Overview– Basic Principle– How it started
• Weiss et al. 1981 and the NOAA SetUp during the SAGA II experiment • Current surface trace gas data – what happened?• NDIR and the GO-system
– CEAS- instruments – opening the field• Basics of functioning• The main players (if you were to buy an instrument) ….• … and how they work (CEAS, CRDS, oa-ICOS; and OF-CEAS)• Examples of gases (and isotopes) to be measured• Ocean community problems ….
– Matrix effects
• Some data from the field2
Overview
Overview II– Some often forgotten issues of equilibrator based measurements
• It is not in equilibrium (never, really)• Response time and equilibrator design• The total gas tension assumption
3
Overview
The Basic principle • Permanently renewed seawater supply
• Recirculating stream of air • Both phases “meet” in a vessel
designed to stimulate rapid gas-water exchange, usually in a counter-flow set-up
• In most case the equilibration vessel is open to the atmosphere, in a setting minimizing contamination by outside air.
• In gas stream analytical device to detect at least one gas species
• Air pressure, salinity , Tinsitu and Tequito be known
• Water vapour content to be known at point of measurement4
Basics
From Gülzow et al., 2011
The Basic principle • Actual measurement: mole fraction of the gas (i) in the gas phase (xi)
• Usually calculation of xi(dry)
• Correction for xi in situ considering changes in solubility due to T-change and assuming 100% water vapor saturation in the exchange vessel
5
Basics
From Gülzow et al., 2011
Takahashi et al., 1993
How it startedRay Weiss et al. 1981Weiss, R.F., 1981. Determinations of carbon dioxideand methane by Dual Catalyst Flame Ionisation Chromatography and nitrous oxide by Electron Capture Chromatography. J. Chrom. Sci. 19, 611-616.
James (Jim) Butler et al. The SAGA II Report (second Soviet-American Gas and Aerosol Experiment; SAGA II)“Five trace gases in the surface water and atmosphere of the West Pacific and East Indian Oceans were measured by automated gas chromatography from May through July 1987”
6
Somehistory
Butler et al., 1988
„The main question we needed to addresswas whether the equilibrator atmosphereproperly represented gas concentrations in the incoming water. We approached thisproblem by conducting some controlledexperiments in the laboratory and applyingthe results to a mass-balance model of theequilibrator, expressed in both differential and finite-increment form. Our objectiveswere to determine (1) the exit coefficient(piston velocity) and the rate constants forgas transfer within the equilibrator, (2) themaximum concentration of gas in theequilibrator atmosphere under steady-stateconditions, (3) the time required forequilibration, and (4) how much theconcentration of gas in the water dropsduring normal sampling. “
„The third GC (not shown) was essentially a Weiss GC configured onlyfor CO2 and CH4“
Somehistory
Non-CO2 Greenhouse gases in the Marine Environment
Oceanic Role as atm. N2O or CH4 source
• Minor for methane, but large uncertainties for shallow waters and Arctic regions
• Approx. 1/3 of natural nitrous oxide emissions, also with considerable uncertainty
• Strong sensitivity to changing redox conditions in the environment (deoxygenation)
• Potential changes in upwelling intensitiy
8
Why
non-
CO
2G
HG
s ?
GHG atm. concentrations and growth rate (NOAA 2018)
Contributors to changed ratdiative forcing, IPCC 2013
Upwelling-induced N2O concentrations in theBenguela Upwelling
System (Arévalo-Martinez et al., 2019)
The global data sets• Methane and nitrous oxide in the
MEMENTO data base (the MarinEMethanE and NiTrous Oxide database)
9
CH4 > 30,000 data entries
Status 2019Surface Measurements (Sampling Depth <10m)
Depth profiles (Sampling Depth > 10m)
N2O > 120,000 data entries
Dat
a G
aps
What made the difference?
• Strong “science pull” driven by the awareness of the oceanic role as a carbon dioxide sink (1st IPCC Report in 1990)
• Non-consuming ideally suited instrument for CO2
LICOR 6251 in 1988, LI-6262 in 1990Wanninkhof et al., 1993, Goyet and Peltzer 1994
• Development of the GO-System as state-of-the art complete system
(Craig tells the history)
12
ND
IR a
ndG
O
Reaching out for other gases
13
Cavityenhancedopticaldetectors
Cavity enhanced absorption spectroscopy
Lambert Beer Law
With I the light intensity after passing the medium, I0 theinitial light intensity, s the absorbers`s cross section, L theoptical pathlength and N the number of absorbing molecules
CEAS use laser pumped cavity to enhance theoptical pathlength and better tuning of absorptionwavelengths.
