max -doas measurements of trace gas and aerosol … · trace gas and aerosol vertical profiles ......

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 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 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


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).


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)


NONO22 Measurements at Measurements at HohenpeiHohenpei ßßenberg, Germanyenberg, Germany

Diurnal variation Diurnal variation –– 9.5.20089.5.2008

Comparison with in situ measurements



Diurnal variation Diurnal variation –– 27.5.0827.5.08

Comparison with in situ measurements

Wind direction


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

]


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

]


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


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


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)


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


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


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


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


Alti

tude

(km

)


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


Alti

tude

(km

)



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


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: 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


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


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


Retrieval of trace gas vertical profilesRetrieval of trace gas vertical profiles

Atmospheric statex


Measurement y





Forward modelling

Inverse modelling


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


Retrieval of aerosol vertical profilesRetrieval of aerosol vertical profiles

Atmospheric statex

Forward model F(SCIATRAN)


Measurement y





Forward modelling

Inverse modelling


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 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


Newly developedNewly developedMAXMAX--DOAS InstrumentationDOAS Instrumentation

Spectrometer unit

Telescope unit



Hohenpeißenberg



Measured vs. modelled NOMeasured vs. modelled NO 22 SCDs SCDs –– 9.5.089.5.08


CCabauwabauw IIntercomparisonntercomparison Campaign of Campaign of NNitrogen itrogen DDioxide measuring ioxide measuring IInstrumentsnstruments

CINDICINDI8.6. 8.6. –– 8.7.20098.7.2009


Comparison of NOComparison of NO 22 ProfilesProfilesduring CINDIduring CINDI


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


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


0.2

0.4

0.6

0.8


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


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


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

max -doas measurements of trace gas and aerosol … · trace gas and aerosol vertical profiles ......

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