Measurements report: The unusual spring 2020 Arctic stratospheric ozone depletion above Ny-Ålesund by ground-based ZSL-DOAS

Of the severe stratospheric ozone depletion events (ODEs) reported over the Arctic, the most severe occurred during the spring of 2020; we analyzed the reasons for this event herein. We retrieved the critical indicator ozone vertical column density (VCD) using zenith scattered light differential optical absorption spectroscopy (ZSL-DOAS) from March 2017 to September 2021 located in Ny-Ålesund, Svalbard, Norway. The average ozone VCD over Ny-Ålesund between March 18 and April 18, 2020, was approximately 274.8 Dobson units (DU), which was about 64.7±0.1% of that in the other years (2017, 2018, 2019, and 2021), and the daily peak difference was 195.7 DU during this period. The retrieved daily averages of ozone VCDs were compared with satellite observations from Global Ozone Monitoring Experiment-2 (GOME-2), a Brewer spectrophotometer, and a Système d’Analyze par Observation Zénithale (SAOZ) spectrometer at Ny-Ålesund; the resulting Pearson correlation coefficients were relatively high at 0.94, 0.86, and 0.91, with relative deviations of 2.3%, 3.1%, and 3.5%, respectively. Polar observations are still inadequate and accurate ZSL-DOAS observations can provide reliable data for polar ozone study. We also considered the chemical components of this process in the Arctic winter of 2019/2020 with the specified dynamics version of the Whole Atmosphere Community Climate Model (SD-WACCM). The SD-WACCM results indicated that both ClO and BrO concentrations peaked in late March, which was a critical factor during the ozone depletion observed in Ny-Ålesund. Chlorine and bromine activation were clearly obvious during the Arctic spring of 2020, whereas the partitioning of bromine compounds was different from that of chlorine. Before activation, chlorine was predominantly present as HCl and ClONO2, whereas bromine was predominantly present as HOBr and BrCl. Particularly, before chlorine activation began, bromine mainly existed as HOBr; however, after chlorine activation, bromine mainly existed in the form of BrCl. By ZSL-DOAS observations, we provided another evidence for unprecedented ozone depletion during the Arctic spring of 2020. The ZSL-DOAS ozone VCD observations can also provide calibration for satellite observations and model simulations, and in the future can provide the support for observations at more Chinese research stations or international local stations in the polar area.

在北极地区已报道的严重平流层臭氧损耗事件（stratospheric ozone depletion events, ODEs）中，2020年春季发生的事件最为剧烈，本文针对该事件的成因展开了分析。本研究于2017年3月至2021年9月期间，在挪威斯瓦尔巴群岛的新奥尔松（Ny-Ålesund）站点，采用天顶散射光差分光学吸收光谱法（zenith scattered light differential optical absorption spectroscopy, ZSL-DOAS）反演得到了关键指标——臭氧垂直柱密度（ozone vertical column density, VCD）。2020年3月18日至4月18日期间，新奥尔松站点的臭氧VCD平均值约为274.8多布森单位（Dobson units, DU），仅为其余年份（2017、2018、2019及2021年）对应时段的64.7±0.1%；该时段内的日峰值差值可达195.7 DU。将反演得到的臭氧VCD日均值，与全球臭氧监测实验二号卫星（Global Ozone Monitoring Experiment-2, GOME-2）、新奥尔松站点的布鲁尔分光光度计及天顶观测分析系统（Système d’Analyze par Observation Zénithale, SAOZ）光谱仪的观测数据进行对比后发现，三者对应的皮尔逊相关系数分别为0.94、0.86及0.91，均处于较高水平，相对偏差则分别为2.3%、3.1%及3.5%。当前极地观测数据仍存在不足，精准的ZSL-DOAS观测可为极地臭氧研究提供可靠的数据支撑。本研究还借助全大气社区气候模型指定动力学分版本（specified dynamics version of the Whole Atmosphere Community Climate Model, SD-WACCM），分析了2019/2020年北极冬季该臭氧损耗过程的化学组分特征。SD-WACCM模拟结果显示，一氧化氯（ClO）与一氧化溴（BrO）的浓度均在3月下旬达到峰值，这是新奥尔松站点观测到臭氧损耗的关键影响因素。2020年北极春季期间，氯与溴的活化过程十分显著，但溴化合物的组分分配模式与氯并不相同。在活化发生前，氯主要以氯化氢（HCl）和硝酸氯（ClONO2）的形式存在；而溴则主要以次溴酸（HOBr）和溴化氯（BrCl）的形式存在。具体而言，在氯活化启动前，溴几乎全部以HOBr的形式存在；而在氯活化发生后，溴则主要以BrCl的形式存在。通过ZSL-DOAS观测结果，本研究为2020年北极春季史无前例的臭氧损耗事件提供了新的佐证。ZSL-DOAS反演得到的臭氧VCD观测数据，还可用于卫星观测与模式模拟的定标工作；未来可为中国更多极地科考站以及国际极地站点的观测研究提供数据支撑。