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-6365 Lidar Remote Sensing CU Boulder April 20, 2016
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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-6365 Lidar Remote Sensing CU Boulder
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)
nC =1
2ΔσC (R)ddRln
NS (λoff ,R)− NB (λoff )NS (λon,R)− NB (λon )
#
$%
&
'(
−1
2ΔσC (R)ddRlnG(λoff ,R)G(λon,R)
GF[ ]
−1
2ΔσC (R)ddRlnβ(λoff ,R)β(λon,R)
B[ ]
−1
ΔσC (R)α(λon,R)−α(λoff ,R)#$ &' E[ ]
−1
ΔσC (R)Δσ Xi
(R) nXi (R)i=1
m
∑ X[ ]
with ΔσC (R) =σC (λon,R)−σC (λoff ,R)
GF = 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) nC =
12ΔσC (R)
ddRln
NS (λoff ,R)− NB (λoff )NS (λon,R)− NB (λon )
#
$%
&
'(
−1
2ΔσC (R)ddRlnG(λoff ,R)G(λon,R)
GF[ ]
−1
2ΔσC (R)ddRlnβ(λoff ,R)β(λon,R)
B[ ]
−1
ΔσC (R)α(λon,R)−α(λoff ,R)#$ &' E[ ]
−1
ΔσC (R)Δσ Xi
(R) nXi (R)i=1
m
∑ X[ ]
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)
−1
2ΔσC (R)ddRlnG(λoff ,R)G(λon,R)
GF[ ]
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.