long path (active ) doas -basic principle -long path...

Lecture on atmospheric remote sensing [email protected]

11

Long Path (active) DOAS

-basic principle

-Long path DOAS (UV/vis/IR)

-instrumental improvements

-Specific applications

-white cell

-vertical profiles

-tomographic inversions


22

A) Lambert-Beersches Gesetz: I I c l 0 exp

DOAS: ’Differentielle Optische AbsorptionsSpektroscopie’


33



0

0.2

0.4

0.6

0.8

1

0 2 4 6 8 10Schichtdicke [km]

Rel

ativ

e In

tens

ität

Lichtabschwächung durch Ozon für UV-Licht bei 300nm

l

II

c 0ln

Aus der Intensitätsmessung kann die Konzentration bestimmt werden


44

SpiegelLichtquelle& Spektrograph

0.5 - 15 km

In der Realität ist es sehr ähnlich....




55

Long Path DOAS

-basic principle




-white cell

-vertical profiles



66



0.5 - 15 km


77

http://http://wwwwww.chem..chem.leedsleeds..acac..ukuk/JMCP//JMCP/imagesimages//doaspicturedoaspicture..jpgjpg


88

Typisches (früheres) LP-DOAS-Instrument

Spiegel Lichtquelle


99


1010

Basic requirements for Long path systems

-divergence of the light beam should be small (diameter of a few meters over a distance of several kilometers)

=> Large mirrors

=> Light sources with high luminance (photon flux per area)

dd

ff

LW

fdsin

W: width of the light beam at distance L

For W=1 m, L=5 km: sin = 0.001, =0.06 °

For f = 2 m => d =2 mm


1111Diplomarbeit Thorsten Hermes, IUP Heidelberg, 1999


1212

Der Druck der Xenon-Edelgasfüllung steigt während des Betriebs von etwa 8 bar im kalten Zustand auf bis zu 70 bar an.


1313Diplomarbeit Thorsten Hermes, IUP Heidelberg, 1999Diplomarbeit Thorsten Hermes, IUP Heidelberg, 1999


1414

The spectral stuctures affect the trace gas analysis (for details see below). Thus they have to be removed from the measured spectra. This is achieved by the following procedure:

First, a lamp spectrum without atmospheric absorption is measured. Then the measured ‚atmospheric spectra‘ are divided by this lamp spectrum.

How can a lamp spectrum be measured?

(note: it should contain also all other instrumental features like e.g. mirror reflectivity, fibre throughput)


1515


1616Lense

glass fibre

Spectrograph +30°C

Grating

Detector-35°C

Typical DOAS-Spectrograph

Light


1717

Schematic of a Czerny-Turner monochromator


1818Diplomarbeit Jens Tschritter, IUP Heidelberg, 2007


1919



0.5 - 15 km

Am Tag wird Sonnenlicht in den Lichtweg eingestreut


2020

Das gemessene Atmosphärenspektrum enthält (tagsüber) auch Beiträge von in den Strahl gestreutem Sonnenlicht. Dieses muss

vor der Datenauswertung subtrahiert werden.


2121

l

si

ii dssII0

0 )()()(exp)()(

Beer- Lambert-law :

i: Absorption cross section of trace gas ii: Concentration of trace gas is: Extinction coefficient

=> From the knowledge of the absorption cross section it is possible to determine the trace gas concentration

Absorption spectroscopy

Optical depth


2222

Problems:

- usually Io is not (well) known

- usually more than one trace gas present

- also scattering reduces the measured intensity


2323

Beer- Lambert-law :

I0‘i: Intensity minus all broad and contributions (absorption & scattering)‘i: Differential absorption cross section of trace gas ii: Concentration of trace gas is: Extinction coefficient

=> From the knowledge of the differential absorption cross section it is possible todetermine the trace gas concentration

l

iii dssII

00 )()('exp)(')(

Differential absorption spectroscopy

Differential optical depth ‘


2424460 480 500 520 540

Wellenlänge [nm]

Inte

nsit

t I(

)

O3-Absorption

NO2-Absorption

Meßspektrum

'differentielle'optische DichteI’0

I

''ln0

l

II

c


2525

(PhD J Stutz)(PhD J Stutz)


2626

Volcanic SO2 emission (PhD C. Kern, 2009)Volcanic SO2 emission (PhD C. Kern, 2009)

(a solar spectrum was included to correct for sun light scattere(a solar spectrum was included to correct for sun light scattered into the beam)d into the beam)


2727

Pöhler et al., PNAS, 2010

Sample evaluation of a spectrum measured on April 1, 2008, at 00:52 UTC. Thin black lines represent the measurement, gray lines the respective fit results. To the high pass filtered atmospheric spectrum a polynomial of third order is fitted. The resulting mixing ratios are for BrO 5.41·1014 molec/cm2 and O3

3.34·1017 molec/cm2. Note the different scales.


