UV-VISIBLE SpectrometersDOBSON and SAOZ

Andrea Pazmiño and Alain SarkissianLATMOS, Institut Pierre Simon Laplace, CNRS-UVSQ-UPMC, Paris, France

Outline• History• The Dobson• The SAOZ

Measurement ofminor atmospheric constituents

Beer-Lambert Law:I = I0 . e -

= -ln(I/I0)

and

= .NL + Ray + Mie

: absorption cross sectionNL : number of molecules in the line-of-sight

N

L

HistoryRegular measurements of ground ozone concentration at Montsouris Park (Albert Lévy)

1877-1907

1979 First measurements from orbit by TOMS

1985 Discovery of the ozone hole (in Antarctica)

OCTOBER

Measurements of ozone by Dobson

at Faraday, Antarctica

1988 First measurement of ozone by SAOZ (Dumont d'Urville, Antarctica) in the frame of the Montreal Protocol (1987)

1924 Dobson develop an ultraviolet spectrograph to measure the integrated ozone (column)

Earth

stratosphere

Measurement ofminor atmospheric constituents

If Nv correspond to the vertical column of constituent x:

Nvx = NLx(SZA,) / AMFx(SZA,)

AMF = Air Mass Factor

Nv

Measurement of atmospheric minor constituentsby UV-visible spectrometry

I / I0 = e -

Beer-Lambert law: = log(I0/I) = . N

Absorption cross sectionN Number of molecules in the line-of-sight

Planet

Satellite

Atmospheric layer

OrbitStar or Sun or Moon

2

1

23

Ground-Based

Spectrometers

Dispersive object

Photodetector

Entrance slit

Output slit

Passive remote sensing and not destructive methodDecomposition of light radiation following → analysis of the spectral distributionCharacterized by its resolution (ability to separate two very close wavelengths)

►

►

►

Dobson and SAOZ spectrometers

DOAS technique (relative) O3, NO2, OClO, PSC Zenith-sky measurements at twilight Measurements in the Visible (Chappuis band)

• Measurements in polar regions in winter • Measurements in all weather conditions• Completely automatic• 20 instruments• >20 years of measurements in Antarctica

Differential absorption (absolute)

O3

Direct Sun measurements Measurements in the Ultraviolet (Huggins band)

• No measurements during polar winter• No measurements in bad weather• Automatic and/or manual• More than 80 instruments• Long series (80 years in Arosa)

Dobson spectrometer

Dobson Measurements

I / I0 = e -

Generally, direct Sun measurements

→ AMF = 1 / cos (SZA)

The Dobson spectrometer uses 2 pairs of wavelengths

= . NL → = . NL

where

= diff. Mie scattering contribution + ( . NL ) O3

O3

Pairs of wavelengths used by Dobson spectrometer

Pair (nm)A 305.5 – 325.4B 308.8 – 329.1C 311.4 – 332.4D 317.6 – 339.8C' 332.4 – 453.6

• Double pairs AD: most reliable for direct Sun measurements → standard

• Double pairs CD: large optical path

O3 = f(T)

Dobson networkFioletov et al., 2008

Dobson measurements: trends

Dobson

Arosa, Switzerland since 1926

Year

Ozo

ne

[DU

]

Conclusion: Dobson► ground-based measurements► direct Sun measurements (generally), or at zenith-sky► measurements in the UV (Huggins band for O3)► uses pairs of wavelengths► differential absorption► simple geometry (AMF = 1/cos(SZA) for direct sun

measurements)==> ++ and --++ good accuracy (< 1-2 %)++ systematic comparison between instruments (calibration)++ 80 instruments++ very long series

-- not in winter at polar regions-- not when bad weather-- needs operator

-- needs correction of absorption cross section of ozone in the UV from temperature

SAOZ spectrometerSystème d’Analyse par Observation Zenithale

Outside Inside

SAOZ Interface

PC

SAOZ

SAOZ spectrometer

ARGOS

shutter, grating, detector

Electronic device

GPS antenna

• Zenith-sky viewing• Twilight geometry• High sensitivity

to the stratosphere (~200 km)

Vertical column calculation:

0SCD RVAMF

O3

SAOZ spectrometer

Slant column Residual amount in reference spectrum

Air mass factor

Comparison of spectrum to the reference spectrum re-adjust the spectrum in wavelength

I/Io

I=Io * e –

Beer-Lambert Law

Signal(I)

Reference (Io)

