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 C 2 H 2 + Atmospheric C 2 H 2 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 band at 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 (  = cm -1 atm -1 )  work at reduced sample pressure; Trapping and pre-concentration (  25) of C 2 H 2 from air; Detection limit for C 2 H 2 is 8 pptv.

A B 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 CRDS3.87 ± 0.22 ppbv GC-FID3.90 ± 0.23 ppbv

Monitoring C 2 H 2 in lab air Wednesday 09/04/08 Sunday 06/04/08

Monitoring atmospheric C 2 H 2

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)

OF-CRDS of single aerosol particles 4  m melamine resin spheres T.J.A. Butler et al., J. Chem. Phys. 126, (2007)

OF-CRDS of single aerosol particles  Mie = 3.8  cm 2  Exp = 3.2  cm 2

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, (2007)

Statistics of aerosol extinction 700-nm diameter polystyrene spheres From Gaussian beam theory, calculate V = cm 3 Mie scattering prediction:  ext = (2.97  0.07)  cm 2 From fit to data:  ext = (2.71  0.05)  cm 2

Aerosol extinction cross sections Particle diameter / nm Size parameter x = 2  r/  exp / cm 2  calc / cm             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 PradhanTimothy Butler Roberto GrilliDaniel Mellon Md. AzizJin Kim EU Marie Curie Early Stage Training Centre BREATHE

Laser beam Water aerosol droplets Height Width OF-CRDS of single aerosol particles T.J.A. Butler et al., J. Chem. Phys. 126, (2007)

~0.4% of the scattered intensity will be re-trapped in the TEM 00 mode of the optical cavity. Differential scattering

Cavity ring-down spectroscopy For an empty cavity: With an absorber:

Allan variance analysis