Transcript ajoe cw crds talk.ppt

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
Detection of atmospheric C2H2
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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 n1 + n3 band at 1535.393 nm
+
• CRDS detection limit in 1 atm air ~ 2.5 ppbv (t = 14 ms)
using DFB diode laser.
CRDS detection limits
• Limiting absorption coefficient:
L tmin
min 
cd t0 t0
tmin
 0.08%
t0
min  1.90 10 9 cm1
• Allan Variance analysis to optimize averaging;
• Pressure broadening (g = 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.
cw CRDS apparatus
A
B
Tests of cw CRDS measurements
• Comparison of cw CRDS
and GC-FID for indoor air
sample
• Apel Reimer standard
mixture of 75 VOCs (C2 –
C11)
cw CRDS
3.87 ± 0.22 ppbv
cw CRDS
8.6  0.6 ppbv
GC-FID
3.90 ± 0.23 ppbv
Manufacturer
8.7  0.05 ppbv
M. Pradhan et al., Appl. Phys. B 90, 1 (2008)
Monitoring C2H2 in lab air
Wednesday 09/04/08
Sunday 06/04/08
Monitoring atmospheric C2H2
Optical properties of aerosol particles
Prior 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.
Optical feedback CRDS
Morville et al., Appl. Phys. B 78, 465 (2004)
-6
Extinction Coefficient / 10 cm
-1
OF-CRDS of single aerosol particles
18
16
14
12
4 mm melamine
resin spheres
10
8
6
4
2
0
0
100
200
Time / ms
T.J.A. Butler et al., J. Chem. Phys. 126, 174302 (2007)
300
400
OF-CRDS of single aerosol particles
18
16
sMie = 3.8  10-7 cm2
14
-6
Extinction Coefficient / 10 cm
18
-6
-1
sExp = 3.2  10-7 cm2
 / 10 cm
-1
12
10
8
6
4
2
16
0
14
260
262
264
266
268
270
272
274
Time / ms
12
10
8
6
4
2
0
0
100
200
Time / ms
300
400
276
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)
Statistics of aerosol extinction
700-nm diameter polystyrene spheres
2
Var    min

sext

V
• From Gaussian beam theory,
calculate V = 0.374 cm3
• Mie scattering prediction:
sext = (2.970.07)  10-9 cm2
• From fit to data:
sext = (2.71 0.05)  10-9 cm2
Aerosol extinction cross sections
Particle
diameter / nm
Size parameter
x = 2pr/l
sexp
/ 10-9 cm2
scalc
/ 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
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
Acknowledgements
Manik Pradhan
Roberto Grilli
Md. Aziz
Timothy Butler
Daniel Mellon
Jin Kim
EU Marie Curie Early
Stage Training Centre
BREATHE
OF-CRDS of single aerosol particles
Laser beam
Water aerosol droplets

  ( vt sin )2 
b2 
 2s 
( t )   2  exp  2 2  exp 

2
 pw L 
w
 w 


Height
Width
T.J.A. Butler et al., J. Chem. Phys. 126, 174302 (2007)
Differential scattering
~0.4% of the scattered intensity will
be re-trapped in the TEM00 mode of
the optical cavity.
Cavity ring-down spectroscopy
For an empty cavity:
L
t0 
c 1  R 
With an absorber:
1 1
t t0
d
 c [ X] s
L
Allan variance analysis
s2A  21 A 2  A12