A New In Situ Laser Long-Path Absorption Instrument for the Measurement of Tropospheric OH Radicals

Abstract

The authors describe a high-resolution long-path differential optical absorption spectrometer developed for the measurement of tropospheric hydroxyl radical concentrations. The instrument uses an atmospheric absorption light path of up to 3000-m total length that is folded into an optical multiple-reflection cell of 20-m base length. A frequency-doubled picosecond laser system serves as a light source, which has a large spectral bandwidth of 0.41 nm (FWHM) centered around 308 nm. In combination with a high-resolution Echelle spectrograph and an optical multichannel detector, a spectral range of 0.26 nm which comprises six OH absorption lines of the A(2) Sigma(+), (nu' = 0) <-- X(2) Pi, (nu '' = 0) transition, can be simultaneously recorded. This large spectral detection range facilitates the subfraction of interfering absorption signals of other atmospheric trace gases (SO2, HCHO, naphthalene) and significantly improves the detection specificity for hydroxyl radicals. A 2 sigma OH detection limit of 1.5 X 10(6) OH cm(-3) for an integration time of 15 minutes and a light pathlength of 2880 m was inferred from held measurements. Using this instrument first,absolute local in situ OH absorption measurements have been performed recently.