45°

Flux

(x10

3 arb

itrar

y un

its)

Wavelength (nm)

SAOZ spectrometer Spectral Analysis

Application of high pass filter

NL

= -ln(I/Io)

NO2O3

To suppress attenuations of lower frequency

(Rayleigh and Mie)1)

Least mean square correlation => NL (measured slant column)

2)

SAOZ spectrometer Spectral Analysis

Optical Thickness

Absorption cross-section

O4 H2O

SAOZ spectrometer Differential Spectral Analysis

NO2O3

0.0200.0150.0100.005

0-0.005-0.010-0.015-0.020-0.025-0.030

residual – NL < 1/1000

Residual Differential Spectrum

ii NLi

NLO3= O3 /O3

200

150

100

50

0

Sla

nt c

olum

n (x

1018

mol

/cm

2)

9080706050

SZA

O3

86°<SZA<91°

SAOZ spectrometer

400

380

360

340

320

300

280

260

Ver

tical

col

umn

(Dob

son)

252015105

AMF

O3 320 DU

86°<SZA<91°

Col

onne

Ver

tical

e (D

U)

Col

onne

obl

ique

(x10

18 m

ol./c

m2 )

Standards SAOZ data : daily vertical columns of O3 and NO2 at twilight

VO3= NLO3+R0/AMFO3

SAOZ networkNy-Alesund

SalekhardZhiganskSodankyla

HarestuaAberystwyth Jungfrau

OHP

Tarawa

Bauru

Rio Gallegos

Reunion

Kerguelen

Rothera

ConcordiaDumont d’Urville

Scoresbysund

Thule

Long term series at Dumont d’UrvilleSAOZ

Ozon

e (D

U)NO

2 (x1

015

mol

/cm

2 )

TOMS : -0,6% SCIA : -0,3% GOME : 0,5% OMI (TOMS) : -1.3% OMI (DOAS) : 0%

Differences

Monthly Mean

Col

onne

ver

tical

e d’

ozon

e (D

U)

(Sat

ellit

e-S

AO

Z)/S

AO

Z (%

)

Comparison with satellites at OHP

SAOZ

Ozone loss: methodology

Comparison of passive O3 of models with SAOZ measurements O3 : accumulated ozone loss2 models : REPROBUS & SLIMCAT

SAOZZhigansk

(CNRS/CAO)

Salekhard(CNRS/CAO)

Sodankyla(CNRS/FMI)

Harestua(BIRA)

NyAlesund(NILU)

Thule (DMI)

ScoresbySund(CNRS/DMI)

Eureka(CNRS/UoT)

Zhigansk(CNRS/CAO)

Salekhard(CNRS/CAO)

Sodankyla(CNRS/FMI)

Harestua(BIRA)

NyAlesund(NILU)

Thule (DMI)

ScoresbySund(CNRS/DMI)

Zhigansk(CNRS/CAO)

Salekhard(CNRS/CAO)

Sodankyla(CNRS/FMI)

Harestua(BIRA)

NyAlesund(NILU)

Thule (DMI)

ScoresbySund(CNRS/DMI)

Zhigansk(CNRS/CAO)

Salekhard(CNRS/CAO)

Sodankyla(CNRS/FMI)

Harestua(BIRA)

NyAlesund(NILU)

Thule (DMI)

ScoresbySund(CNRS/DMI)

Eureka(CNRS/UoT)

Sensibility of O3 loss to history of stratospheric temperature=> high annual variability

Warm winters: 5-13% or 25-60 DU (eg. 1998/99, ) Cold winters: 20-30% or 90-140 DU (eg. 2007/08, )

Arctic ozone loss

SAOZ

- ground-based measurements- zenith-sky measurements at twilight- ozone measured in the visible Chappuis band- differential absorption- complicated geometry (AMF = f(SZA,))

++ good accuracy (<3% for ozone)++ >20 years series in polar regions++ measurements in polar regions in winter when ozone hole forms++ completely automatic++ measurements in all weather conditions++ simultaneously measurements of NO2, OClO, BrO and PSC detection

-- needs radiative transfer calculation (AMF)-- sensible to vertical profiles that generates seasonal variation

SAOZ data of O3 & NO2 in real time (current year) & consolidate (already validated by the Principal Scientist and sent to NDACC) :

- http://saoz.obs.uvsq.fr/SAOZ-RT.html

- http://saoz.obs.uvsq.fr/SAOZ_consol_v2.html

Conclusions: SAOZ