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MAXMAX--DOAS Measurements of DOAS Measurements of Trace Gas and AerosolTrace Gas and Aerosol
Vertical ProfilesVertical Profiles
Udo FrieUdo Frie ßßInstitute of Environmental PhysicsInstitute of Environmental Physics
University of HeidelbergUniversity of HeidelbergGermanyGermany
MAXMAX--DOAS trace gas and aerosol profilingDOAS trace gas and aerosol profiling
OutlineOutline
• MAX-DOAS: The idea
• Retrieval techniques
• MAX-DOAS instrumentation
• Retrieval of trace gas vertical profiles
• Retrieval of aerosol vertical profiles
• Summary
MAXMAX--DOAS trace gas and aerosol profilingDOAS trace gas and aerosol profiling
MAXMAX--DOAS Instrumentation:DOAS Instrumentation:TelescopeTelescope
• 2D scanner (elevation and azimuth) using fused silica prisms allowing to collect light from any direction in the sky
• Brushless servo motors with position encoder and transmission enabling high accuracy in focusing
• Achromatic optics with enhanced aluminium coating
• No polarisation sensitivity due to fibre optics
• Diffusor plate for direct sun measurements
• Integrated calibration lamps
MAXMAX--DOAS trace gas and aerosol profilingDOAS trace gas and aerosol profiling
MAXMAX--DOAS Instrumentation:DOAS Instrumentation:Spectrometer UnitSpectrometer Unit
• Three temperature stabilised spectrographs covering the full UV/Vis wavelength range with high spectral resolution
• Embedded PC allowing fully autonomous measurements and remote control over TCP/IP
• Indoor operation under stable conditions
390 - 425 nm 400 - 607 nm 600 - 789 nm
MAXMAX--DOAS trace gas and aerosol profilingDOAS trace gas and aerosol profiling
Zenith-sky measurements:Sensitivity strongly weighted towards the stratosphere
MultiMulti --Axis DOASAxis DOAS
Multi-Axis measurements:
Increasing light path through the troposphere with decreasing elevation angle
Spectrograph
Zenith
Sun
Stratosphere
TroposphereSpectrograph
Zenith
Sun
Stratosphere
Troposphere
45°
20°
10°5°2°
DOAS (Differential Optical Absorption Spectroscopy) of scattered sunlight yields the integrated concentration of trace gases along the atmospheric light path
Trace gas and aerosol vertical profiles can be retrieved using inverse methods (i.e., optimal estimation).
MAXMAX--DOAS trace gas and aerosol profilingDOAS trace gas and aerosol profiling
Trace Gases Trace Gases detectable by detectable by
UV/Vis scattered UV/Vis scattered light DOASlight DOAS
300 400 500 600 700 800
Br2
ClO
OBrO
H2O
HONO
(CHO)2
HCHO
O2
O4
OClO
OIOI2
IOBrO
NO3
NO2SO
2
O3 Vis
Wavelength [nm]
O3 UV
(log)
MAXMAX--DOAS trace gas and aerosol profilingDOAS trace gas and aerosol profiling
NONO22 Measurements at Measurements at HohenpeiHohenpei ßßenberg, Germanyenberg, Germany
Diurnal variation Diurnal variation –– 9.5.20089.5.2008
Comparison with in situ measurements
MAXMAX--DOAS trace gas and aerosol profilingDOAS trace gas and aerosol profiling
NONO22 Measurements at Measurements at HohenpeiHohenpei ßßenberg, Germanyenberg, Germany
Diurnal variation Diurnal variation –– 27.5.0827.5.08
Comparison with in situ measurements
Wind direction
MAXMAX--DOAS trace gas and aerosol profilingDOAS trace gas and aerosol profiling
Retrieval of NORetrieval of NO 22 vertical profilesvertical profilesCabauw Intercomparison Campaign, June 2009Cabauw Intercomparison Campaign, June 2009
5 6 7 8 9 10 11 12 13 14 15 16 17 18 190
1
2
3
4
NO2 Profiles - 24.06.2009 - Vis
Time [UT]
Alti
tude
[km
]
0.0
1.0
2.0
3.0
4.0
5.0
6.0
NO
2 mix
ing
ratio
[ppb
]
MAXMAX--DOAS trace gas and aerosol profilingDOAS trace gas and aerosol profiling
