Sequestration of HNO3 in polar stratospheric clouds and chlorine activation as monitored by ground-based Fourier transform infrared solar absorption measurements

Solar absorption infrared spectra measured with the ground-based Michelson Interferometer for Passive Atmospheric Sounding-Laboratory Model (MIPAS-LM) at the Esrange research station near Kiruna, North Sweden, were analyzed for zenith column amounts of stratospheric trace gases. Time series of HF, HCl, ClONO2, HNO3, and O-3 measured in the winter of 1991-1992, the summer of 1992, and the winter of 1992-1993 were studied for anomalies indicating a perturbed stratospheric chemistry in the Arctic winter. Indications of chlorine activation were found in both winters, inside the stratospheric polar vortex as well as at the vortex edge, as was indicated by the ratio of stratospheric HCl to the HF zenith column amount. This ratio decreased from values higher than 3 for chemically undisturbed stratospheric conditions to 1.1 - 1.3 in January 1992 and 0.8 in February 1993, inside the polar vortex when maximum chlorine activation took place. Sequestration of. HNO3 in polar stratospheric clouds (PSCs) was found on several days of observation inside the polar vortex; it leads to a decrease of the HNO3 zenith column amounts by up to 50% on February 10, 1993. From comparison to model calculations we conclude that the HNO3 loss;can only be explained by formation of nitric acid trihydrate (NAT) PSCs on January 18, 1992, and February 10, 1993, whereas on January 19, 1992, January 28, 1993, and February 11, 1993, the formation of ternary solutions can also explain the HNO3 loss.