The Moon as a light source for FTIR measurements of stratospheric trace gases during the polar night: Application for HNO3 in the Arctic

Abstract

A Fourier Transform Infrared (FTIR) spectrometer has been used with the Moon as an IR light source to derive column densities of stratospheric trace gases. The measurements were performed during the polar night in the Arctic winter 1992/1993 at the Network for Detection of Stratospheric Change (NDSC) station in Ny‐Ålesund (79°N, 12°E) located at Spitsbergen. Because of the emission of the Moon itself it was possible to use not only the Indium Antimonide (InSb) detector for the wavenumber region 2600–4000 cm−1 but also the less sensitive Mercury Cadmium Telluride (MCT) detector covering the interval 600–1000 cm−1. Solar OH lines, missing in the lunar spectra, demonstrate the contribution of the Moon's emission effect to the overall infrared intensity. The measurements could be performed for about a week around the full Moon. Spectra were recorded at 0.01 cm−1 or 0.02 cm−1 resolution. It was possible to derive zenith column densities of N2O, CH4, C2H6, HF, HCl, O3, NO2, HNO3, and ClONO2. They were retrieved using a version of the FASCOD2 program for personal computers or the retrieval of atmospheric trace gas profiles (RAT) program. Spectral data were taken from the HITRAN92 database. Lunar observations for HNO3, performed in December 1992 and February 1993, are compared with solar observations from March 1992 and March–July 1993 performed at the same location. Our measurements at 79°N agree with literature results for this trace gas. Owing to the occurrence of the polar night and polar summer, the seasonal cycle in the column densities of HNO3 is found to be more pro‐nounced in the Arctic than at midlatitudes. Although polar stratospheric clouds (PSCs) were frequently observed throughout the winter, the data do not indicate a significant removal of HNO3 from the atmosphere by sedimentation via PSCs. A denitrification, the irreversible removal of NOx, as observed during the winter in Antarctica, did not occur.