NONO22 surface mixing ratiosurface mixing ratioIn situ versus MAXIn situ versus MAX --DOASDOAS
Cabauw Intercomparison CampaignCabauw Intercomparison Campaign
17.6 18.6 19.6 20.6 21.6 22.6 23.6 24.6 25.6
0
5
10
15
20
25
30
Date 2009
EMPA in situ MAX-DOAS
NO
2 sur
face
mix
ing
ratio
[ppb
]
MAXMAX--DOAS trace gas and aerosol profilingDOAS trace gas and aerosol profiling
6 8 10 12 14 16 180.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
HCHO Profiles - 23.06.2009
Time [UT]
Alti
tude
[km
]
0.0
0.5
1.0
1.5
2.0
2.5
HC
HO
mix
ing
ratio
[ppb
]
Retrieval of formaldehyde vertical profilesRetrieval of formaldehyde vertical profilesCabauw Intercomparison Campaign, June 2009Cabauw Intercomparison Campaign, June 2009
MAXMAX--DOAS trace gas and aerosol profilingDOAS trace gas and aerosol profiling
Aerosol retrievalAerosol retrievalusing Ousing O 44 absorptionabsorption
• MAX-DOAS measurements of a trace gas with a known vertical profile:� Contain information on the light path
through the atmosphere
� Allow to gain information on atmospheric aerosols
• Most suitable trace gas for aerosol retrieval in the UV/Vis is the oxygen collision complex O4:� Numerous absorption bands, easy
to detect with DOAS
� O4 concentration proportional to the square of the O2 concentration
� Scale height of O4 profile: ~4km
350 400 450 500 550 600 6500
2
4
6
8
10
O4
abso
rptio
n cr
oss
sect
ion
[arb
. uni
ts]
Wavelength [nm]
Absorption cross section of O4
MAXMAX--DOAS trace gas and aerosol profilingDOAS trace gas and aerosol profiling
Sensitivity to observation parametersSensitivity to observation parameters
• Elevation α: The atmospheric light path and thus the optical depth of O4 generally increase with decreasing elevation
� Information on aerosol extinction profile
• Wavelength λ: The visibility (average scattering distance along line of sight) and thus the O4 optical depth generally decreases with decreasing wavelength.
� Information on wavelength dependence of aerosol extinction (Angstrom coefficient)
� Further information on aerosol extinction profile
• Relative azimuth β: Scanning in different azimuth directions yields O4 optical depth as a function of scattering angle
� Information on angular dependence of scattering (phase function and single scattering albedo)
MAXMAX--DOAS trace gas and aerosol profilingDOAS trace gas and aerosol profiling
Sensitivity to observation parametersSensitivity to observation parameters
• Elevation α: The atmospheric light path and thus the optical depth of O4 generally increase with decreasing elevation
� Information on aerosol extinction profile
• Wavelength λ: The visibility (average scattering distance along line of sight) and thus the O4 optical depth generally decreases with decreasing wavelength.
� Information on wavelength dependence of aerosol extinction (Angstrom coefficient)
� Further information on aerosol extinction profile
• Relative azimuth β: Scanning in different azimuth directions yields O4 optical depth as a function of scattering angle
� Information on angular dependence of scattering (phase function and single scattering albedo)
MAXMAX--DOAS trace gas and aerosol profilingDOAS trace gas and aerosol profiling
Sensitivity to observation parametersSensitivity to observation parameters
• Elevation α: The atmospheric light path and thus the optical depth of O4 generally increase with decreasing elevation
� Information on aerosol extinction profile
• Wavelength λ: The visibility (average scattering distance along line of sight) and thus the O4 optical depth generally decreases with decreasing wavelength.
