Optical Remote Sensing with DIfferential Absorption Lidar (DIAL)

Part 1: Theory

Christoph Senff

CIRES, University of Colorado & NOAA/ESRL/CSD/Atmospheric Remote Sensing Group

http://www.esrl.noaa.gov/csd/groups/csd3/

Guest lecture for ASEN-6519 Lidar Remote Sensing CU Boulder

November 19, 2014

Outline

Ø  DIAL concept

Ø  A short history of DIAL

Ø  DIAL equation

Ø  Precision & accuracy of DIAL retrieval

Ø  Dual-DIAL technique

Atmospheric gases measured with DIAL

§  H2O §  O3

§  SO2

§  NO2, NO §  NH3

§  CH4

§  CO2

§  Hg

§  VOCs (Volatile Organic Compounds) §  Toluene, Benzene

First DIAL measurements Richard M. Schotland (“The father of DIAL”) 1964 – Measured vertical profiles of water vapor by thermally tuning a ruby laser on and off the water vapor absorption line at 694.38 nm. Only 4 years after invention of ruby laser !

Major milestones in the history of DIAL

Maiman: Invention of the laser

1964 1960 1981 1978

Shumate & Menzies: First Airborne DIAL (column O3)

Browell: First airborne H2O DIAL

Space-based DIAL

Schotland: First H2O DIAL measurements

1977

Megie: First O3 DIAL

1995

Browell: First autonomous DIAL (LASE H20 DIAL on ER-2 aircraft)

Present ??

τ (R) = α (r) dr0

R∫

T (R) = exp −τ (R)[ ] = exp − α(r) dr0

R∫#

$%&'(

α (R) , m−1"# $%

Refresher

Extinction coefficient:

Optical depth / thickness:

Transmission:

DIAL equation (1)

[ ] [ ]

∑+=

+⎥⎦⎤

⎢⎣⎡−Δ= ∫

iiabsmolTot

BR

TotLS

rinrrrwith

NRGdrrRARRNRN

)(),(),(),( ,,

02 )(),()(),(2exp),()(),(

λσλαλα

λλληλαλβλλ

( )

( )[ ] ⎥⎦

⎤⎢⎣

⎡−−×

⎥⎦⎤

⎢⎣⎡ −−×

⎥⎦⎤

⎢⎣⎡ −−×

=−

−

∫ ∑

∫

∫

=

drrnrr

drrnrr

drrr

RRGNRRGN

RNRNRNRN

R m

iXonXoffX

CR

onCoffC

Ronoff

ononononL

offoffoffoffL

onBonS

offBoffS

iii01

0

0

)(),(),(2exp

)(),(),(2exp

),(),(2exp

),(),()()(),(),()()(

),(),(),(),(

λσλσ

λσλσ

λαλα

λβλληλ

λβλληλ

λλ

λλ

Single scattering, elastic backscatter LIDAR equation:

Take ratio of LIDAR equations for online and offline wavelengths λon and λoff :

Number density of constituent C

DIAL equation (2)

[ ]

[ ]

[ ] [ ]

[ ]

),(),()(

)()()(

1

),(),()(

1),(),(

ln)(2

1),(),(

ln)(2

1

)(),()(),(

ln)(2

1

1

RRRwith

XRnRR

ERRR

BRR

dRd

R

GRGRG

dRd

R

NRNNRN

dRd

Rn

offConCC

m

iXX

C

offonC

on

off

C

on

off

C

onBonS

offBoffS

CC

ii

λσλσσ

σσ

λαλασ

λβ

λβ

σ

λ

λ

σ

λλ

λλ

σ

−=Δ

ΔΔ

−

−Δ

−

Δ−

Δ−

⎥⎦

⎤⎢⎣

⎡

−

−

Δ=

∑=

G = differential geometrical factor B = differential backscatter E = differential extinction X = interfering constituents

How to choose an appropriate absorption line for DIAL (1) Extinction of online wavelength due to absorption by constituent C must be neither too small or too large. Absorption too strong Absorption too weak Best precision in nC when: (Remsberg & Gordley, 1978)

⎥⎦⎤

⎢⎣⎡−∝ ∫

R

ConConS drrnrRN0

)(),(2exp),( λσλ

1.1)(),(),( max

0max == ∫ drrnrR Con

R

Con λσλτ

λoff

λon

NS

R R

NS

λoff

λon

How to choose an appropriate absorption line for DIAL (2) Example: Ozone 1.1)(),(),(

max

0max == ∫ drrnrR Con

RCon λσλτ

222max

max318

1083.1)(1.1)(

:310280

333

33

mRn

kmRandmnorppbvmrFor

onOOonO

OO

−

−

×=⇒=

=×==

λσλσ

λon

λoff

288 nm

299 nm

Precision of DIAL measurements

Simple “back of the envelope” calculation: Even modest precision of 5% requires high SNR. SNR can be increased by

averaging on/offline signals time- and range-wise.

