Detection of tropospheric OH radicals by long‐path differential‐optical‐absorption spectroscopy: Experimental setup, accuracy, and precision

Abstract

Abstract. This paper describes a newly developed long-path differential-optical-absorptionspectroscopy instrument used for the measurement of tropospheric OH radicals. The instrument consists of a high resolution echelle spectrometer in conjunction with a multiple-reflection cell of 38.5 m base length and a UV laser light source that provides a spectral line width of 0.41 nm. Local in situ absorption measurements at total path lengths of either 1.85 or 3.1 km can be performed. The simultaneous observation of six atmospheric OH rotational absorption lines (Qi(2), Q21(2), R2(2), Ql(3), Q21(3), and Pl(1)) around 308 nm allows OH measurements with high specificity. A new method to accurately determine the precision and the detection limit of each individual OH measurement data point is presented. Presently, a 2(;-detection limit of 1.5x106 OH -3 cm is achieved (based on 1.85 km absorption path length and about 6 min integration time), which corresponds to a minimum detectable optical density of 2.5x10 -5. The absolute instrumental accuracy was calculated to be better than 6.5%, which emphasizes the qualification of the longpath absorption technique as an absolute method. Examples of field experiments are reported to illustrate the present performance.

Investigation of the formaldehyde differential absorption cross section at high and low spectral resolution in the simulation chamber SAPHIR

“…The Multichannel Scanning Technique (MCST) is used to reach the required signal to noise ratio . One air spectrum is recorded within 135 s. A comparison of measured and modelled MCST spectra is given by Hausmann et al (1997) who found a maximum 1.7% error contribution to the accuracy of the measurement. The deconvolution of the measured spectrum is performed by fitting a trigonometric background consisting of 16 terms and three reference spectra, namely OH, HCHO, and an so far unidentified absorber X (Schlosser et al, 2006).…”

“…An effective cross section for broad-band-DOAS can be calculated using any of these cross sections by convolution with an appropriate instrument function. For the high-resolution DOAS (HR-DOAS) instrument designed to detect tropospheric OH radicals (Hausmann et al, 1997;Brauers et al, 2001;Schlosser et al, 2006) no literature data at sufficiently high spectral resolution (2.7 pm) is available. However, a comparison between the highest resolution literature data of Smith et al (2006) and the HR-DOAS data is given in the discussion of this paper .…”

Investigation of potential interferences in the detection of atmospheric RO<sub><i>x</i></sub> radicals by laser-induced fluorescence under dark conditions

“…Only one instrument currently exists for atmospheric OH detection, which is installed in the atmosphere simulation chamber SAPHIR in Jülich. Details of the instrument can be found elsewhere (Hausmann et al, 1997;Schlosser et al, 2007Schlosser et al, , 2009.…”

Comparison of OH concentration measurements by DOAS and LIF during SAPHIR chamber experiments at high OH reactivity and low NO concentration

“…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.…”

“…Besides OH, also formaldehyde, sulfur dioxide, and naphthalene can be simultaneously detected. Details of the instrument can be found elsewhere (Hausmann et al, 1997;Schlosser et al, 2007Schlosser et al, , 2009. 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 0.5 nm.…”