CRDS
Oa-ICOS
OF-CEAS
???
14
Cavityenhancedopticaldetectors
Cavity enhanced absorption spectroscopy
• ALL work with tunable lasers and fulfill absorption or ring-down experiments over a range of wavelengths.
• ALL work at reduced pressure and stable temperature, and detect water vapor to deal with peak broadening.
• Off-axis Integrated Cavity Output Spectroscopy (oa-ICOS) and optical feedback-cavity enhanced absorption spectroscopy (OF-CEAS) both measure absorption through transmission; CRDS does not.
• Cavity ringdown spectroscopy (CRDS) and OF-CEAS work on locked laser modes, oa-ICOS does not.
CRDS
OA-ICOS
OF-CEAS
???
Reaching out for other gases
Cavity Ring Down Spectroscopy • Measures “decay” of a loaded resonator, and derives concentration from the decay time (or the decay time with absorber in relation to the decay time without absorber)
• Reality: tunable laser allows measurement in non absorbing wavelengh window, rather than without absorber15
CRDS
a is the absorption coefficient, σ is the absorption cross section ofthe absorber, N is the absorber's number density, co is speed oflight, t and to the ringdown time with/without absorbing gas
I(t, λ) = I0 e-t/τ( λ)
Off axis Integrated Cavity Output Spectroscopy
16
Oa-ICOS
• Off-axis configuration reduces need for precise optical alignment
• Increases own maintenance possibilities• No modes, continuous spectrum is possible• Requires relatively large cavity (i.e. volume)• Avoids optical feedback• Potentially less sensitive to changes in T, P,
vibrations etc. oa-ICOS
Reaching out for other gases
17
Opt
ical
Fee
dbac
k -C
EAS
OF-CEAS
• OF-CEAS induces laser bandwith narrowing and laser locking to the cavity modes
• Optimal laser-cavity coupling• Currently two models, but payoff on CO2
performance for dual gas analyser too high
Is there a “best technology”?
19
Bes
t ins
trum
ent?
Maybe not• ICOS ATC, after intensive testing, accepts (only)
Picarro G2301 (CO2, CH4, H2O) and G2401 (CO, CO2, CH4, and H2O) for CO2 and CH4
• But only LGR N2OCM-913 for N2O• In principle, oa-ICOS should have advantages in
terms of vibration issues and „do it yourself“ needs.
• Oa-ICOS has largest inner volume, though• New players on the market (like Miro Analytics,
an EMPA (Switzerland) offspring with supportfrom ETH Zürich
• Ocean community might have other qualitycriteria
Peak broadening and matrix effects
20
Peak
bro
aden
ing
Asorption Peak Shape controls
• Minimum Peak widths given by Heisenberg uncertainty principle (neglible)
• Movement of gas molecules leads tosymmetric Doppler broadening (Gauss shaped)
• Pressure or collisional broadening occursowing to collisions between the particles, which disturb the emission and absorptionprocess, depending on pressure, cross section, and velocity (Lorenz shaped)
• Pathlength between collisions short comparedto wavelength => Peak narrowing
• Resulting profile is called Galatry Profile (Galatry, 1963)
• Most instruments measure the peak heigth, which is depending on peak broadening
Δ" Δ# = ℎ2'
Ocean surface applications
Difficultiesforoceanapplications
• Change of bulk gas composition– Changes in O2/Ar/N2 ratio– MATRIX effect
• Larger dynamic range than atmosphericcommunity
– Choice of instrument– Issues with calibration gases
Becker et al. 2012;-0.3 ppm CO2 for a 380ppm gas at 2% oxygenoversaturation
Trace Gas System for RV-based operations• 3 laser-based detection
systems• Los Gatos GGA (CH4 and CO2)
• Los Gatos N2O/CO Analyser
• Picarro 2131 (CO2, d13CCO2)
• High flow through equilibration system
• Parallel setup of internal sensor pumps
• Additional short circuit air pump
• High water flow-through (8.5 L/min)
• Coupled bubble-type shower head type
reactor
• High exchange in resulting foam level
CEA
S ap
plic
atio
non
RVs
Benguela upwelling
• All year with varying intensity, different water characteristics in the North and the South
• 3 hydrographic sectionsperpendicular to the coast (A,B,D), plus offshore transects C,E (filaments)
• Generally lower oxygenconcentrations in the upwelledwaters in the north of the workingarea
E
Exam
ple
I -B
engu
ela
Spatial patterns• Temperature
• Cold inshore surface waters• Indications of filaments 100s of km
offshore
4
Potential temperature (top) and westward velocity (bottom) sections along the Namibian upwelling front as measured during cruise M99 in August 2013 (uncalibrated data). Red dots on top of the upper figure mark the location of UCTD profiles; numbers are UCTD profile numbers. ADCP data were averaged to 10 min ensembles. Acknowledgements We like to thank Captain Klaus Bergmann, his officers and the crew of RV Meteor for their support of our measurement programme, and their hospitality, in particular towards the participating students. The ship time of RV Meteor was provided by the Deutsche Forschungsgemeinschaft within the core program METEOR/MERIAN. Financial support for the project was provided though the German Ministry of Education and Research (SACUS-SPACES). We also benefited from financial contributions by the research institutes involved.
