

Air mass factor (AMF) depends on the viewing geometry, the scattering properties of the atmosphere, and the vertical distribution of the absorber

Requires an Radiative Transfer model and a Chemical Transport

Model (CTM)

0

= ( ) ( )GAMF AMF S z w z dz

w(z): satellite sensitivity (“scattering weight”), determined from radiative transfer model including clouds and aerosolsS(z): normalized mixing ratio (“shape factor”) from CTMAMFG: geometric air mass factor (no scatter)

Palmer et al., 2001

Accounting for scattering weights is far less important for stratospheric species (eg. BrO in the stratosphere)

GEOMETRIC AIR MASS FACTOR

SZAv

SZA=solar zenith anglev=satellite viewing angle

d1 d2

dv

Slant distance = d1+d2

= dv/cos(SZA)+dv/cos(v)= dv [sec(SZA)+sec (v) ]

AMFG=d/dv = sec(SZA)+sec(v)

Values of the geometric air mass factor typically ~ 2-2.5 Vertical column is ~ ½ of slant column purely from geometry

GOME sensitivityw()

HCHO mixing ratioprofile S() (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

Account for vertical instrument sensitivity (scattering increases towards the surface, inhibits the view of the lower atmosphere) + how this is convolved with the distribution of species X

Example from GOME formaldehyde (HCHO) measurements

WHAT ARE AMFs SENSITIVE TO?

Scattering weights are most sensitive to the surface albedo and aerosol loading

Palmer et al., 2001

• Increasing A allows more solar radiation into the lower atmosphere = increasing observational sensitivity• Relatively insensitive to angle between sun & satellite (E): with larger angle physical path increase (AMFG larger) but scattering along the path reduces the sensitivity • Aerosols increase the sensitivity to HCHO in this example: increasing AOD from 0.1 to 1.0 (typical range) increases AMF by 30%

Scattering decreases the sensitivity to species AMF is typically ½ AMFG

HOW VARIABLE ARE AMFs?

If most of the species X is in the BL, where the instrument is not as sensitive, the AMF will be lower to compensate ocean AMFs are higher than over land

* We should all be grateful that journals no longer charge for on-line colour

Also see particularly low

AMFs in California due to

shallow BL

Continuing with our HCHO example…

Palmer et al., 2001

Given this variability, it is inappropriate to use single S(z) for tropospheric species

RETRIEVAL CONSIDERATIONS

1. CLOUDS: Cloud droplets scatter radiation and complicate the interpretation…Generally try to filter for < 40% cloudy conditions to ensure higher quality retrievals

2. AEROSOLS: Important sensitivity to aerosols means it’s important to include these in the scattering weight calculations.

3. ARTIFACTS: GOME solar diffuser plate bias: daily varying global bias – tricky correction!

4. STRATOSPHERIC CONTRIBUTION: For species with significant part of the column in the stratosphere, must develop a technique to remove this contribution.

4. SHAPE FACTOR: continual improvement of shape factors from model. Also shape factors may vary at spatial scales higher than represented by models…

THE ROLE OF CLOUDS

Clouds enhance sensitivity to species above clouds and reduce (obscure) sensitivity for below cloud

Early retrievals tried to limit cloud contamination by keeping Fcloud<40%

Later techniques: more sophisticated approach to separately estimate AMF for cloudy & clear scenes and combine based on cloud fraction

Advantages:(1)correct cloud effects on backscatter (2) retrieve in partly cloud scenes

Martin et al., 2002

a a c c

a c

AMF R (1 f ) AMF R fAMF

R (1 f ) R f

a=clear-skyc=cloudyR=reflectivityf=cloud fraction

VARIABILITY OF AMFs and CLOUDS

July AMFs for NO2

Clear-sky AMFs

Actual AMFs (accounting for clouds)

Martin et al., 2002

AMFc>AMFa when little NO2 is below the cloud (oceans) AMFc<AMFa when cloud obscures BL NO2 (land)

Note high spatial variability in the cloud correction

STRATOSPHERIC CONTRIBUTION MUST BE REMOVED FOR SOME SPECIES….

