Comparative of total ozone column observed under high solar zenith angles at Zhongshan station, Antarctica

Daily minimum solar zenith angle (SZA)≥80° during or near the period of polar night (May-July) at Zhongshan station of China (69.37°S, 76.38°E) in Antarctic restricts ground-based observations of total column (TOC) by using direct solar light (DS), which limits further accurate understanding of TOC seasonality, especially in the autumn and winter. To address this issue, the following four types of TOC observations at SZA≥80°: ① Brewer ozone spectrophotometer DS, TOC_{DS (≥80°)}, ② focused sun-disk (FS), TOC_{FS (≥80°)}, ③ focused moon disk (FM), TOC_FM and ④ SAOZ spectrometer zenith twilight observations, TOC_SAOZ, made from 2009 to 2023 are analyzed with reference to the Brewer DS observations within SZA of 72°, 80°), i.e., TOC_{DS (72, 80°))}.The results show that, on average, TOC_DS(≥80°) are lower by (5.5±9.5(1σ)) DU or (2.1±4.1(1σ)) % when SZA is 81°, 81.5°) while TOC_{FS (≥80°)} are higher by (2.8±4.3(1σ)) DU or (1.75±4.3(1σ))% when SZA is 80°, 84.5°) . The fixed stratospheric ozone layer height (~22 km) in the air mass factor (μ) calculation causes a decreased μ with increasing SZA, which is the main reason for the higher TOC _{FS (≥80°)} . On average, TOC_FM is lower by (6.8±16.7(1σ)) DU or (2.2±6.8(1σ))% while TOC_SAOZ is lower by (12.1±12.9(1σ)) DU or (4.4%±5.0(1σ))% on average. The longer optical path of twilight in the atmosphere together with the nature latitudinal distribution of TOC are the reasons for lower TOC_SAOZ values. The significant diurnal variation of TOC during the 'ozone hole' (TOC≤220 DU) period increases the differences between those observed TOC at SZA≥80° and the reference. It is confirmed that the annual 'ozone hole' at Zhongshan Station starts in August. The average TOC is within 270—290 DU while the minimum value can be ~230 DU, indicating that ozone depletion has already occurred during the polar night when the polar vortex has developed. The secondary seasonal lower TOC in autumn (March), which are due to the influence of the nature stratospheric Brewer-Dobson circulation, is similar to the lowest TOC in autumn in the middle and high latitudes of the northern hemisphere. FS observations should be preferentially applied when the SZA≥80°, and FM should be implemented because it is the only effective way during the polar night for routine TOC observations.