24�S 28�S
Pot. temperature and westward velocity on a UCTD transect between 24�S and 28�S
Exam
ple
I -B
engu
ela
Spatial patterns• Temperature
• Cold inshore surface waters• Indications of filaments 100s of km
offshore• pCO2
• Highest partial pressures nearshore• Front between 25�S and 26�S
further offshore
Exam
ple
I -B
engu
ela
Spatial patterns• Temperature
• Cold inshore surface waters• Indications of filaments 100s of km
offshore• pCO2
• Highest partial pressures nearshore• Front between 25�S and 26�S
further offshore• Nitrous oxide
• Comparable patterns to pCO2• Moderate oversaturation of 100 %
Exam
ple
I -B
engu
ela
Spatial patterns• Temperature
• Cold inshore surface waters• Indications of filaments 100s of km
offshore• pCO2
• Highest partial pressures nearshore• Front between 25�S and 26�S further
offshore• Nitrous oxide
• Comparable patterns to pCO2• Moderate max. oversaturation of 100 %
• Methane• Moderate max. oversaturation of 200 %• High concentrations bound to inshore
upwelled waters
Exam
ple
I -B
engu
ela
The BALTIC VOS Ferrybox SystemFinnlinesM/S Finnmaid
• Greenhouse gas measurements: pCO2 (twice) and CH4
• Installed alongside pre-existing Finnish Alg@linesystem (Real time algal monitoring in the Baltic Sea)
Description: Gülzow et al., 2011
t: 226 s for CO2, 676 s for CH4
Overview (Methane)• Continuous measurement of CH4 using oa-ICOS on the VOS
Finnmaid• Unique spatiotemporal coverage• Main drivers: SST, mixed layer thickness, upwelling, thermocline
stability
Extended data set from Gülzow et al., 2013
23
!
!
Fig. 20. Methane concentrations (dots) and hydrographic parameters at a station in the central Bornholm Basin in De-cember 2009 (left), and August 2010 (right, with high resolution sampling of the lower 5m). Note jump in methane scale. Bottom waters were characterized by inflow of oxygenated waters at the bottom in December and anoxic conditions in summer, in conjunction with an increase of dissolved methane concentrations from 20 to 80 nM over this period of time.
!
!
Fig. 21. Schematic of a system for the contin-uous measurement of CH4 and pCO2 in sur-face waters using off-axis ICOS. The system is installed on the ferry Finnmaid run by Finn-lines.
!