� Information on wavelength dependence of aerosol extinction (Angstrom coefficient)
� Further information on aerosol extinction profile
• Relative azimuth β: Scanning in different azimuth directions yields O4 optical depth as a function of scattering angle
� Information on angular dependence of scattering (phase function and single scattering albedo)
MAXMAX--DOAS trace gas and aerosol profilingDOAS trace gas and aerosol profiling
Comparisons with RamanComparisons with Raman LidarLidar
Raman lidar:
� diurnal variation of the range corrected signal
MAX-DOAS:
� diurnal variation of the aerosol extinction profile
Intercomparison measurements in Cabauw, May 2008
Lidar data courtesy of A. Apitouley, RIVM
MAXMAX--DOAS trace gas and aerosol profilingDOAS trace gas and aerosol profiling
Raman lidar:
� diurnal variation of the range corrected signal
MAX-DOAS:
� diurnal variation of the aerosol extinction profile
Comparisons with RamanComparisons with Raman LidarLidar
Intercomparison measurements in Cabauw, May 2008
Lidar data courtesy of A. Apitouley, RIVM
MAXMAX--DOAS trace gas and aerosol profilingDOAS trace gas and aerosol profiling
6. 6. ComparisonsComparisons : Sun Photometer: Sun Photometer
0.00
0.04
0.08
0.12
0.16
0.20
0.24
0.28
0.32
08:00 09:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 08:00 09:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00
0.00
0.04
0.08
0.12
0.16
0.20
0.24
0.28
0.32
08:00 09:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:000.00
0.04
0.08
0.12
0.16
0.20
0.24
0.28
0.32
08:00 09:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:000.00
0.04
0.08
0.12
0.16
0.20
0.24
0.28
0.32
10.05.200809.05.2008
08.05.2008
07.05.2008
Time (UTC)
Time (UTC)
AO
D at 550 nm
Sun Photometer DOAS
AO
D a
t 550
nm
Intercomparisonmeasurements in Cabauw, May 2008
Sun photometer data courtesy of B. Henzing, TNO
MAXMAX--DOAS trace gas and aerosol profilingDOAS trace gas and aerosol profiling
6. 6. ComparisonsComparisons : : RamanRaman LidarLidar
Intercomparisonmeasurements in Melpitz, June2008
Lidar data courtesy of D. Althausen, IFT
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
0.00 0.02 0.04 0.06 0.08 0.10 0.12
09:30-10:00 UTC of 10.06.2008
Aerosol extinction (km-1)
Alti
tude
(km
)
a priori MAX-DOAS retrieved MAX-DOAS retrieved Raman lidar
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
0.00 0.02 0.04 0.06 0.08 0.10 0.12
10:30-11:00 UTC of 10.06.2008
Aerosol extinction (km-1)
Alti
tude
(km
)
a priori MAX-DOAS retrieved MAX-DOAS retrieved Raman lidar
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
0.00 0.02 0.04 0.06 0.08 0.10 0.12
12:30-13:00 UTC of 10.06.2008
Aerosol extinction (km-1)
Alti
tude
(km
)
a priori MAX-DOAS retrieved MAX-DOAS retrieved Raman lidar
MAXMAX--DOAS trace gas and aerosol profilingDOAS trace gas and aerosol profiling
SummarySummary
• Multi-Axis DOAS measurements allow for retrieving trace gas and aerosol vertical profiles and optical properties
• Typical vertical resolution of 50 – 200 m resolve the structure of the boundary layer
• Measurements can be performed with simple and cost effective fully automated instrumentation
• Inherently self- calibrating
• Simultaneous measurement of aerosols and numerous trace gases in the entire UV/Vis range
• MAX-DOAS is an important tool for satellite validation
• Potential for integration in world wide remote sensing networks
MAXMAX--DOAS trace gas and aerosol profilingDOAS trace gas and aerosol profiling
Parameters Affecting Parameters Affecting DOAS MeasurementsDOAS Measurements
• Lambert-Beer law:• Light path through the atmosphere and trace gas
absorption are determined by:– Viewing geometry (SZA, elevation, azimuth)– Wavelength (dependency of light path and extinction on λ)– Aerosol extinction– Trace gas profile– ...
• DOAS measurements contain (indirect) information on the atmospheric state (e.g., trace gas an aerosol profile)
• Established method for the retrieval of atmospheric parameters: Optimal Estimation
( )∫ ⋅++⋅−⋅= dssksks mreII )()()()(0 )()( ρλσλλ
MAXMAX--DOAS trace gas and aerosol profilingDOAS trace gas and aerosol profiling
MAXMAX--DOAS: The IdeaDOAS: The Idea• Spectrally resolved observations of
scattered sunlight in the UV/Vis along different lines of sight
• Detection of various trace gases (NO2, BrO, HCHO, …) by identifying their individual absorption features
• Analysis of spectra based on the Lambert- Beer law:
I0(λ), I(λ): incident and transmitted intensityσ(λ): absorption cross sectionρ(s): trace gas concentrationkr(s), km(s): Rayleigh and Mie extinction coefficients
• Basic quantity measured by DOAS is the slant column density (SCD) of an absorber, i.e. the integrated concentration along the light path:
• Problem: “Length” of light path is difficult to determine, requires radiative transfer modelling
Zenith
Stratosphere
Boundary layer
α=45°
α=20°
α=10° α=5° α=2°
Instrument
Sun
Θ
∫ ⋅= dssS )(ρ
( )∫ ⋅++⋅−⋅= dssksks mreII )()()()(0 )()( ρλσλλ
MAXMAX--DOAS trace gas and aerosol profilingDOAS trace gas and aerosol profiling
Remote sensing of atmospheric trace gases: Remote sensing of atmospheric trace gases: DDifferential ifferential OOptical ptical AAbsorption bsorption SSpectroscopy pectroscopy
(DOAS)(DOAS)When sampling the light intensity on a discrete wavelength grid λk (and neglecting the pressure and temperature dependence of the absorption cross section), the Lambert- Beer law can be solved numerically by minimising
∑ ∑ ∑
⋅+⋅+−≡k i n
nnikikk k
cSII λλσλλχ )()(ln)(ln 02
to determine the integrated concentrations along the light path (Slant Column Density, SCD):
∫ ⋅≡L
ii dssS0
)(ρ
The polynomial Σcnλkn removes the broad-banded
spectral structure caused by Rayleigh- and Mie-scattering. Thus only compounds with high frequent absorption features can be detected. The high frequent parts of σ and τ are referred to as the differential absorption cross section and optical density σ’ and τ’.
