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

+

• 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.

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

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cm1090.1

%08.0

AB

cw CRDS apparatus

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

Monitoring C2H2 in lab air

Wednesday 09/04/08

Sunday 06/04/08

Monitoring atmospheric C2H2

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.

Optical feedback CRDS

Morville et al., Appl. Phys. B 78, 465 (2004)

OF-CRDS of single aerosol particles

4 m melamine resin spheres

0 100 200 300 400

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Ext

inct

ion

Coe

ffici

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cm-1

Time / ms

T.J.A. Butler et al., J. Chem. Phys. 126, 174302 (2007)

0 100 200 300 400

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Ext

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OF-CRDS of single aerosol particles

260 262 264 266 268 270 272 274 276

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/ 1

0-6 cm

-1

Time / ms

Mie = 3.8 10-7 cm2

Exp = 3.2 10-7 cm2

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

• 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

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

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

AcknowledgementsManik Pradhan Timothy ButlerRoberto Grilli Daniel MellonMd. Aziz Jin Kim

EU Marie Curie Early Stage Training Centre BREATHE

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)

~0.4% of the scattered intensity will be re-trapped in the TEM00 mode of the optical cavity.

Differential scattering

Cavity ring-down spectroscopy

Ldc ]X[

011

For an empty cavity:

With an absorber:

R10

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Allan variance analysis 212212

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