Measurement of trace atmospheric constituents by cw cavity ring-down spectroscopy

1. 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

2. 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 n1 + n3 band at 1535.393 nm • CRDS detection limit in 1 atm air ~ 2.5 ppbv (t = 14 ms) using DFB diode laser. +

3. CRDS detection limits • Limiting absorption coefficient: • 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.

4. B A cw CRDS apparatus

5. 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) M. Pradhan et al., Appl. Phys. B 90, 1 (2008)

6. Monitoring C2H2 in lab air Wednesday 09/04/08 Sunday 06/04/08

7. Monitoring atmospheric C2H2

8. 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.

9. Optical feedback CRDS Morville et al., Appl. Phys. B 78, 465 (2004)

10. OF-CRDS of single aerosol particles 4 mm melamine resin spheres T.J.A. Butler et al., J. Chem. Phys. 126, 174302 (2007)

11. OF-CRDS of single aerosol particles sMie = 3.8  10-7 cm2 sExp = 3.2  10-7 cm2

12. 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)

13. Statistics of aerosol extinction 700-nm diameter polystyrene spheres • 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

14. Aerosol extinction cross sections

15. 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

16. Acknowledgements Manik Pradhan Timothy Butler Roberto Grilli Daniel Mellon Md. Aziz Jin Kim EU Marie Curie Early Stage Training Centre BREATHE

17. OF-CRDS of single aerosol particles Laser beam Water aerosol droplets Height Width T.J.A. Butler et al., J. Chem. Phys. 126, 174302 (2007)

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

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

20. Allan variance analysis