Significant fraction of the column is in the stratosphere

(note poleward increase)

General approach:(1)Pick low-tropospheric NO2 region Pacific

* If assume NO tropospheric contribution here might remove too much!(2)Assume the stratospheric contribution is longitudinally invariant(3)Subtract stratospheric contribution from total slant column

1978 1995 2002 2005 2007

TOMS/

Nimbus 7 (78-94) Meteor-3 (78-94) ADEOS (96) EP (96-06)

GOME/ERS-2 OMI/AURASCIMACHY/ENVISAT

Total O3 (derived tropospheric column), AI, SO2

Global coverage ~dailyUV-vis

O3, NO2, HCHO, BrO, OClO, H2O, SO2

Global coverage 3 daysUV-vis

O3, NO2, HCHO, BrO, OClO, H2O, SO2

Daily Global coverageUV-vis

O3, NO2, HCHO, BrO, OClO, H2O, SO2, CO, CH4

Global coverage 3 daysUV-vis-near IR

ATMOSPHERIC MEASUREMENTS FROM UV-VISIBLE

GOME-2/ METOP-A

TOTAL OZONE MAPPING SPECTROMETER

(1979-2006)

1 day of dataLaunched onboard several platforms, last in the series was EP-TOMS (NASA)

EP-TOMS HORIZONTAL COVERAGE: 39 km x 39 km nadir footprint with 1365 km cross-track scanningGlobal coverage ~ daily

EP-TOMS OVERPASS TIME: ~11:16 equator cross-over

TOMS INSTRUMENT1979-2006

PRODUCTS:O3 columnAerosol IndexUV radianceSO2

MEASUREMENT TECHNIQUE:Monochromator (SBUV)–6 wavelength bands (309, 313, 317, 322, 331, 360 nm)

Chappuis band

Huggins band

Harrtley band

GLOBAL OZONE MONITORING EXPERIMENT (GOME)

Launched Apr. 1995

HORIZONTAL COVERAGE: 40 km x 320 km nadir footprint with 960 km cross-track scanning (GOME-2 40 km x80 km)Global coverage in ~3 days (GOME-2 ~1day+)

OVERPASS TIME: ~10:30 equator cross-over (note only daytime in UV-visible provides useful data)

1 day of data

Launched onboard ERS-2 (ESA)

GOME INSTRUMENTLaunched Apr. 1995

PRODUCTS:O3 (column & profile)NO2

HCHOBrOOClOH2OSO2

MEASUREMENT TECHNIQUE:Scanning Spectrometer–Spectral range: 240-790 nm (covered in 4 channels) –Spectral resolution: 0.2-0.4nm

GOME-2

SCANNING IMAGING ABSORPTION SPECTROMETER FOR ATMOSPHERIC

CARTOGRAPHY(SCIAMACHY)Launched Mar 2002

HORIZONTAL COVERAGE: 30 km x 60 km nadir footprint with 1000 km cross-track scanningGlobal coverage in ~3 days

OVERPASS TIME: ~10:00 equator cross-over

1 day of dataLaunched onboard Envisat (ESA)

SCIAMACHY INSTRUMENTLaunched Mar 2002

MEASUREMENT TECHNIQUE:Imaging Spectrometer (very similar to GOME)–Spectral range: 240-314, 309-3405, 394-620, 604-805, 785-1050, 1000-1750, 1940-2040 and 2265-2380nm–Spectral resolution: 0.2-1.5nm

THREE VIEWING GEOMTRIES FOR SCIAMACHY

(1) Nadir(2) Limb(3) Solar occultation

OZONE MONITORING INSTRUMENT (OMI)

Launched July 2004

HORIZONTAL COVERAGE: 13 km x 24 km nadir footprint with 2600 km cross-track scanning Daily Global coverage

OVERPASS TIME: ~13:30 equator cross-over

1 day of dataLaunched onboard EOS-Aura (NASA)

Reduced pixel size is a big advantage! (less cloud contamination)

OMI INSTRUMENTLaunched July 2004

MEASUREMENT TECHNIQUE:Imaging spectrometer (CCD detector) –Spectral range: 270-314nm, 306-380nm, 350-500nm–Spectral resolution: 0.45-1.0nm

PRODUCTS:O3 (column & profile)NO2

HCHOBrOOClOH2OSO2

EFFECT OF SPATIAL RESOLUTION ON TROPOSPHERIC MEASUREMENTS

Even at SCIAMACHY spatial resolution details are lost

Courtesy: Andreas Richter* OMI is 13x24 km2 GOME-2 is 80x40 km2

OMI instrumentMarch 2006NASA INTEX-B aircraft mission

Boersma et al. [2008]

