1. UV-VISIBLE Spectrometers
DOBSON and SAOZ
Andrea Pazmiño and Alain Sarkissian
LATMOS, Institut Pierre Simon Laplace, CNRS-UVSQ-UPMC, Paris, France
Outline
History
The Dobson
The SAOZ

2. Measurement of minor atmospheric constituents
Beer-Lambert Law: I = I0 . e -
 = -ln(I/I0)
and
 = .NL + Ray + Mie
 : absorption cross section
NL : number of molecules in the line-of-sight N L

3. Measurements of ozone by Dobson
History
Regular measurements of ground ozone concentration at Montsouris Park (Albert Lévy)
1924
Dobson develop an ultraviolet spectrograph to measure the integrated ozone (column)
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)

4. minor atmospheric constituents Nvx = NLx(SZA,) / AMFx(SZA,)
Measurement of
minor atmospheric constituents Nv
Earth
stratosphere
If Nv correspond to the vertical column of constituent x:
Nvx = NLx(SZA,) / AMFx(SZA,)
AMF = Air Mass Factor

5. Measurement of atmospheric minor constituents
by UV-visible spectrometry
I / I0 = e -
Beer-Lambert law:  = log(I0/I)
 =  . N
 Absorption cross section
N Number of molecules in the line-of-sight 2
Satellite
Orbit
Star or Sun or Moon 3 2
Planet
Ground-Based
Atmospheric layer 1

6. Spectrometers Photodetector Dispersive object Entrance slit
Output slit ►
Passive remote sensing and not destructive method
Decomposition of light radiation following  → analysis of the spectral distribution
Characterized by its resolution
(ability to separate two very close wavelengths) ► ►

7. Dobson and SAOZ spectrometers
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)
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

8. Dobson spectrometer

9. Dobson Measurements O3 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

10. Pairs of wavelengths used by Dobson spectrometer
= f(T)
Pair
 (nm) A
305.5 – 325.4 B
308.8 – 329.1 C
311.4 – 332.4 D
317.6 – 339.8 C'
332.4 – 453.6
Double pairs AD: most reliable for direct Sun measurements → standard
Double pairs CD: large optical path

11. Dobson network
Fioletov et al., 2008

12. Dobson measurements: trends
Arosa, Switzerland since 1926
Dobson
Ozone [DU]
Year

13. 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

14. Système d’Analyse par Observation Zenithale
SAOZ spectrometer
Système d’Analyse par Observation Zenithale

15. SAOZ spectrometer SAOZ SAOZ Interface PC ARGOS Outside Inside
shutter, grating, detector
Electronic device
GPS antenna
ARGOS
Outside
Inside

16. SAOZ spectrometer O3 Zenith-sky viewing Twilight geometry
High sensitivity
to the stratosphere (~200 km)
Vertical column calculation:
Slant column
Residual amount in reference spectrum
Air mass factor

17. SAOZ spectrometer Spectral Analysis
Comparison of spectrum to the reference spectrum
 re-adjust the spectrum in wavelength
 I/Io
Reference (Io)
45°
I=Io * e –t
Beer-Lambert Law
Flux (x103 arbitrary units)
Signal
(I)
Wavelength (nm)

18. SAOZ spectrometer Spectral Analysis
Optical Thickness
Absorption cross-section
NO2 O3 t
= -ln(I/Io) s 1)
Application of high pass filter
Dt = Ds NL
To suppress attenuations of lower frequency
(Rayleigh and Mie) 2)
Least mean square correlation => NL (measured slant column)

19. SAOZ spectrometer Differential Spectral Analysis
NO2 O4
H2O
Dti
Dsi NLi
Residual Differential Spectrum
0.020
0.015
0.010
0.005
-0.005
-0.010
-0.015
-0.020
-0.025
-0.030 Dt
residual Dt = Dt – S Ds NL
< 1/1000

20. Standards SAOZ data : daily vertical columns of O3 and NO2 at twilight
SAOZ spectrometer
NLO3= DtO3 /DsO3
VO3= NLO3+R0/AMFO3
320 DU
Colonne Verticale (DU)
Colonne oblique (x1018 mol./cm2)
86°<SZA<91°
86°<SZA<91°
Standards SAOZ data : daily vertical columns of O3 and NO2 at twilight

21. SAOZ network Thule Ny-Alesund Sodankyla Zhigansk Scoresbysund
Salekhard
Zhigansk
Sodankyla
Harestua
Aberystwyth
Jungfrau
OHP
Tarawa
Bauru
Rio Gallegos
Reunion
Kerguelen
Rothera
Concordia
Dumont d’Urville
Scoresbysund
Thule

22. Long term series at Dumont d’Urville
SAOZ
Long term series at Dumont d’Urville
Ozone (DU)
NO2 (x1015 mol/cm2)

23. Comparison with satellites at OHP
SAOZ
Comparison with satellites at OHP
TOMS : -0,6% SCIA : -0,3% GOME : 0,5% OMI (TOMS) : -1.3% OMI (DOAS) : 0%
Differences
Monthly Mean
Colonne verticale d’ozone (DU)
(Satellite-SAOZ)/SAOZ (%)

24. Ozone loss: methodology
SAOZ
Ozone loss: methodology
Comparison of passive O3 of models with SAOZ measurements
O3 : accumulated ozone loss
2 models : REPROBUS & SLIMCAT

25. SAOZ
Arctic ozone loss
Sensibility of O3 loss to history of stratospheric temperature
=> high annual variability
Warm winters: 5-13% or DU (eg. 1998/99, )
Cold winters: 20-30% or DU (eg. 2007/08, )

26. Conclusions: 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) : - -