Volcanic SO2 Flux Variability and Pre-Eruptive Degassing at Stromboli and Etna: DOAS Traverse and Satellite TROPOMI Comparison Over 3 Years

This study investigates SO2 flux variability, pre-eruptive degassing signals, and comparison between ground-based DOAS traverse measurements and satellite TROPOMI retrievals at Stromboli and Etna over 36 months within the context of volcanology and atmospheric remote sensing, an area of growing scientific importance given its implications for volcanic SO2 early warning integration with civil protection, satellite-ground cross-calibration protocols, and Mediterranean volcanic emission budgets for climate modeling. Using car-mounted DOAS traverse at 1-hour cadence during field campaigns (120 campaign days at each volcano), satellite TROPOMI SO2 column (1 km x 5.5 km pixel) daily retrieval, and cross-sensor correlation with 24-hour temporal matching, we examine shallow magma degassing releasing SO2 from sulphur-rich melt ascending to Stromboli conduit; Etna paroxysmal lava fountain episodes preceded by 24-72 h of elevated passive SO2 flux indicating magma recharge from deeper reservoirs in 36-month dataset (January 2022 - December 2024): Stromboli 284 DOAS traverse days, Etna 284 campaign days, TROPOMI 1,080 orbital passes covering both volcanoes drawn from Stromboli volcano (Aeolian Islands, 38.8 N, 15.2 E) and Mount Etna (Sicily, 37.7 N, 15.0 E) with DOAS traverse along fixed routes at 1-km standoff distance from plume centerline. Results indicate that DOAS-TROPOMI correlation r=0.84 at Etna (better geometry) and r=0.68 at Stromboli (emission height uncertainty); pre-eruptive SO2 anomalies detected 24-48h before 84% of Etna paroxysms (n=24); Stromboli mean SO2 flux 84 t/day vs. Etna 1,240 t/day during non-eruptive periods (p < 0.001), with r=0.84 DOAS-TROPOMI at Etna; 84% paroxysm pre-detection; Etna flux 1,240 vs. Stromboli 84 t/day as the primary quantitative benchmark. Concordance between primary and confirmatory measurement approaches exceeded 93%, validating the analytical framework. These findings contribute empirically to volcanology and atmospheric remote sensing and carry actionable implications for the design of programs and policies targeting volcanic SO2 early warning integration with civil protection, satellite-ground cross-calibration protocols, and Mediterranean volcanic emission budgets for climate modeling.