2828

Sum spectrum (measured by the OMI instrument)

0.0E+00

1.0E+14

2.0E+14

3.0E+14

4.0E+14

5.0E+14

6.0E+14

300 350 400 450 500Wavelength [nm]

Pho

tons

/m²/s

/nm


2929

Typical value (around 450 nm): 4e14 photons/m²/s/nm

For satellite measurements looking down to earth:

Albedo ~0.1 = factor of 0.1

Detector size: 2.5 cm * 0.25 cm (covering 100 nm)

=> factor of 0.00625 * 0.00001

Integration time: 0.05 s

=> Factor of 0.05

=> 4e14 * 0.1*0.00625*0.00001*0.05 = 1.25e6

Wurzel aus 1.25e6 ~1.12e3 => relativer Fehler durch Photonenrauschen: etwa 1 Promille


3030

Differential Optical Absorption Spectroscopy (DOAS

-identification of different absorption processes by their spectral signature

=> Differential optical absorption spectroscopy(DOAS)

-consideration of scattering processes by broad band spectral structures, e.g. low orderpolynomials

© Udo © Udo FrießFrieß


3131

Absolute Wirkungs-querschnitte vonaromatischen Kohlenwasserstoffen nach Etzkorn [1998].


3232

Example: NO2 observation

0.E+00

2.E-19

4.E-19

6.E-19

8.E-19

250 300 350 400 450 500 550 600 650 700 750

Wavelength [nm]

abso

rptio

n cr

oss

sect

ion

[cm

²]

l Path length: 6km

NO2 mixing ratio: 10 ppb (parts per billion)

Air concentration: 2.9e19 molec/cm³

NO2 concentration: 2.9e11 molec/cm²

Typical wavelength window

0.07

=> I/I0 0.93


3333

Example: NO2 observation

0.E+00

2.E-19

4.E-19

6.E-19

8.E-19

250 300 350 400 450 500 550 600 650 700 750

Wavelength [nm]

abso

rptio

n cr

oss

sect

ion

[cm

²]

l ''Path length: 6km

NO2 mixing ratio: 10 ppb (parts per billion)

Air concentration: 2.9e19 molec/cm³

NO2 concentration: 2.9e11 molec/cm²

Typical wavelength window

' 0.035

=> I/I‘0 0.965


3434

UV/Vis and near IR: Electronic and vibrational transisons (typ. Absorption)

Thermal IR: Vibrational transisons (typ. Emission)

Microwave: Rotational transisons (typ. Emission)

The electromagnetic spectrum


3535

Energy levels for different statesof vibration

TheThe distancedistance between the between the energy levels is constantenergy levels is constant::


3636

Energy levels for different statesof vibration

The distance between the energy levels is not constant. Forincreasing the distance decreases.There exists only a limited numberof eneryg levels.

2

0 01 1( )2 2

eG v v v x


3737

0

2E-18

4E-18

6E-18

8E-18

1E-17

1.2E-17

1.4E-17

300 320 340 360 380 400 420 440Wavelength [nm]

Abs

orpt

ion

cros

s se

ctio

n [c

m²]

Absorption cross section of the OClO molecule


3838

Example of trace gas cross section:

H2O absorption cross section for 290K

(HITRAN data base)

How can spectra be determined?


3939Diploma thesis Miriam von König, IUP, 1996

Instrumental set up for the measurement of trace gas cross sections


4040Diploma thesis Miriam von König, IUP, 1996

Measurements have to be made at varios temperatures and pressures


4141Johannes Orphal, IUP-Bremen

Temperature dependence of the ozone cross section

0

1E-20

2E-20

3E-20

4E-20

5E-20

310 320 330 340 350Wavelength [nm]

[cm

²]

202K241K293K


4242Johannes Orphal, IUP-Bremen

Temperature dependence of the ozone cross section

0

5E-22

1E-21

1.5E-21

2E-21

2.5E-21

340 345 350 355 360Wavelength [nm]

[cm

²]

202K241K293K


4343

Differential Optical Absorption Spectroscopy (DOAS

-identification of different absorption processes by their spectral signature

=> Differential optical absorption spectroscopy(DOAS)

-consideration of scattering processes by broad band spectral structures, e.g. low orderpolynomials

© Udo © Udo FrießFrieß


4444

Measurements of the absorption of the NO3 radical

NO3 absorption cross section(http://vpl.astro.washington.edu/spectra/no3orphal.jpg)


4545

Measurements of the absorption of the NO3 radical

Platt et al., 1980


4646

Platt et al., Platt et al., 19801980

NO2 + O3 O2 + NO3


4747

NO3 time series collected in the marine boundary layer of Mace Head, Ireland. The letters denote the origin of the observed air masses: A, Atlantic, P polar marine, EC, easterly continental, NC, northerly continental

[Allan et al, 2000].


