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Measurement of trace atmospheric constituents by cw cavity ring-down spectroscopy
A.J. Orr-Ewing, M. Pradhan, R. Grilli, T.J.A. Butler, D. Mellon, M.S.I. Aziz and J. Kim
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Detection of atmospheric C2H2
+
• Atmospheric C2H2 has mostly anthropogenic sources• Atmospheric lifetimes ~ days to weeks• Tracer for polluted air masses• Mixing ratios 0.8 – 2.5 ppbv in rural areas• Monitor via P(17) line of 1 + 3 band at 1535.393 nm
• CRDS detection limit in 1 atm air ~ 2.5 ppbv ( = 14 s) using DFB diode laser.
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CRDS detection limits
• Limiting absorption coefficient:
• Allan Variance analysis to optimize averaging;• Pressure broadening ( = 0.073 cm-1 atm-1) work at
reduced sample pressure;• Trapping and pre-concentration ( 25) of C2H2 from air;
• Detection limit for C2H2 is 8 pptv.
0
min
0min
cd
L
19min
0
min
cm1090.1
%08.0
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AB
cw CRDS apparatus
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Tests of cw CRDS measurements
• Apel Reimer standard mixture of 75 VOCs (C2 – C11)
• Comparison of cw CRDS and GC-FID for indoor air sample
cw CRDS 8.6 0.6 ppbv
Manufacturer 8.7 0.05 ppbv
M. Pradhan et al., Appl. Phys. B 90, 1 (2008)
cw CRDS 3.87 ± 0.22 ppbv
GC-FID 3.90 ± 0.23 ppbv
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Monitoring C2H2 in lab air
Wednesday 09/04/08
Sunday 06/04/08
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Monitoring atmospheric C2H2
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Optical properties of aerosol particlesPrior work by Atkinson (Portland), Ravishankara (NOAA), Strawa (NASA-Ames) and others on aerosol extinction by CRDS.
• Statistical fluctuations for low particle number densities dominate the uncertainty in extinction measurements.
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Optical feedback CRDS
Morville et al., Appl. Phys. B 78, 465 (2004)
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OF-CRDS of single aerosol particles
4 m melamine resin spheres
0 100 200 300 400
0
2
4
6
8
10
12
14
16
18
Ext
inct
ion
Coe
ffici
ent /
10-6
cm-1
Time / ms
T.J.A. Butler et al., J. Chem. Phys. 126, 174302 (2007)
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0 100 200 300 400
0
2
4
6
8
10
12
14
16
18
Ext
inct
ion
Coe
ffici
ent /
10-6
cm-1
Time / ms
OF-CRDS of single aerosol particles
260 262 264 266 268 270 272 274 276
0
2
4
6
8
10
12
14
16
18
/ 1
0-6 cm
-1
Time / ms
Mie = 3.8 10-7 cm2
Exp = 3.2 10-7 cm2
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Measurements for multiple particles
• Poisson statistics to treat variance of extinction• Allow for Gaussian intensity profile• Extinction depends on positions of particles in laser beam• Phase of cavity standing wave has further effects
J.L. Miller and AJOE, J. Chem. Phys. 126, 174303 (2007)
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Statistics of aerosol extinction
700-nm diameter polystyrene spheres
• From Gaussian beam theory, calculate V = 0.374 cm3
• Mie scattering prediction: ext = (2.970.07) 10-9 cm2
• From fit to data:ext = (2.71 0.05) 10-9 cm2
V
Var ext2min
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Aerosol extinction cross sections
Particle diameter / nm
Size parameter x = 2r/
exp
/ 10-9 cm2
calc
/ 10-9 cm2
707 8.5 1.35 0.01 2.71 0.05 2.97 0.07
499 6.5 0.95 0.01 0.485 0.010 0.49 0.01
404 5.9 0.77 0.01 0.15 0.07 0.146 0.004
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Conclusions
• Quantitative trace gas sensing in pptv – ppbv range.
• Mid-IR sources (e.g., DFG, QCLs) may improve detection limits for VOCs and other compounds.
• Aerosol optical extinction – quantitative retrieval of optical properties for size-selected particles.
• At higher extinctions, variance of fits to ring-down decays becomes significant.#
• A major challenge is to separate scattering and absorption losses.
# K.K. Lehmann and H. Huang, private communication
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AcknowledgementsManik Pradhan Timothy ButlerRoberto Grilli Daniel MellonMd. Aziz Jin Kim
EU Marie Curie Early Stage Training Centre BREATHE
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Laser beam
Water aerosol droplets
2
2
2
2
2 w)sinvt(exp
wb2exp
Lw2)t(
Height Width
OF-CRDS of single aerosol particles
T.J.A. Butler et al., J. Chem. Phys. 126, 174302 (2007)
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~0.4% of the scattered intensity will be re-trapped in the TEM00 mode of the optical cavity.
Differential scattering
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Cavity ring-down spectroscopy
Ldc ]X[
011
For an empty cavity:
With an absorber:
R10
cL
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Allan variance analysis 212212
A AA