NO

420 nm

NO2

MAPPING OF NOx EMISSIONS FROM SPACEusing measurements of tropospheric NO2 columns

satellitevalidationspirals

NOx is mainly from fossil fuel combustion; limiting precursor for ozone formation

TROPOSPHERIC NO2 RETRIEVAL

Data analysis:1. Cloud screening2. DOAS retrieval of total slant columns3. Subtraction of clean Pacific sector to derive tropospheric slant columns 4. Application of tropospheric airmass factor to compute tropospheric vertical column

ERROR ANALYSIS FOR TROPOSPHERIC NO2

min error

[Martin et al., 2002]

HIGHER SPATIAL RESOLUTION FROM SCIAMACHY

Launched in March 2002 aboard Envisat

Potential for finer resolution of sources, but need to account for transport

320x40 km2 60x30 km2

K. Folkert Boersma (KNMI)

TROPOSPHERIC NO2 FROM OMI

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

MEASUREMENT OF NO2 SHIPPING EMISSIONS FROM SCIAMACHY

Ship emissions:• large source of

NOx, SOx and aerosols

• relevant input into marine boundary layer

• well defined NO2-patterns in Red Sea and Indian Ocean in SCIAMACHY data

• consistent with pattern of shipping emissions

Courtesy: Andreas Richter

MAPPING OF REACTIVE HYDROCARBON EMISSIONS FROM SPACE

using measurements of formaldehyde columns

Millet et al. [2008]

Biogenic isoprene is the main reactive hydrocarbon precursor of ozone…and a major source of organic particles

hydro-carbons

340 nm

formaldehyde

FITTING OF HCHO SLANT COLUMNS

FROM GOME SPECTRA

s = 1.0 ± 0.3 x1016 cm-2

s = 3.0 ± 0.4 x1016 cm-2

s = 8.4 ± 0.7 x1016 cm-2

Fitting uncertainty of4x1015 molecules cm-3

corresponds to ~ 1 ppbv HCHO in lowest 2 km

Chance et al. [2000]

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

SEASONALVARIATION 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]

A NEEDLE IN A HAYSTACK: DERIVING TROPOSPHERIC

OZONE FROM TOMS

[Fishman and Larson, 1987; Fishman et al., 2008]

Issues:• high uncertainty• seasonal averages only• does not extend to high latitudes

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

Optimal estimation (MAP) also used for retrievals of ozone profiles in the UV-vis

COMPARING SENSITIVITY OF OZONE RETRIEVALS IN IR vs UV-VIS

TOMS: SO2 MEASUREMENTS

SO2 has an absorption band in one of the TOMS channels (which can interfere with the ozone retrieval)

If concentrations are high enough SO2 can be retrieved

http://toms.umbc.edu/archives.html Courtesy: Andreas Richter

Example: volcanic eruptions such as Mount Hekla (Iceland) on Feb 27, 2000

GOME: TROPOSPHERIC BrO IN THE ANTARCTIC

Low ozone events in Arctic and Antaractic Spring are correlated with high BrO in the BL (catalytic cycle for ozone destruction = parallel to strat chemistry)

GOME BrO Sep. 7 - 9, 1996

[Wagner and Platt, 1999]

Courtesy: Andreas Richter

GOME data provided the first information of the spatial and temporal distribution of polar springtime BrO

GLYOXAL COLUMNS: WHAT DO THEY MEAN?

SCIAMACHY data suggest a large marine source of glyoxal (not seen in models)

[Wittrock et al., 2006]

SCIAMACHY RETRIEVALS OF CO First measurements of vertical column of CO with sensitivity down to the surface.Unfortunately long averaging times required to combat noisy retrievals.

[Buchwitz et al., 2007]

NEAR-IR CLIMATE GAS RETRIEVALS: SCIAMACHYRetrieve column averaged mixing ratios, denoted XCH4 and XCO2. They are computed by normalizing the measured greenhouse gas columns by the measured total airmass (number of air molecules per cm2) obtained from, e.g., simultaneously measured O2 columns.

Clearly visible are major methane source regions such as wetlands (e.g. Siberia, tropics) and rice fields (e.g. China)

Northern hemispheric carbon dioxide during March-June, where CO2 is

relatively high mainly due to release of CO2 to the atmosphere by decaying

vegetation, and July-October, where CO2 is relatively low mainly due to

uptake of atmospheric CO2 by growing vegetation

[Buchwitz et al., 2007]