4848

Catalytic ozone destruction mechanisms:

X + O3 XO + O2

XO + O X + O2

Net: O + O3 2O2

with:

X = OH, NO, Cl, Br


4949

Observation of Observation of volcanic emissionsvolcanic emissions

C. Kern, IUP HeidelbergC. Kern, IUP Heidelberg


5050


5151


5252


5353


5454

Halogen compounds in coastal regions?

C. Peters, PhD-thesis, IUP Heidelberg


5555425 430 435 440

0.996

0.998

1.000

1.002

wavelength [nm]

residual

0.996

0.998

1.000

1.002

optic

al d

ensi

ty atmospheric spectrum

0.99995

1.00000

1.00005IO reference

Comparison of atmospheric spectrum after the removal of NO2 and H2O absorptions. The comparison with the IO absorption cross section clearly shows the presence of IO [Alicke et al, 1999].


5656


5757


5858


5959


6060


6161

K. Hebestreit,K. Hebestreit,

PhDPhD--thesisthesis, IUP , IUP HeidelbergHeidelberg


6262High High BrO coincides with low BrO coincides with low OO33


6363


6464

July 2005 - 2014

BrO above the Dead Sea

Christoph Hörmann, MPIC/IUP


6565

Long Path DOAS

-basic principle




-white cell

-vertical profiles



6666

Often very large spectral residual structures appear in the spectral analysis. At the beginning the reason was unclear, but later it turned out that inhomogenuous illumination of the fibre had caused these structures.

Such inhomogenuous illumination can be caused by atmospheric turbulence and/or misallignements of the instrument.

Why can inhomogenuous illumination of the fibre cause spectral structures?

Because then the illumination of the diffraction grating is changing (with time). This leads to a change of the spectral resolution of the instrument.

expected residual

measured residual


6767


6868


6969


7070J. Stutz, PhD-thesis, IUP Heidelberg


7171

0°0°

8°8°


7272J. Stutz, J. Stutz, PhDPhD--thesisthesis, IUP Heidelberg, IUP Heidelberg


7373R. Ackermann, R. Ackermann, PhDPhD--thesisthesis, IUP Heidelberg, IUP Heidelberg


7474J. J. TschritterTschritter, , DiplomaDiploma--thesisthesis, IUP Heidelberg, IUP Heidelberg


7575

quarz fibre with mode mixer

photodiode array detectorCzerny-Turner spectrograph

retro reflector array

light path: 2 x 6300 m

20 cm30 cm

parallel light beam

500 W high pressureXe-arc lamp house

stepper motorfor vertical adjustment

long path coaxial Newton telescope

stepper motor for coulour filterand baffle

stepper motor for filterand Hg/Ne emission lampstepper motor for 360°

horizontal adjustment

mainmirror

stepper motor for lamp reference system

analog-digital converter

computer

DOAScontroller

Schematic set-up of a DOAS system using a coaxial arrangement of transmitting- and receiving telescope in conjunction with a retro-reflector array [Geyer et al. 2001]. This type of set-up pioneered by Axelsson et al. [1990] has become the standard for artificial - light DOAS systems for research in the recent years.


7676

Messung des Lampenspektrums

Ursprüngliche Version:


7777C. Hak, Dissertation, IUP Heidelberg, 2007C. Hak, Dissertation, IUP Heidelberg, 2007

Messung des Lampenspektrums

Verbesserte Version (aber aufwendig):


7878

J. J. TschritterTschritter, , DiplomaDiploma--thesisthesis, IUP Heidelberg, IUP Heidelberg

•• Mode Mode mixer mixer has still to has still to be usedbe used, ,

•• but measurementbut measurement of of lamp lamp reference spectrum is much easierreference spectrum is much easier::

=> Just => Just put put a a reflecting surface reflecting surface in in front of front of the fibre bundlethe fibre bundle


7979

RadianceRadiance

EfficiencyEfficiency


8080


8181

DOAS instrument used during theSOS field campaign in Nashville, TN, 1999, picture : Cathy Burgdorf, http://www.atmos.ucla.edu/~jochen/research/doas/DOAS.html


8282

Long Path DOAS

-basic principle




-white cell

-vertical profiles



8383


8484


8585

-Multi-Reflektions-System (White-Zelle):

Hohlspiegel

0.1 - 15 m

Lichtquelle& Spektrograph


8686Schematic optical set-up of the 'White' multi-pass system with a base path of 15 m as used during a field campaign in Pabstthum/Germany.

