Field Methods of Monitoring Atmospheric Systems Chp. 3 – UV-Visible Differential Optical Absoprtion Spectroscopy Copyright © 2006 by DBS

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Introduction Most widely used technique –Surface, airborne and satellites Active: Light source can be artificial (e.g. Xe lamp or laser) –Long-path (LP-DOAS) or active DOAS –Multi-pass cell DOAS Passive: Light source is extra-terrestrial (sun, moon, stars) –Direct –Indirect (scattered by moleules (Rayleigh) or particles (Mie))

Desirable Characteristics 1. Sensitive to work at low concentrations –Mixing ratios < 1 ppt ~ 2.4 x 10 7 molecules -3 ) at 1 atm, 293 K 2. Able to measure a particular species with high specificity –Avoids interferences from similar compounds 3. Measure with high time resolution –Species are not spatially homogeneous

Principles of DOAS The Beer-lambert law OD i = σ i c i l OD i = optical density, σ i = absorption cross-section, c i = concentration, l = pathlength (for species i) Corrections are made for Rayleigh and Mie scattering

Principles of DOAS Differential absoprtion spectroscopy Unattenuated atmospheric spectrum (I 0 ) reduced by Rayleigh and Mie scattering –Cannot directly apply Beer-Lambert Differential absorption can be used if the absorption spectrum of the species of interest contains marked structure within a fairly narow wavelegth range (10 nm or less) σ i = σ i ’ + σ i s Where σ i = absolute cross-section, σ i ’ = differential absorption cross-section, σ i s = slowly varying component Differential structure is much smaller than absolute OD i’ ’ = σ i ’ c i l ‘fingerprint’ of molecular species

Principles of DOAS Selecting Wavelength regions Transmission is limited by O 3, O 2 absorption and Rayleigh scattering Require significant absorption structures –More imporatnts than size of absolute differential absorption cross-sections Spectral characteristics of light source may interfere

Principles of DOAS Application to atmospheric measurements Heard, 2006

DOAS Using Artificial Light Sources Broad-band light sources l

h

DOAS Using Artificial Light Sources Laser sources l

Cavity ring-down spectroscopy l

DOAS Using Scattered Sunlight Spectroscopic principles Spectroscopic principles Zenith-viewing geometry: Retrieval of NO 3 viewing profiles Multi axis DOAS

DOAS Using Scattered Sunlight Zenith-viewing geometry: Retrieval of NO 3 viewing profiles l

DOAS Using Scattered Sunlight Multi axis DOAS l

DOAS Using Scattered Sunlight Tom-DOAS LP-DOAS measures total column abundance Spatial distribution gained by using intersecting light paths and applying tomographic analysis 2-D map of vehicle exhaust NO 2 Pundt et al. (2005)

DOAS Using Scattered Sunlight Airborne MAX-DOAS Upward-viewing telescope scans atmosphere above flight altitue Downward telescopes collects scattered sunlight from both above and below

Summary

Further Reading Journal articles Petritoli, A. (2002) Platt, U. (1994) Differential optical absorption spectroscopy (DOAS), in Sigrist, M.W. (ed.), Air monitoring by spectroscopy techniques, John Wiley, London, pp Plane, J.M.C. and Smith, N. (1995) Automatic monitoring by differential and optical absorption spectroscopy, in Clark, R.J.H. & Hester, R.E. (eds.), Spectrosc. Environ. Sci., John Wiley, London, pp Pundt (2005)

Text Books Finlayson-Pitts, B.J., and Pitts, J.N. (2000) Chemistry of the Lower Atmosphere: Theory, Experiments and Applications. Academic Press, San Diego. Heard, D.E. (ed.) (2006) Analytical Techniques for Atmospheric Measurement. Blackwell Publishing. Hollas, J.M. (1996) Modern Spectroscopy. John Wiley & Sons Ltd., New York.