DataSheet1_v1_Quantifying Light Dilution in Ultraviolet Spectroscopic Measurements of Volcanic SO2 Using Dual-Band Modeling.DOCX

High precision and accuracy in volcanic SO2 emission rate quantification is critical for eruption forecasting and, in combination with in-plume gas ratios, quantifying global volcanic emission inventories. Light dilution, where scattering of ultraviolet light dilutes plume SO2 absorbance signals, has been recognized for more than 50 years, but is still not routinely corrected for during gas flux quantification. Here we use modeling and empirical observations from Masaya volcano, Nicaragua, to show that light dilution produces: i) underestimates in SO2 that can reach a factor of 5 and, at low column densities, cause little impact on standard retrieval fit quality, even for heavily diluted spectra; ii) retrieved SO2 amounts that are capped by a maximum value regardless of the true amount of SO2, with this maximum amount being reduced as light dilution increases. Global volcanic volatile emission rates may therefore be significantly underestimated. An easily implementable dual-waveband analysis provides a means to detect, and in clear sky conditions, correct dilution effects directly from the spectra, opening a path to more accurate SO2 quantifications.

精准定量火山二氧化硫（SO₂）排放率，对于火山喷发预报具有关键意义；结合火山羽流内的气体比值，还可用于构建全球火山排放清单。紫外线散射导致火山羽流中SO₂吸光度信号被稀释的光稀释（Light dilution）现象，早在50余年前便已被学界发现，但在气体通量定量过程中，该现象仍未得到常规校正。本研究基于尼加拉瓜马萨亚火山的建模与实测观测数据，证实光稀释会引发两类问题：i) SO₂排放量的低估幅度可达5倍，且在柱浓度较低时，即便光谱被严重稀释，该现象对标准反演拟合质量的影响也微乎其微；ii) 反演得到的SO₂量会被固定上限值限制，不受实际SO₂排放量影响，且该上限值会随光稀释程度加剧而降低。因此，全球火山挥发物排放率可能被显著低估。一种易于实现的双波段分析方法，可用于检测光稀释效应，且在晴空条件下可直接从光谱数据中校正该效应，为实现更精准的SO₂定量分析开辟了可行路径。