Italian Space Agency (ASI), Via del Politecnico s.n.c., 00133 Rome, Italy.
Mathematics Department, University of Rome Tor Vergata, Via della Ricerca Scientifica 1, 00133 Rome, Italy.
Sensors (Basel). 2021 Oct 21;21(21):6991. doi: 10.3390/s21216991.
Sulfur dioxide (SO) degassing at Strombolian volcanoes is directly associated with magmatic activity, thus its monitoring can inform about the style and intensity of eruptions. The Stromboli volcano in southern Italy is used as a test case to demonstrate that the TROPOspheric Monitoring Instrument (TROPOMI) onboard the Copernicus Sentinel-5 Precursor (Sentinel-5P) satellite has the suitable spatial resolution and sensitivity to carry out local-scale SO monitoring of relatively small-size, nearly point-wise volcanic sources, and distinguish periods of different activity intensity. The entire dataset consisting of TROPOMI Level 2 SO geophysical products from UV sensor data collected over Stromboli from 6 May 2018 to 31 May 2021 is processed with purposely adapted Python scripts. A methodological workflow is developed to encompass the extraction of total SO Vertical Column Density (VCD) at given coordinates (including conditional VCD for three different hypothetical peaks at 0-1, 7 and 15 km), as well as filtering by quality in compliance with the Sentinel-5P Validation Team's recommendations. The comparison of total SO VCD time series for the main crater and across different averaging windows (3 × 3, 5 × 5 and 4 × 2) proves the correctness of the adopted spatial sampling criterion, and practical recommendations are proposed for further implementation in similar volcanic environments. An approach for detecting SO VCD peaks at the volcano is trialed, and the detections are compared with the level of SO flux measured at ground-based instrumentation. SO time series analysis is complemented with information provided by contextual Sentinel-2 multispectral (in the visible, near and short-wave infrared) and Suomi NPP VIIRS observations. The aim is to correctly interpret SO total VCD peaks when they either (i) coincide with medium to very high SO emissions as measured in situ and known from volcanological observatory bulletins, or (ii) occur outside periods of significant emissions despite signs of activity visible in Sentinel-2 data. Finally, SO VCD peaks in the time series are further investigated through daily time lapses during the paroxysms in July-August 2019, major explosions in August 2020 and a more recent period of activity in May 2021. Hourly wind records from ECMWF Reanalysis v5 (ERA5) data are used to identify local wind direction and SO plume drift during the time lapses. The proposed analysis approach is successful in showing the SO degassing associated with these events, and warning whenever the SO VCD at Stromboli may be overestimated due to clustering with the plume of the Mount Etna volcano.
二氧化硫（SO）在斯特龙博利火山的排放与岩浆活动直接相关，因此其监测可以提供有关喷发类型和强度的信息。意大利南部的斯特龙博利火山被用作案例研究，以证明哥白尼哨兵-5 前哨（Sentinel-5P）卫星上的 TROPOspheric Monitoring Instrument（TROPOMI）具有合适的空间分辨率和灵敏度，可对相对较小规模、近乎点状的火山源进行局部尺度的 SO 监测，并区分不同活动强度的时期。从 2018 年 5 月 6 日至 2021 年 5 月 31 日，利用 UV 传感器收集的斯特龙博利火山上空的 TROPOMI 二级 SO 地球物理产品的整个数据集，利用专门编写的 Python 脚本进行处理。开发了一种方法学工作流程，以提取给定坐标处的总 SO 垂直柱密度（VCD）（包括 0-1、7 和 15 公里处三个不同假设峰的条件 VCD），并根据 Sentinel-5P 验证团队的建议进行质量过滤。对主火山口和不同平均窗口（3×3、5×5 和 4×2）的总 SO VCD 时间序列的比较证明了所采用的空间采样标准的正确性，并为在类似的火山环境中进一步实施提出了实际建议。尝试了一种在火山上检测 SO VCD 峰值的方法，并将检测结果与地面仪器测量的 SO 通量水平进行了比较。SO 时间序列分析补充了上下文 Sentinel-2 多光谱（在可见光、近红外和短波红外）和 Suomi NPP VIIRS 观测提供的信息。目的是正确解释 SO 总 VCD 峰值，当它们 (i) 与现场测量的中等到非常高的 SO 排放以及火山观测站公告中已知的排放相吻合时，或者 (ii) 尽管 Sentinel-2 数据中可见活动迹象，但在无显著排放期间发生时。最后，通过 2019 年 7 月-8 月的爆发、2020 年 8 月的重大爆炸和 2021 年 5 月的近期活动期间的每日时间推移，进一步研究了时间序列中的 SO VCD 峰值。利用欧洲中期天气预报中心再分析 v5（ERA5）数据中的每小时风记录，识别时间推移期间的当地风向和 SO 羽流漂移。所提出的分析方法成功地显示了与这些事件相关的 SO 排放，并在由于与埃特纳火山的羽流聚类而可能高估斯特龙博利 SO VCD 时发出警告。
