ClONO2 total vertical column abundances above the Jungfraujoch Station, 1986-1994: Long-term trend and winter-spring enhancements

Total vertical column abundances of chlorine nitrate (ClONO2) have been retrieved from 0.006 cm(-1) resolution solar absorption spectra recorded at the International Scientific Station of the Jungfraujoch (ISSJ) in the Swiss Alps (altitude 3.58 km, latitude 46.5 degrees N, longitude 8.0 degrees E) on 105 days between June 1986 and November 1994. The analysis is based on spectral fittings of the ClONO2 nu(4) band Q branch at 780.21 cm(-1) and the interferences occurring in the same spectral region. The ISSJ measurements show a regular long-term increase in the ClONO2 column with an occasional factor of 2 to 3 enhancements during the midwinter to early spring. Excluding data from this time of the year, the ISSJ database reflects a linear rate of increase and la uncertainty equal to (4.0 +/- 0.7)% yr(-1) referenced to 1990.0. The corresponding ClONO2 total vertical columns for mid-1986 and mid-1994 are equal to 0.92 and 1.26 x 10(15) molecules cm(-2), respectively. The high ClONO2 columns and high HF/HCl column ratios sometimes measured during winter indicate the occasional presence of chemically processed air above the station. This is corroborated by trajectories calculated for the stratospheric air masses sounded on these occasions.