Istituto Nazionale di
Ottica Applicata (INOA)
TUNABLE DIODE LASER (TDL)
SPECTROMETERS for IN-SITU TRACE
GAS MEASUREMENTS
Consiglio Nazionale
delle Ricerche (CNR)
GOAL: MEASUREMENT OF GAS CONCENTRATION BY MEANS OF THE MOLECULAR ABSORPTION OF A LASER BEAM
The in-situ concentration of trace gases is performed by collecting an air sample into a cell and by measuring, with an intensity detector, the absorption of the laser radiation when the laser beam passes
through the cell. Different molecules absorb laser radiation at different frequencies.
I
I
IN
LASER BEAM WITH INTENSITY IIN
BEER-LAMBERT LAW:
OUT
DETECTOR
LASER BEAM WITH INTENSITY IOUT
L
cell of length L containing the gas under analysis
IIN = IOUT e
- measurements of IIN and IOUT ⇒ calculation of the absorption coefficient α
−α L
- the gas CONCENTRATION can be obtained from α (known some molecular parameters)
α : absorption coefficient of the gas
TDL-QCL: Tunable Diode Laser Spectrometer with Quantum Cascade Laser (QCL) for CO and/or N2O in-situ measurements
The instrument TDL-QCL is derived from a previous version: COLD (Cryogenically Operated Laser Diode) which was employed onboard the M55 Geophysica stratospheric aircraft (for HNO3, N2O, and CO)
THE COLD INSTRUMENT:
Dw: liquidnitrogen dewar
MC
RC
45 cm
L
E
Dw
RC
E
D2
D: detector
RC: reference cell
L
MC
E: etalon
COLD IMPROVEMENTS:
weight: 55 Kg
COLD Î TDL-QCL
the liquid-nitrogen cooled lead salt
diode laser will be replaced by a
room-temperature single mode
QUANTUM CASCADE LASER (QCL)
Dw
MC: multipass cell
D1
NEWS !!!
D1
L: diode laser
D2
Laser source: single mode lead salt diode laser LiquidNitrogen cooled and temperature stabilized (≈ 110 0K)
@ 4 - 5 μm
Multipass cell: astigmatic Herriott cell (New Focus
5611), optical path = 36 m
Detector 1 (signal): HgCdTe (Graseby Infrared),
Liquid-Nitrogen cooled
Detector 2 (reference): InSb (Hamamatsu), cooled by
Peltier
Etalon: ZnSe, Free Spectral Range = 0.07 cm-1
Transient Digitizer: 5 Msample/s, 12 bit, 1024 points
for single acquisition, 2 input channels, 6000 averages
(Licel GbR)
Technique: direct absorption
55 cm
IN FLIGHT COLD PERFORMANCES: Ö Sensitivity: 4 10-4 (for CO: 3 ppbV) Õ
Ö Accuracy: 6 - 9 % Õ
Ö Time resolution: 4 s Õ
ADVANTAGES OF QCL WITH RESPECT TO COOLED LEAD SALT DIODE LASER:
COLD reference:
‹ much higher power of QCL ⇒ increase of signal-to-noise ratio ⇒ increase of the sensitivity
‹ reduction of the instrument size by removing the cryogenic apparatus
Ö Precision: 1 % Õ
Viciani et al., Applied Physics B, 90,
581-592 (2008).
‹ long time operation
ALTO: Tunable Diode Laser Spectrometer with Distributed Feedback Laser (DFB) for CH4, CO2, etc. measurements
The instrument ALTO (Airborne Laser Tunable Observer) was employed onboard the M55 Geophysica aircraft for CH4 measurements (other species may be measured with different laser diodes)
THE ALTO INSTRUMENT:
26 cm
MC
PC
D
PC:electronics and computer;
L: diode laser; MC: multipass cell
L
D: detector; RC: reference cell
D
RC
V
MC
PC
V: gas valve
L
D
Laser source: single mode room temperature
DFB laser (@ 1.651 μm)
Multipass cell: home-made Herriott cell, optical
path = 25 m
Detectors: InGaAs (room temperature)
Technique: direct absorption or frequency
modulation
60 cm
ALTO reference: D’Amato et al.,
Applied Physics B, 75, 195-202 (2002).
V
weight: 25 Kg
IN FLIGHT ALTO PERFORMANCES: Ö Sensitivity for CH4: 60 ppbV Õ Ö Accuracy: 10-15 % Õ Ö Time resolution: 5 s Õ Ö Precision: 5 % Õ
SOME RESULTS OBTAINED with COLD (for CO) and ALTO (for CH4) during CAMPAIGNS ONBOARD THE M55 GEOPHYSICA AIRCRAFT
CO - OZONE (O3) CORRELATIONS
M55 altitude (km)
BRAZIL
IRREVERSIBLE
MIXING OF
STRATOSPHERIC and
TROPOSPHERIC AIR
due to TROPICAL
DEEP CONVECTION
CO concentration (ppbV)
COLD INSTRUMENT
References: - Konopka et al., Atmospheric Chemistry and Physics 7, 3285 (2007).
- Viciani et al., Applied Physics B, 90, 581 (2008).
CONTACTS:
FRANCESCO D’AMATO ([email protected]) and
2.0
20
1.6
16
1.2
12
0.8
8
0.4
4
0.0
55000
60000
65000
0
70000
M55 altitude (km)
CO concentration (ppbV)
evidence of local
convection activity
(FRESH CONVECTION)
+ remote convection
(AGED CONVECTION)
from a biomass burning
region in Africa.
CH4 DATA
CH4 concentration (ppmV)
In Brazil:
TROCCINOX-2
(Araçatuba, Brazil)
flight of the
01/02/2005
EVIDENCE OF VERY
FINE STRUCTURES:
intrusion of
stratospheric air in
the troposphere
(observed from
several instruments)
ZOOM
troposhere
COLD INSTRUMENT
TROCCINOX-2
flight of the
05/02/2005
time (UTC)
Concentration (ppbV)
Concentration (ppbV)
evidence of local
convection activity
(FRESH
CONVECTION).
TROCCINOX-2
campaign
(Araçatuba, Brazil)
FOZAN INSTRUMENT
M55 altitude (km)
In Australia:
stratosphere
M55 altitude (km)
CO vertical profiles
during 2005 tropical
campaigns:
TROCCINOX-2
(Araçatuba, Brazil) and
SCOUT-O3 (Darwin,
Australia)
O3 concentration (ppbV)
AUSTRALIA
FINE STRUCTURES
CO concentration (ppbV)
VERTICAL PROFILES
COLD: CO
FOZAN: O3
80 s → 16 km
typical ALTO
result
Time UTC (s)
SILVIA VICIANI ([email protected]).
CNR-INOA, Largo E. Fermi, 6, I-50125 Firenze (ITALY), www.inoa.it