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.

Ground based in‐situ measurements of tropospheric hydroxyl radicals were conducted by folded long‐path laser absorption as part of the field campaign POPCORN in August 1994. The OH instrument used an open optical multiple‐reflection cell of 38.5 m base length through which the laser beam was passed up to 80 times. The broadband emission of a short‐pulse UV laser together with a multichannel detection system allowed the simultaneous observation of six OH absorption lines in a spectral interval of Δλ≃0.24 nm at 308.1nm (A²Σ+,υ′ = 0← X²Π,υ″ = 0 transition). Along with the OH radicals, the trace gases SO2, HCHO, and naphthalene were measured by this technique. The large spectral detection range covered a multitude of rotational absorption lines of these trace gases which were all used for multicomponent analysis, thus allowing a specific and sensitive detection of tropospheric OH radicals. An average 2σ detection limit of 1.5 × 106 OH/cm³ for an integration time of 200 seconds and an absorption light path length of 1848 m was determined from the field measurements. In total, 392 OH data were obtained by long‐path absorption during 16 days of field measurements. The observed OH concentrations reached peak values of 13 × 106 cm−3 at noon.

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