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AOSC 634
Air Sampling and Analysis
Lesson
Cavity Ring Down Spectroscopy
A source of terrific sensitivity.
Copyright Dickerson 2015
Quick Review
How lasers work
Light Amplification by Stimulated Emission of Radiation.
Generate light with great spatial and spectral coherence.
Produce tight pattern over distance
Narrow wavelength interval.
Requires:
A population inversion
Resonant optical cavity to generate a standing wave.
How lasers work
A gain medium must be pumped into an inversion inside a
resonant cavity.
The medium could be
Solid Nd-YAG, (Yttrium Aluminum Garnet) or ruby.
Liquid such as a dye
Gas such as a mixture of He and Ne or N2 and CO2.
Pumping can be supplied by a flash lamp, electrical
discharge, or chemical reaction.
The resonant cavity is usually a highly (99.9%) reflective and
a semi reflective (99%) mirror.
Example: He-Ne laser
• A mixture of He and Ne (10:1 at ~1 hPa) is exposed to a
high voltage, low current electric field (think neon sign).
• The electric field generates a plasma, mostly of excited He
atoms.
• The excited He atoms collide with Ne and transfer their
energy.
• The excited Ne atoms relax to the ground state, emitting
light.
How lasers work
In the plasma, He is excited to the metastable 21S0 state that just
happens to have the same energy as Ne with an electron in the 5s
orbital. The population of excited states exceeds that of the ground
state, forming an inversion. The transition from 5s to 3p is rapid and
is accompanied by emission of 633 nm (red) light.
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A pair of mirrors provides the optical cavity.
A standing wave is formed.
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Cavity ringdown uses a laser and
a laser cavity.
Effective path lengths of km’s are
possible.
Example: NO2
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CRDS Method
11 1 
N    
c  o 
t
I (t )  I o exp  
 o 
t
I (t )  I o exp  
 
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Chemiluminescence is the Customary
Method to Measure NOx or NOY
A. 375oC Mo Catalyst
– Actually measures NOy
– Widely used in NOx monitoring
networks
– 50 pptv/2 min detection limit
– 10% uncertainty
B.
–
–
–
Zero
Air
Photolysis
Specific to NO2
LEDs impart little heat
Conversion efficiency <100%
Cooled PMT
NO + O3 NO2 +
O3
O3
O3 O3
O3
NO2 NO
Sample
Air
Exhaust
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Variety of Methods to Measure NO2
DOAS3
TDLAS4
A specific, reliable, fast, economical method for
monitoring
NO2 in rural and urban environments is
1
MIESR
needed
LIF2
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CRDS Method
• Empty Cavity:
• R = mirror reflectivity
• l = length of the cavity
• K = coefficient of miscellaneous
scattering or absorption
dI
c
t
  I [(1  R )  K ] I (t )  I o exp(  )
dt
l
0
• Cavity with Analyte:
dI
c
  I [(1  R )  Nl  K ]
dt
l
t
I (t )  I o exp(  )

11 1 
N    
c  o 
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Los Gatos NO2 CRDS instrument.
Laser out
Mirror
Cavity
Mirror
Detector
Inlet Outlet
Laser
30 cm
Inlet
Pressure
Controller
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NO2
Molecule
Absorption5
Cross Section
[cm2 molecule-1]
SO2
3x10-24
PAN
5x10-21
O3
2x10-23
H2 O
1x10-25
Kirsme et al.,
JGR (1997)
• Single wavelength diode 405 nm laser
– Technique is insensitive to lamp fluctuations
• Highly reflective, low curvature mirrors result in ~ 1 km
path-lengths
– Highly sensitive
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Before the instrument is reliable:
• Characterize Instrument
–
–
–
–
Drift
Zero
Calibration
Response Time
• Modify Instrument for increased sensitivity
– Remove interferences: Water, Particles
– Establish a background: τo
• Compare to chemiluminescence
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Modifications: Chemical zero.
• FeSO4
– Cheap
– Drawback: powder,
H2O sensitive
• Metal oxide O3
Scrubber
– Efficient and already
in easy to install
casing
– Proprietary mixture of
metal oxides
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Gas Phase Titration Calibration
GPT: Mixed a small flow
of NO-in-Nitrogen
with excess ozone
Calculated [NO2] from:
•
Monitored change in
ozone (equal to
[NO2])
•
From standard
dilution flow rates.