( )( )

!400%5,05.0:

111

11),(),(

)(21

),(),(),(),(

ln)(2

1

,2

2

=⇒==Δ

Δ=⇒

Δ=

ΔΔ=

=ΔΔ

=ΔΔ

≈ΔΔ

=

−=⎥⎥⎦

⎤

⎢⎢⎣

⎡

Δ+

Δ+

ΔΔ=

∑

SNRnnExample

nnSNR

SNRSNRRnnn

NNSNRwith

SNRRNN

RRNRN

RRn

NNNwithRNRRNRNRRN

RRn

CC

CCCCC

C

CCji ji

ji

CC

BSoffon

onoff

CC

δτ

δττσδ

δσδ

σλ

λδ

σδ

λλ

λλ

σ

5.15.05.05.0

5.05.0

,:

−− ΔΔ∝ΔΔ∝ΔΔ∝

=⇒=

RtnandRtSNRRtNSinceNSNRNNstatisticsPoisson

Cδ

δ

Accuracy of DIAL measurements (1)

[ ]

[ ]

[ ] [ ]

[ ]XRnRR

ERRR

BRR

dRd

R

GRGRG

dRd

R

NRNNRN

dRd

Rn

m

iXX

C

offonC

on

off

C

on

off

C

onBonS

offBoffS

CC

ii∑=

ΔΔ

−

−Δ

−

Δ−

Δ−

⎥⎦

⎤⎢⎣

⎡

−

−

Δ=

1)()(

)(1

),(),()(

1),(),(

ln)(2

1

),(),(

ln)(2

1

)(),()(),(

ln)(2

1

σσ

λαλασ

λβ

λβ

σ

λ

λ

σ

λλ

λλ

σ

Accuracy affected by:

Ø  How well is absorption cross section known? Ø  Improper correction of signal offsets, e.g. background light Ø  Geometrical factor different for λon and λoff

Ø  Differential backscatter & extinction not properly corrected Ø  Interfering species not taken into account

Accuracy of DIAL measurements (2)

[ ]GRGRG

dRd

R on

off

C ),(),(

ln)(2

1λ

λ

σΔ−

Effect of differential geometrical factor on O3 retrieval

Differential geometrical factor:

1;),(),(

1),(),(

<≠

==

GRGRG

RGRG

offon

offon

λλ

λλ

[ ]

[ ] [ ]

AerosolRayleighAerosolRayleigh

offonC

on

off

C

ERRR

BRR

dRd

R

αααβββ

λαλασ

λβ

λβ

σ

+=+=

−Δ

−

Δ−

,

),(),()(

1),(),(

ln)(2

1

Accuracy of DIAL measurements (3)

Ø  For ozone DIAL retrieval, backscatter and extinction correction is necessary due to large Δλ. Ø  βAerosol and αAerosol have to be determined from offline signal data and wavelength dependence of β and α have to be guessed.

Differential backscatter & extinction:

λon

λoff

288 nm

299 nm

Accuracy of DIAL measurements (4)

βAerosol simulation

O3 retrieval simulation

Wrong assumptions about aerosol parameters can introduce significant errors in O3 retrieval !

Dual-DIAL concept

λ1 λ2 λ3

2 DIAL wavelength pairs: λ1 / λ2 and λ2 / λ3

Dual-DIAL minimizes aerosol interference (1)

[ ]

( )[ ] [ ]

22

11

221121

2211

2

2

1

1

2*

2*

1*

1*

0''

/:2,/:1,)(

'),(),(),(),()(

1

'),(),(

ln),(),(

ln)(2

1

),(),(

ln),(),(

ln)(2

1

offon

offon

offonoffonCCC

offonoffonC

on

off

on

off

C

onS

offS

onS

offS

CC

CforEB

pairDIALpairDIALCRwith

ERRCRRR

BRR

CRR

dRd

R

RNRN

CRNRN

dRd

Rn

λλ

λλ

λλλλσσδσ

λαλαλαλαδσ

λβ

λβ

λβ

λβ

δσ

λ

λ

λ

λ

δσ

−

−=≈=

Δ−Δ=

−−−−

⎥⎦

⎤⎢⎣

⎡−−

⎥⎦

⎤⎢⎣

⎡−=

Ø  No correction of differential aerosol effects needed and residual errors are small.

Ø  However, precision of DIAL retrieval is degraded.

Dual-DIAL minimizes aerosol interference (2)

O3 retrieval simulation

Selected References

DIAL history (slides 5 - 6)

Schotland, R. M., 1974: Errors in the Lidar Measurement of Atmospheric Gases by Differential Absorption, J. Appl. Meteorol., 13, 71-77. Shumate, M. S., R. T. Menzies, W. B. Grant, and D. S. McDougal, 1981: Laser Absorption Spectrometer: Remote Measurement of Tropospheric Ozone, Appl. Opt., 20, 545-553. Pelon, J. and G. Megie, 1982: Ozone Monitoring in the Troposphere and Lower Stratosphere: Evaluation and Operation of a Ground-Based Lidar Station, J. Geophys. Res., 87, 4947-4955. Megie, G. J., G. Ancellet, and J. Pelon, 1985: Lidar Measurements of Ozone Vertical Profiles, Appl. Opt., 24, 3454-3463. Browell, E. V., S. Ismail, W. B. Grant, 1998: Differential Absorption Lidar (DIAL) Measurements from Air and Space, Appl. Phys. B, 67, 399 – 410. Ismail, S., E. V. Browell, R. A. Ferrare, S. A. Kooi, M. B. Clayton, V. G. Brackett and P. B. Russell, 2000: LASE Measurements of Aerosol and Water Vapor Profiles During TARFOX, J. Geophys. Res., 105, D8, 9903-9916. How to choose a DIAL absorption line? (slides 10 - 11)

Remsberg, E. E. and L. L. Gordley, 1978: Analysis of Differential Absorption Lidar from the Space Shuttle, Appl. Opt., 17, 624-630. Aerosol correction & DUAL-DIAL (slides 15 - 19)

Browell, E. V., S. Ismail, and S. T. Shipley, 1985: Ultraviolet DIAL measurements of O3 profiles in regions of spatially inhomogeneous aerosols, Appl. Opt., 24, 2827-2836. Wang, Z., H. Nakane, H. Hu, and J. Zhou, 1997: Three-Wavelength Dual Differential Absorption Lidar Method for Stratospheric Ozone Measurements in the Presence of Volcanic Aerosols, Appl. Opt., 36, 1245-1252.