350 355 360 365 370 375 380 385
-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0
σ '( λ ) = σ ( λ ) - σ b ( λ )
B σ ' [1
0 -1
9 c
m 2 ]
λ [nm]
4.5
5.0
5.5
6.0
σ b ( λ )
σ ( λ ) A
σ [1
0 -1
9 c
m 2 ]
The Optical Density is defined as
−=⋅≡
)(
)(ln)()(
0 λλλσλτ
I
IS
MAXMAX--DOAS trace gas and aerosol profilingDOAS trace gas and aerosol profiling
Retrieval of trace gas and aerosol vertical profile s:Retrieval of trace gas and aerosol vertical profile s:Inverse ModellingInverse Modelling
Atmospheric statex
Forward model F(Radiative transfer model)
Simulated Measurement y = F(x)
Measurement y
Inverse model (based on F)
Estimate for atmospheric statex
Measurement error Sε
Error of state vectorS
Forward modelling
Inverse modelling
A priori state vectorxa, Sa
Optimal Estimation[Rodgers, 1990]
The Maximum A Posteriori (MAP) solution is determined iterativelyby minimising
[ ] [ ] [ ] [ ]444 3444 214444 34444 21
priori a from vector state ofDeviation
tmeasuremen actual from modelled ofDeviation
ˆˆ)ˆ()ˆ( 112aa
Ta
T xxSxxxFySxFy −−+−−= −−εχ
x
MAXMAX--DOAS trace gas and aerosol profilingDOAS trace gas and aerosol profiling
Retrieval of trace gas vertical profilesRetrieval of trace gas vertical profiles
Atmospheric statex
Simulated Measurement y = F(x)
Measurement y
Inverse model (based on F)
Estimate for atmospheric statex
Measurement error Sε
Error of state vectorS
Forward modelling
Inverse modelling
A priori state vectorxa, Sa
Forward model F(Radiative transfer model)
The measurement vector
Trace gas SCDsat differentelevation angles α
=)(
)( 1
mS
S
y
α
αM
The state vector
Trace gas verticalprofile
=)(
)( 1
nz
z
x
ρ
ρM
MAXMAX--DOAS trace gas and aerosol profilingDOAS trace gas and aerosol profiling
Retrieval of aerosol vertical profilesRetrieval of aerosol vertical profiles
Atmospheric statex
Forward model F(SCIATRAN)
Simulated Measurement y = F(x)
Measurement y
Inverse model (based on F)
Estimate for atmospheric statex
Measurement error Sε
Error of state vectorS
Forward modelling
Inverse modelling
A priori state vectorxa, Sa
The measurement vector
O4 optical depth
Relative intensity
=
)(),(
)(),(
),(
),(
0
1011
4
114
mmm
mmO
O
II
IIy
λαλ
λαλαλτ
αλτ
M
M
The state vector
Aerosol extinctionprofile
Aerosol opticalproperties- phase function- single scattering albedo- size distribution
=
r
n
q
q
zk
zk
x
M
M
1
1
)(
)(
MAXMAX--DOAS trace gas and aerosol profilingDOAS trace gas and aerosol profiling
MAXMAX--DOAS Instrumentation DOAS Instrumentation for longfor long --term Measurements:term Measurements:
RequirementsRequirements
• Large wavelength range to cover many trace gases
• Sufficient spectral resolution (0.5 – 1.5 nm)
• High mechanical stability of spectrograph unit � Indoor operation, temperature stabilisation
• High detector sensitivity to achieve low detection limit
• Flexible telescope unit to observe light from any direction in the sky
• Direct sun- and moonlight capability
• Fully autonomous operation
• Self-calibration capabilities
MAXMAX--DOAS trace gas and aerosol profilingDOAS trace gas and aerosol profiling
Newly developedNewly developedMAXMAX--DOAS InstrumentationDOAS Instrumentation
