Solar absorption measurements of stratospheric OH in the UV with a Fourier-transform spectrometer

Abstract

We performed solar absorption measurements of OH in the UV using a Fourier-transform spectrometer (FTS). The experiments were carried out in the high Arctic at Ny- Alesund (79 degrees N, 12 degrees E) during the summer of 1996. We accomplished the analysis in two ways: (1) by studying single solar-absorption spectra recorded in the middle of the solar disk and (2) by utilizing the Doppler shift of two spectra, recorded on the east and west sides of the solar disk. The results of both analysis methods agree and give total columns of approximately 6 x 1013 molecules cm-2 for solar zenith angles of 60 degrees . To find out the main noise contribution in the spectra, we compared the measured and calculated signal-to-noise ratios (SNR 's). During clear-sky conditions the photon noise determines the total SNR. However, because a FTS is extremely sensitive to source fluctuations, conditions that were already slightly cloudy increased the scintillation noise, preventing OH analysis. The noise contribution caused by the instrumental sampling process itself was found to be negligible; even through two sampling positions had to be interpolated between the laser zero crossings.