Diurnal variability and night detection of stratospheric hydroxyl radical from far infrared emission measurements

We report the measurement of the stratospheric hydroxyl radical (OH) performed with a balloon-borne far-infrared Fourier Transform Spectrometer. Continuous observations were made, on a mid-latitude air mass, between 15 km and about 38 km, with a cycling limb-scanning observation technique. The measurement period commenced before the local noon and ended just before sunrise on the following day, permitting nearly complete observations of the diurnal OH cycle. A retrieval analysis of the daytime observations has produced a set of altitude-distribution profiles of the volume-mixing ratio, from 22 to 38 km, that provides a picture of the variation of the radical as a function of the solar zenith angle at 1 km altitude steps. Above 38 km column density values have been determined. The observations taken at sunset show the expected rapid decay of OH. The nighttime data also show residual spectral features in correspondence to OH transitions. A spectroscopic analysis, carried out with the help of simulated spectra, indicates that these spectral features are most likely due to the OH transitions and hence these data provide the first positive detection of OH in the stratosphere in darkness. The retrieval analysis of the nighttime spectral features has established that the nighttime OH distribution is peaked around 36 km with a mixing ratio of 3.2 +/- 2.0 ppt; values for the column density are also determined above and below this altitude. (C) 2001 Elsevier Science Ltd. All rights reserved.