Spectrometer unit
Telescope unit
MAXMAX--DOAS trace gas and aerosol profilingDOAS trace gas and aerosol profiling
NONO22 Measurements at Measurements at HohenpeiHohenpei ßßenberg, Germanyenberg, Germany
Hohenpeißenberg
MAXMAX--DOAS trace gas and aerosol profilingDOAS trace gas and aerosol profiling
NONO22 Measurements at Measurements at HohenpeiHohenpei ßßenberg, Germanyenberg, Germany
Measured vs. modelled NOMeasured vs. modelled NO 22 SCDs SCDs –– 9.5.089.5.08
MAXMAX--DOAS trace gas and aerosol profilingDOAS trace gas and aerosol profiling
CCabauwabauw IIntercomparisonntercomparison Campaign of Campaign of NNitrogen itrogen DDioxide measuring ioxide measuring IInstrumentsnstruments
CINDICINDI8.6. 8.6. –– 8.7.20098.7.2009
MAXMAX--DOAS trace gas and aerosol profilingDOAS trace gas and aerosol profiling
Comparison of NOComparison of NO 22 ProfilesProfilesduring CINDIduring CINDI
MAXMAX--DOAS trace gas and aerosol profilingDOAS trace gas and aerosol profiling
NONO22 profiling profiling –– Averaging KernelsAveraging Kernels
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
-0.2 0.0 0.2 0.4 0.6 0.8 1.0Averaging Kernel
Alti
tude
[km
]Altitude [km]
0.00.20.40.60.81.01.21.41.61.8
MAXMAX--DOAS trace gas and aerosol profilingDOAS trace gas and aerosol profiling
Aerosol retrieval:Aerosol retrieval:Comparison of measured and modelled Comparison of measured and modelled
OO44 dSCD and intensitydSCD and intensity
0.00
0.01
0.02
08 10 12 14 16
measured retrieved
08 10 12 14 16 18
0.2
0.4
0.6
0.8
intensity, 577 nm
intensity, 477 nm
3.000 6.000 11.00 21.00 90.00
intensity, 360 nm
0.00
0.02
0.04
O4 optical depth, 577 nm
O4 optical depth, 477 nm
0.2
0.4
0.6
0.8
O4 optical depth, 360 nm
Elevation2° 5° 10° 20° 90°
08 10 12 14 160.00
0.04
0.08
Time (UTC) of 07.05.2008
Time (UTC) of 07.05.2008
Intensity (a.u.)
O4 o
ptic
al d
ensi
ty
08 10 12 14 16 18
0.2
0.4
0.6
0.8
Comparison of measured and
retrieved O4 optical depths and intensities,
for the 07.05.2008 in Cabauw
MAXMAX--DOAS trace gas and aerosol profilingDOAS trace gas and aerosol profiling
Aerosol Retrieval ResultsAerosol Retrieval Results
Single retrieved aerosol extinction
profiles and corresponding
averaging kernels,
for the 07.05.2008 in Cabauw
0
1
2
3
40.0 0.1 0.2
retrieved a priori
07:00 UTC
Alti
tude
(km
)
Aerosolextinction (km-1)0.0 0.1 0.2
11:00 UTC09:00 UTC
0.0 0.1 0.2
0.0 0.1 0.2
13:00 UTC
0.0 0.1 0.2
15:00 UTC
0
1
2
3
40.0 0.1 0.2
17:00 UTC
0
1
2
3
0.0 0.5
Averaging Kernel
Altitude (km
)
Altitude (km) 0.1 0.3 0.5 0.7 0.9 1.1 1.3 1.5 1.7 1.9 2.1 2.3 2.5 2.7 2.9 3.1 3.3 3.5 3.7 3.9
0.0 0.5 0.0 0.5 0.0 0.5 0.0 0.5 0.0 0.50
1
2
3
MAXMAX--DOAS trace gas and aerosol profilingDOAS trace gas and aerosol profiling
6. 6. ComparisonsComparisons : Sun Photometer: Sun Photometer
0.10 0.15 0.20 0.25 0.30
0.10
0.15
0.20
0.25
0.30
0.35
15%
30%
1:1
15%
30%
45%
07.05. 08.05. 09.05. 10.05.
AO
D fr
om D
OA
S
AOD from Sun Photometer
45%
Aerosol optical depth in May 2008
� AOD from MAX-DOAS
measurements are
underestimated by ~15%
compared to Sun Photometer
values
Intercomparisonmeasurements
in Cabauw, May 2008