H.-P. Dorn

In‐situ detection of tropospheric OH radicals by folded long‐path laser absorption. Results from the POPCORN Field Campaign in August 1994

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.

Intercomparison of tropospheric OH radical measurements by multiple folded long‐path laser absorption and laser induced fluorescence

An intercomparison of in-situ OH measurements by differential optical absorption spectroscopy (DOAS) and laser-induced fluorescence spectroscopy (LIF) was carried out in August 1994 in a clean rural environment in North-East Germany. A large data set of temporally overlapping OH measurements with well defined measurement errors was obtained and compared. Both instruments encountered the same air masses, except when the wind came from NNW and caused a perturbation of the DOAS measurements. Excluding that wind sector, the weighted regression analysis of 137 data pairs (70% of all available data pairs) yields a linear relationship between the DOAS and LIF measurements with a correlation coefficientr = 0.90. The unity slope (1.01±0.04) and the non-significant intercept (0.28±0.15) × 106 cm−3 demonstrate that both OH instruments agreed excellently in their calibrations and accurately measured OH.

Tropospheric OH concentration measurements by laser long-path absorption spectroscopy

Tropospheric OH radicals have been detected by broadband laser long-path absorption spectroscopy. This technique has been improved by application of a low-noise multichannel spectrometric detector to record OH absorption spectra. The simulataneous observation of at least three rotational lines (Q1 (2), Q21 (2), R2 (2) or Q1 (3), Q21 (3), P1 (1)) of the OH (A2Σ+ ← X2Π) (0,0) vibrational band at 308 nm and the spectra evaluation by fitting OH reference spectra for proper atmospheric conditions allows a specific OH concentration determination. Furthermore, the broad spectral range of the detector system facilitates the identification and elimination of interfering absorption structures, which are produced by atmospheric trace gases like SO2, CH2O, CS2 or naphthalene in polluted air. Diurnal OH concentration profiles have been observed during various field campaigns showing different temporal shapes. These are in qualitative agreement with the primary net production process of OH from ozone u.v.-photolysis and the primary net destruction of hydroxyl radicals by the reaction with NO2 at high NOx levels.