Ground-based zenith scattered light differential optical absorption spectroscopy (DOAS) measurements were performed in summer and autumn (27 May–30 November) 2020 at Golmud (94°54′ E, 36°25′ N; 2807.6 m altitude) to investigate the abundances and temporal variations of ozone (O₃) and its depleting substances over the northern Tibetan Plateau (TP). The differential slant column densities (dSCDs) of O₃, nitrogen dioxide (NO₂), bromine monoxide (BrO), and chlorine dioxide (OClO) were simultaneously retrieved from scattered solar spectra in the zenith direction during the twilight period. The O₃ vertical column densities (VCDs) were derived by applying the Langley plot method, for which we investigated the sensitivities to the chosen wavelength, the a-priori O₃ profile and the aerosol extinction profile used in O₃ air mass factor (AMF) simulation as well as the selected solar zenith angle (SZA) range. The mean O₃ VCDs from June to November 2020 are 7.21 × 10¹⁸ molec·cm⁻² and 7.18 × 10¹⁸ molec·cm⁻² at sunrise and sunset, respectively. The derived monthly variations of the O₃ VCDs, ranging from a minimum of 6.9 × 10¹⁸ molec·cm⁻² in October to 7.5 × 10¹⁸ molec·cm⁻² in November, well matched the OMI satellite product, with a correlation coefficient R = 0.98. The NO₂ VCDs at SZA = 90°, calculated by a modified Langley plot method, were systematically larger at sunset than at sunrise as expected with a pm/am ratio of ~1.56. The maximum of the monthly NO₂ VCDs, averaged between sunrise and sunset, was 3.40 × 10¹⁵ molec·cm⁻² in July. The overall trends of the NO₂ VCDs were gradually decreasing with the time and similarly observed by the ground-based zenith DOAS and OMI. The average level of the BrO dSCD_90°–80° (i.e., dSCD between 90° and 80° SZA) was 2.06 × 10¹⁴ molec·cm⁻² during the period of June–November 2020. The monthly BrO dSCD_90°–80° presented peaks in August and July for sunrise and sunset, respectively, and slowly increased after October. During the whole campaign period, the OClO abundance was lower than the detection limit of the instrument. This was to be expected because during that season the stratospheric temperatures were above the formation temperature of polar stratospheric clouds. Nevertheless, this finding is still of importance, because it indicates that the OClO analysis works well and is ready to be used during periods when enhanced OClO abundances can be expected. As a whole, ground-based zenith DOAS observations can serve as an effective way to measure the columns of O₃ and its depleting substances over the TP. The aforementioned results are helpful in investigating stratospheric O₃ chemistry over the third pole of the world.

Recent research on stratospheric ozone indicates signs of ozone recovery, but on the other hand, ozone recovery is also expected to be delayed by many aspects (e.g climate change). Therefore, it is important to monitor continuously stratospheric trace gases to predict the future evolution of the Arctic ozone and other trace gases which are involved in the ozone depletion chemistry. OClO is well known as an indicator of the stratospheric chlorine activation and can be measured using remote sensing techniques.In this study, we present long-term measurements of OClO slant column densities at Kiruna, Sweden (67.84°N, 20.41°E) which were obtained from the ground-based zenith sky DOAS instruments since 1997. The measurement site is located north of the polar circle in which the variability of the OClO abundance depends on the state of stratospheric chlorine activation but also whether the polar vortex is located above the measurement site.The aim of this study is to give an overview of the measured stratospheric OClO abundance for 19 years, and to investigate the dominant parameters affecting ozone and OClO during periods of stratospheric chlorine activation. One particular focus is on the parameters which trigger the activation and de-activation at the beginning and the end of the polar winter.To do so, we compare the general dependencies of OClO on other trace gases and meteorological conditions.