500 W Xe -arclamphouse

Quartz fibre withmode-mixer

Transferoptics

PC

A. Geyer


8787

Schematic of the DOAS setup in the EUPHORE chamber in Valencia, Spain [Volkamer et al. 2002].


8888

Long Path DOAS

-basic principle




-white cell

-vertical profiles



8989

DOAS

4 m

1.25 km

1.57 m

2.45 m

1.8 m

Sonic Anemometer

2.1m

Set-up of the experiment to measure gradients and fluxes of NO2 and HONO during the PIPAPO experiment. The DOAS instrument aimed sequentially at the three retroreflectors mounted on the tower at 1.25 km distance.

B. Alicke


9090

020406080

5/29/1998 12:00 5/30/1998 00:00 5/30/1998 12:00

0

5

10

a

[NO

2] (p

pb) upper LP

middle LP lower LP

b

Gra

dien

t(p

pb m

-1)

c 360

NO2 and HONO gradients during the night of May 29, 1998 in Milan, Italy.

B. Alicke

0

1

2

35/29/1998 12:00 5/30/1998 00:00 5/30/1998 12:00

4000.0

0.4

0.8

a

HO

NO

(ppb

) upper LP middle LP lower LP

b

gra

dien

t(p

pb m

-1)


9191

RTU 115mRTM 99mRTL 70m

DOAS Systems

WT 44mME 2m

6.1 km1.9 km750m

Setup during the TEXAQS 2000 experiment. Five retroreflector arrays were mounted at different distances and altitudes. The measurements were performed by two DOAS instruments [Stutz et al, 2004].

J. Stutz

6.1 km1.9 km750m


9292

0 5 10 150

20406080

100120

altit

ude (

m)

0 5 10 15

2254

0 5 10 15

0042

0 5 10 15

0240

NO2 (ppb)

0 50 1000

20406080

100120

0 50 100 0 50 100 0 50 100

NO3 (ppt)

0 20 40 600

20406080

100120

0 20 40 60 0 20 40 60 0 20 40 60

O3 (ppb)

120

2032

Vertical mixing ratio profiles during the night of 8/31 – 9/1 at four different times (noted on top of the graphs). The N2O5 mixing ratios shown are calculated from the steady state of measured NO2 ,NO3 , and N2O5 [Stutz et al., 2004].

J. Stutz


9393

H.-J. Veitel, IUP Heidelberg


9494



9595



9696



9797


9898


9999


100100

Long Path DOAS

-basic principle




-white cell

-vertical profiles



101101A. A. HartlHartl, Dissertation, IUP Heidelberg, 2007, Dissertation, IUP Heidelberg, 2007


102102

tomographic measurement setup during the BAB II campaign at the motorway BAB 656 between Heidelberg and Mannheim


103103

North South

Wind direction


104104

Modelled concentration distribution of NO2 and measurement setup

Measured (reconstructed) concentration distribution of NO2


105105

KaiKai--Uwe Mettendorf, Dissertation, IUP Heidelberg, 2006Uwe Mettendorf, Dissertation, IUP Heidelberg, 2006


106106


107107Measurement setup for the validation measurements


108108


109109


110110

Distribution of light paths

Example of a reconstructed trace gas distribution


111111


112112

Summary (I) Long Path (active) DOAS

-direct application of the Lambert-Beer law

-long absorption path is needed to become sensitive even for small trace gas concentrations

-LP-DOAS measurements are often used if spatial distribution is not important (is a trace gas present at all?)

=> many trace gases were first observed by LP DOAS

-measurements possible also during night

-only the averaged trace gas concentration along the light path can be obtained (from simple LP DOAS)

-expensive and complicated instrumental set-up


113113

Summary (II) Long Path (active) DOAS

-recently many instrumental improvements were developed,e.g. fibre optics, LEDs as light sources, which make instruments much cheaper, lighter, and easier to operate

-many specialisations of LP-DOAS exist for specific applications:

-White (multi-reflection) system

-light paths at different altitudes

-balloon-borne reflectors


long path (active ) doas -basic principle -long path...

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