AB: full basin mixinginducing escape fromsediments
GB: small ASE fluxes except forupwelling events
GoF: mixed layerdeepening andseasonal icecoverage
A gl
imps
e on
the
data
54
56
58
60
12 16 20 24 28Lon °E
Lat °
N
55
56
57
58
59
60
2010 2011 2012 2013 2014 2015 2016 2017Year
Lat °
N
2.5 3 3.5 4 4.5 5 6 7 8 10 12
cCH4 [nmol/L]
Reliability
Webb et al. 2016• Systematic evaluation of
equilibrator response times
• 21�C, 1L/min gas flow, • CH4 response time always larger
than CO2 response time• Ratio smallest for membrane-type
equilibrators
RESP
ONS
ETI
MES
Theoretical assessment is difficult• Schneider et al., 2007 suggests t
scales with solubility
• Webb et al.2016:„The mechanism of equilibration in the marbleand showerhead types is turbulent transfer acrossthe air-water surface, whereas gaseous transfervia permeation occurs in the three membraneequilibrators.“
• Szyler 2015 suggests dependency on degree of exchange per “path” through the equilibration vessel
Deg
ree
ofeq
uilib
rium
Time (s)
Comparison of degree of equilibrium for gases with 2 response times(20s, solid line; 300s, dotted line; difference, dashed line; from Szyler, 2015)
RESP
ONS
ETIMES
37
Problem• Sparingly soluble gases equilibrate slower
• In particular methane distribution can be patchy with l small scale differences
• CO quickly reacts to irradiation
• CO2 and N2O usually quite fast and similar
• Response time: system-dependent• For tCH4 / tCO2 : ~ 18 to 3
(Webb et al., 2016)
• Equilibrator always in disequilibrium(Johnson et al., 1999)
February 1st to 2nd
2014
Stepexperiment
Time [s]
0 500 1000 1500 2000
CH
4 [pp
m]
1,0
1,5
2,0
2,5
3,0
3,5
4,0
N2O
[ppb
]
200
400
600
800
1000
1200
1400
CO
2 [pp
m]
1000
2000
3000
4000
5000
6000
CH4N2OCO2
But first of all:
This istru
e for CO2as well ....
RESP
ONS
ETIMES
38W
HAT
ISM
EANT
BY2 µ A
TMAC
CURA
CY
So what is a correct value?
•Implications for“Quality Goals”,i.e. SOCAT, ICOS
Longitude (�E)
40TIMELA
GCO
RREC
TION
•Equilibrator always in disequilibrium
(Johnson et al., 1999)
•Bears potential to derive the “true” water value, assuming t is well known and deconvolution scheme is established
41
Correction for equilibration time
corrected methane concentration at
time t+1
Equations introduced in Johnson et al., 1999.Derived following Miloshevich, 2004.
observed apparent methane concentrations
for time t+1 and preceding measurement at time t
time interval between consecutive measurements at times t and t+1, small enough
that the methane concentration can be assumed to be approximately constant
gas and equilibrator (and T and S?) dependent
equilibrator time constant
TIM
ELA
GCO
RREC
TIO
N
43
Upwelling event: spatial lag
27th August 2014, transect direction is north to south
ALIGNING
UPWELLING
SIGNA
LS
47
Upwelling event: spatial lag
T decreasing (entering upwelling), CH4increasing.
ALIGNING
UPWELLING
SIGNA
LS
48
Upwelling event: spatial lag
T decreasing (entering upwelling), CH4increasing.
T increasing (leaving upwelling), no corresponding decrease in CH4
Rehder et al.: Continuous measurement of CH4 and pCO2 in the Baltic Sea using oa-ICOS on a VOSAL
IGNING
UPWELLING
SIGNA
LS
49
Upwelling event: spatial lag
T decreasing (entering upwelling), CH4increasing.
T increasing (leaving upwelling), no corresponding decrease in CH4
T steady (no longer in upwelling region), CH4decreasing with time lag
Rehder et al.: Continuous measurement of CH4 and pCO2 in the Baltic Sea using oa-ICOS on a VOSAL
IGNING
UPWELLING
SIGNA
LS
Response Time• VGas minimization not an option• Reduction of gas in equilibration
chamber reduces fraction of residence of gas in the active exchanging part
• Enhancement of k only option => get bigger
11
3.1 Stufenexperiment
Für das Stufenexperiment wurden zwei ca. 900 l fassende Wasserbassins mit Wasser
unterschiedlicher Methankonzentration gefüllt. Eine Wassertonne wurde mit normalen
Leitungswasser gefüllt, die andere Tone wurde mit zuvor Methanübersättigten Wasser versetzt.
Hierzu wurde 1 l Wasser in einer Schottflasche 5 Minuten mit 100% Methan begast. Aus der Flasche
wurden genau 40 ml entnommen und in die Wassertonne gegeben, welche anschließend mit
Leitungswasser auf 700 l aufgefüllt wurde. Um einen Austausch der beiden Wasserbassins mit der
Umgebungsluft zu verhindern, wurden Styroporplatten mit gasdichter Folie ummantelt und als
schwimmender Deckel auf die beiden Wasseroberflächen gegeben. Mit einer Druckwasserpumpe
wurde das Wasser zum Equilibrator gepumpt. Die Pumpe ließ sich über einen Druckminderer in ihren
Durchflussgeschwindigkeiten regeln. Als erstes wurde das Wasser aus der Tonne mit dem normalen
Leitungswasser zum Equilibrator gepumpt. Wenn sich ein konstanter Methangehalt eingestellt hat,
wurde mittels Dreiwegeventil zur zweiten Tonne umgeschaltet, welche ebenfalls bis zur Einstellung
eines konstanten Wertes gepumpt worden ist. Dann wurde erneut zum normalen Leitungswasser
umgeschaltet um so die Abklingkurve zu erhalten.
