DOAS

Differential Optical Absorption Spectroscopy

The OH-DOAS Instrument

The unique Jülich DOAS instrument was deployed in field experiments in the past (Brandenburger et al., 1998; Brauers et al., 2001) and is now permanently installed at the SAPHIR chamber. It provides inherently calibration-free measurements of tropospheric OH radicals with an 1σ accuracy of 6.5% (Hausmann et al., 1997) and is therefore accepted as a reference technique for OH.

Besides OH also formaldehyde, sulfur dioxide, and naphthalene can be simultaneously detected. Details of the instrument and comparison with other OH detection techniques were published by Hausmann et al., 1997, Schlosser et al., 2007/2009, and Fuchs et al. 2012.

The output of a dye laser (616.08 nm, pulse duration 800 fs) is frequency doubled in an external BBO crystal to generate broad-band UV radiation at 308.04 nm with a bandwidth of approximately 0.5nm. The dye laser is synchronously pumped by a picosecond, passively mode-locked, diode-pumped Nd:YAG laser with internal frequency doubling to 532 nm. The repetition rate is 82.2 MHz with a timing jitter of less than 1 Hz controlled by a phase-locked-loop feedback system.

The absorption signal is detected using a high resolution Echelle spectrometer (spectral resolution Δλ = 2.7 pm, f = 1.5 m), which is coupled to a cooled photodiode array detector. The spectral detection interval of 0.25 nm comprises five OH absorption lines.

The absorption path in the SAPHIR chamber has a length of 2240 m and is realized by a passively stabilized optical multiple reflection cell (modified White cell type), whose mirrors have a distance of 20m. The UV laser power is held well below 1-2 mW, in order to suppress significant self-generation of OH radicals, which could be produced by the UV radiation field within the White cell at high ozone concentrations. Additionally, the level of OH is checked at the end of each experiment, when the chamber roof is closed and the OH concentration is expected to be zero.

The detection limit and the precision of each measurement is mainly limited by the residual structures in the spectra. Absorption spectra are evaluated using the Multi-Channel-Scanning-Technique (MCST) (Brauers et al., 1995), which enables the detection of minimal optical densities of the order of 1x10^-5 (RMS) in 100 s integration time.

The total measurement time for one data point is about 200 s due to the acquisition time of the spectra, time needed to turn the spectrograph’s grating, and additional computing time. The precision of each single measurement is calculated from the spectral residuum as described in Hausmann et al. (1999). A statistical analysis of spectra acquired during zero air periods of experiments in 2011 revealed a mean 1σ detection limit of 7.3x10⁵ cm^-3.

The accuracy of the instrument depends on the stability of the spectral resolving power and the repeatability of the wavelength scanning mechanism of the Echelle spectrometer. Both are periodically checked by comparison of the OH reference spectrum that is used for data evaluation with actually measured spectra of OH. For this purpose, OH radicals are formed within the White cell by photolysis of water vapor by the 185 nm radiation of a low pressure mercury lamp. Although the OH-DOAS instrument is optimized for the detection of hydroxyl radicals, the instrument is also used for the intercomparison of formaldehyde (HCHO) detection instruments.References

References

Brandenburger, U., T. Brauers, H. P. Dorn, M. Hausmann, and D. H. Ehhalt (1998), In-situ measurements of tropospheric hydroxyl radicals by folded long-path laser absorption during the field campaign POPCORN, J. Atmos. Chem., 31(1-2), 181-204.

Brauers, T., M. Hausmann, A. Bister, A. Kraus, and H. P. Dorn (2001), OH radicals in the boundary layer of the Atlantic Ocean 1. Measurements by long-path laser absorption spectroscopy, J. Geophys. Res., 106(D7), 7399-7414.

Hausmann, M., U. Brandenburger, T. Brauers, and H. P. Dorn (1997), Detection of tropospheric OH radicals by long-path differential-optical-absorption spectroscopy: Experimental setup, accuracy, and precision, J. Geophys. Res., 102(D13), 16011-16022.

Schlosser, E., Bohn, B., Brauers, T., Dorn, H.-P., Fuchs, H., Häseler, R., Hofzumahaus, A., Holland, F. Rohrer, F., Rupp, L.O., Siese, M., Tillmann, R., Wahner, A.  (2007), Intercomparison of two hydroxyl radical measurement techniques at the Atmosphere Simulation Chamber SAPHIR, J. Atmos. Chem., 56(2), 187-205., ,

Schlosser, E., Brauers, T., Dorn, H.-P., Fuchs, H., Häseler, R., Hofzumahaus, A., Holland, F., Wahner, A., Kanaya, Y., Kajii, Y., Miyamoto, K., Nishida, S., Watanabe, K., Yoshino, A., Kubistin, D., Martinez, M., Rudolf, M., Harder, H., Berresheim, H., Elste, T., Plass-Dülmer, C., Stange, G., Schurath, U., (2009), Technical Note: Formal blind intercomparison of OH measurements: results from the international campaign HOxComp, Atmos. Chem. Phys., 9(20), 7923-7948

Fuchs, H., Dorn, H.-P., Bachner, M., Bohn, B., Brauers, T., Gomm, S., Hofzumahaus, A., Holland, F., Nehr, S., Rohrer, F., Tillmann, R., Wahner, A. (2012)
Comparison of OH concentration measurements by DOAS and LIF during SAPHIR chamber experiments at high OH reactivity and low NO concentration
Atmos. Meas. Tech., 5, 1611-1626