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Summary of Performance Statistics
Detection Response Power
Limit
Time
Draw
Internal
Dimensions
Pressure
Weight
NO2 to
NO
[pptv]
[s]
[W]
[torr]
WxHxD
[cm]
[kg]
CRDS Analyzer
60 (60 s)
18 (95%)
90
170
42.5 x 21.9
x 58.4
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NOAA
Chemiluminescence
100 (60 s)
3 (95%)
1000
30
42 x 33 x
58.4
~30
Photolysis
Thermo
Electron Corp.
Model 42i TL
50 (120 s)
60
300
200-450
42.5 x 21.9
x 58.4
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Hot Mo
The fixed internal pressure, low power draw, and compact size
of the CRDS instrument makes it ideal for aircraft use at
altitudes up to ~10 km.
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Intercomparison Study
• Measured alongside the NOAA
chemiluminescence device from January 516, 2009 outside the UMD laboratory
window
– Sampling next to parking lot close to car
exhaust (very dirty!)
– Calibrated both instruments every other day
using GPT
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NO
NO2 by Chemi.
NO2 by CRDS
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y = 0.96x + 0.28
y = 0.93x - 0.61
R = 0.995
R = 0.982
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• Fluctuations in
the power supply
resulted in
variations in the
pumping speed,
and instrument
sensitivity
• Sharp increase in
CRDS noise
associated with
particles entering
the cavity
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References
• Modification of a Commercial Cavity Ring-Down Spectroscopic NO2
Detector for Enhanced Sensitivity, P. Castellanos, W. T. Luke, P. J. Kelley,
J. W. Stehr, S. H. Ehrman, and R. R. Dickerson, Rev. Sci. Instrum., 80(11),
DOI: 10.1063/1.3244090, 2009.
• Evaluation of the use of a commercially available cavity ringdown
absorption spectrometer for measuring NO2 in flight, and observations over
the Mid-Atlantic States during DISCOVER-AQ, L.C. Brent, W.J. Thorn, M.
Gupta, B. Leen, J.W. Stehr, H. He, H.L. Arkinson, A. Weinheimer, C.
Garland, S.E. Pusede, P.J. Wooldridge, R.C. Cohen, R.R. Dickerson, J.
Atmos. Chem., DOI 10.1007/s10874-013-9265-6, 2013.
• High-accuracy continuous airborne measurements of greenhouse gases (CO2
and CH4) using the cavity ring-down spectroscopy (CRDS) technique, Chen,
H.; Winderlich, J.; Gerbig, C.; et al. ATMOSPHERIC MEASUREMENT
TECHNIQUES Volume: 3 Issue: 2 Pages: 375-386, 2010.
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PICARRO G2401
Picarro GHG analyzer
CRDS Analyzer
CO2IR
+ laser
CO + CH4 + H2O
• Near
The world’s only field-deployable analyzer capable of
• 0.25
m cell
with
pathtrace
length
measuring
the four main
atmospheric
gases
simultaneously and continuously.
up to 20 km.
• Wavelength scanning for
selectivity.
• Ringdown time is
independent of laser
intensity.
• Precision in ppb range.
•
Global #1 in precision, accuracy, and portability
•
Capable of meeting WMO Data Quality Objectives for CO, CO2 & CH4
•
Guaranteed lowest drift of any continuous greenhouse gas measurement instrument
•
Unique water correction automatically reports dry gas mol fraction
Meeting
the
WMO
Standard:
The
World
Meteorological Organization has published exact
requirements on the precision of greenhouse gas and
carbon monoxide measurements made at its Global
Atmosphere Watch (GAW) stations and the G2401
provides the level of precision needed to meet these
standards. The unique combination of continuous 4species
measurement,
high
precision,
field
deployability, and long-term reliability makes the G2401
the instrument of choice for greenhouse gas
measurements.
Picarro’s Patented CRDS Technology: The heart of
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Picarro CO2 Detector
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• Wavelength is scanned to measure ringdown time
on and off absorption peaks.
• Temperature, pressure, and water vapor are
monitored for correction.
• Advertised uncertainty 0.050 ppm (out of 400).
• Outstanding stability and sensitivity – at a price.