Abbildung 7: Beispiel eines Stufenexperiments bei einem Durchfluss von 9 l/min und einem zusätzlichen Gasdurchfluss von 4 l/min.
Im Moment des Wechsels der beiden Wasserbassins beginnt der Methangehalt in der Gasphase sich
dem Gleichgewichtspartialdruck des jeweils anderen Wasserbassinsgehalts anzunähern. Er bewegt
sich von der Konzentration p1 zum Zeitpunkt t0 mit der Geschwindigkeit pt zu p2 zum Zeitpunkt t1
bzw. von p2 zurück auf p1. Die Geschwindigkeit hängt vom Grad des Ungleichgewichts und der
Effizienz des Gasaustausches ab.
0,00000
5,00000
10,00000
15,00000
20,00000
25,00000
0 1000 2000 3000 4000 5000
[CH4
] in
ppm
Zeit in s
Stufenexperiment Flow 9 l/min
[CH4] ppm
[CH4] ppm
[CH4] ppm
p1 bei t=t0
p2 bei t=t1
p1 at t=t0
p2 at t=t1
Time (s)
14
Abbildung 10: Zeitkonstanten für Methan der verschiedenen Stufenexperimente
Man erkennt in Abbildung 10 ganz deutlich den Trend, dass mit zunehmender
Wasserdurchflussmenge sich das Gleichgewicht zwischen Wasser und Gasphase schneller einstellt.
Dies scheint jedoch kein linearer Trend zu sein sondern durch andere Faktoren eher limitiert.
Auffällig ist hier, dass ein zusätzlicher Gasstrom bei geringer Durchflussmenge mehr Einfluss auf die
Zeitkonstante nimmt als bei hoher. Ebenso erstaunlich ist, dass der geringere zusätzliche
Gasdurchfluss bei hohen Wasserdurchflussmengen mehr Gewicht hat als der, mit der höheren
zusätzlichen Gasdurchflussmenge.
Dass der Einfluss des zusätzlichen Gasdurchflusses bei hohen Wasserdurchflussmengen nicht mehr
den gravierenden Einfluss nimmt, lässt sich durch Beobachtung des Experiments erklären. Bei dem
Wasserflow von nahezu 11 l·min-1 bildeten sich auf der Wasseroberfläche im Equilibrator
Turbulenzen. Das Wasser schäumte beinahe. So hat man natürlich den Headspace automatisch
verkleinert und zusätzlich eine relativ große Austauschfläche geschaffen.
0
100
200
300
400
500
600
700
800
900
1000
3 5 7 9 11
τin
s
Durchfluss l/min
Vergleich τ CH4
250 ml/min Gasdurchsatz
4,7l l/min Gasdurchsatz
2,5l l/min Gasdurchsatz
Water flow (L/min)
Optimized (lab, aboard):t < 30 s (CO2)t < 230 s (CH4)
FASTER
EQUILIBR
ATIONBY
HARD
WAR
E
Take home messages– Surface equilibration technique is straightforward, but a few known
issues are usually neglected (response time, total pressure bias)– New sensors allow for measurement of a large variety of gases, e.g.
nitrous oxide and methane– Deconvolution schemes for time lag so far not usally used, and maybe
not needed in most cases– Response time, which can be assessed upon switch from calibration
mode to surface water measurement, should be part of metadata wherever possible
– CEAS measurement requires standard gases in natural air balance– Matrix effects complicate measurements, in particular addressing stable
isotope ratios
Take
aw
ays
Next generation instrumentation
55
- Instrumentation after ICOS funding– 3 laser-based detection systems– Los Gatos GGA (CH4 and CO2)– Los Gatos N2O/CO Analyser– Picarro 2131 (CO2, d13CCO2)– Spectrophotometric pH (BONUS PINBAL)
– Double oxygen optode
- High flow through system– Parallel setup of internal sensors – Additional short circuit air pump– High water flow-through (8.5 L/min)
61
Flow scheme, mainequilibration system withtwo Los Gatos gas sensors; Equilibration system withmain and help equilibrator
62
Flow scheme, secondequilibration set up withmain and helpequilibrator, one Picarrogas analyser, 2 oxygenoptodes and pH-spectrophotometer
Water mass characteristics
• Intermediate water of sectionA (north) distinct
• Intermediate water of sectionA with higher oxygen levels, both vs T and vs Depth
• Obviously coldest waters atsurface in section B
64
Step experimentdeconvolution
Data:Sampling rate 5stCH4 = 555sRed: „true concentration“Blue: recorded data
Time resolution t/5
Time resolution t/2
Time resolution t/10
115 km at 25knTIMELA
GCO
RREC
TION
65
The following notes briefly outline the calculations used to convert ring down times into absorption intensity. This is done automatically in Picarro CRDS analyzers and is included here only for completeness.The light signal at the photodetector is given byI(t, λ) = I0 e-t/τ( λ)
Where I0 is the transmitted light intensity at the time the laser is switched off and τ(λ) is the ring down time constant. For a given wavelength, λ, the decay rate,R (λ) = 1/τ(λ)is proportional to the optical losses inside the cavity and equal to the empty-cavity decay rate plus a factor dependent on the sample absorption:R (λ,C) = 1/(λ) = R (λ,O) + cε(λ)Cwhere R(λ) = 1/τo(λ) is the empty-cavity decay rate. The effective path length of the measurement is given byLeff = cτo(λ)where c is the speed of light. For typical mirrors having a reflectivity of 99.995% and scattering losses of less than 0.0005 percent, Leff can be over 10 km. For a cell length of 25cm, this is a pathlength enhancement factor of over 20,000.The sample absorption can be written as:α(λ) = ε(λ)CWhere ε is the extinction coefficient and C is the concentration. This can be found by taking the difference between the decay rates of an empty cavity (C = 0) and a cavity containing a sample, i.e.:α(λ) = 1/c[R(λ,c) - R(λ,0)]If the absorption cross section and lineshape parameters of the sample are known, then the concentration of the sample can be readily computed. For a more detailed mathematical treatment of cavity ring down spectroscopy, see K.W. Busch and M.A. Busch (1997). "Cavity ring-down spectroscopy: An ultratrace absorption measurement technique." ACS Symposium Series 720, Oxford.
This scheme of comparing the ring down time of the cavity without anyabsorbing gas, with the ring down time when a target gas is absorbing light isaccomplished not by removing the gas from the cavity, but rather by using a laser whose wavelength can be tuned. By tuning the laser to different wavelengths where the gas absorbs light, and then to wavelengths where thegas does not absorb light, the "cavity only" ring down time can be compared tothe ring down time when a target gas is contributing to the optical loss withinthe cavity. In fact, the laser is tuned to several locations across the targetgas's spectral absorption line (and ring down measurements are conducted at all these points) and a mathematical fit to the shape of that absorption line iswhat is actually used to calculate the gas concentration.
At Picarro we solved this problem by developing and patenting our ownwavelength monitor. It can measure absolute laser wavelength to a precision more than 1000 times narrower than the observed Doppler-broadened linewidth for small gas phase molecules. We then use this in a novel way - specifically, we lock the laser to the wavemeter, which wethen actively tune to known wavelengths. The result is higher spectralprecision than in any commercial spectrometer - laser-based orotherwise. This spectral precision is the key to the ultra-precise fitting oflineshapes and line heights necessary to reach parts per trillionconcentration sensitivity.
CEAS • Measures Absorption from the mirror-transmitted intensity
68
ND
IR a
ndG
O
oa-ICOS :This approach deliverssuperior performance, yet is orders-of-magnitude less sensitive to internal alignment of components and tovariations in local temperature andpressure
In addition, variations in the concentrations of a gas with a permanent dipole (like CO2 or H2O) can even cause subtlechanges in the shape of a non-overlapped line, forexample CH4 — a well-known effect called pressurebroadening.
Cavity Ring Down Spectroscopy • Cw pumping until threshold
• Patented wavelength monitor measureabsolute laser wavelength to a precisionmore than 1000 times narrower than theobserved Doppler-broadened linewidth forsmall gas phase molecules.
• Laser locked to the wavemeter, which wethen actively tune to known wavelength.
69
CRDS