sulfur_dioxide

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Responsible:Niccolò Calandri

Sulfur Dioxide — Sentinel-5P TROPOMI SO2

Overview

The Sulfur Dioxide (SO₂) layer measures the vertical column density of SO₂ in the atmosphere using the TROPOMI instrument aboard Sentinel-5P. SO₂ is a reactive gas with significant impacts on both human health and climate:

Health effects: respiratory irritation, exacerbation of asthma and cardiovascular disease
Climate effects: SO₂ oxidizes to sulfate aerosols which have a net cooling effect (negative radiative forcing) and serve as cloud condensation nuclei

Major SO₂ sources:

Source	Contribution	Examples
Anthropogenic	~70 %	Coal combustion, oil refining, metal smelting, shipping
Natural	~30 %	Volcanic degassing, geothermal vents

SO₂ has a short atmospheric lifetime of 1–2 days, making it an effective tracer of local emission sources and volcanic activity. The native resolution is ~1113 m, with output at 500 m.

Calculation Methodology

Retrieval algorithm:

The SO₂ vertical column is retrieved using DOAS in the UV spectral range (312–326 nm):

Spectral fitting of SO₂ absorption features, accounting for interference from O₃ and other absorbers
Background correction to remove the clean-atmosphere SO₂ signal
Air mass factor computation for different SO₂ layer heights (planetary boundary layer, middle troposphere, upper troposphere/stratosphere)
Quality filtering based on cloud fraction, solar zenith angle, and fit quality

SO₂ products are provided for three assumed layer heights:

PBL (0–1 km): for anthropogenic near-surface emissions
TRM (5–10 km): for elevated volcanic plumes
STL (15 km): for stratospheric volcanic injections

Data Sources

Code	Name	Provider	Type	Coverage	Resolution	Availability
`COPERNICUS/S5P/OFFL/L3_SO2`	Sentinel-5P TROPOMI SO2 L3	ESA/Copernicus	Atmospheric column	Global	~1113 m	2018-06 — present

Documentation: GEE S5P SO2

Technical Notes

SO₂ retrievals have higher noise than NO₂ or O₃ due to weaker absorption features in the UV
Volcanic SO₂ plumes can produce very high column values (> 10 DU) easily detectable from space
Anthropogenic emissions are typically 1–2 orders of magnitude lower than volcanic plumes
The assumed SO₂ layer height significantly affects the air mass factor and retrieved column
Maritime shipping lanes are clearly visible in SO₂ data due to high-sulfur fuel emissions (decreasing since IMO 2020 regulations)

References

Veefkind, J.P. et al. (2012). "TROPOMI on the ESA Sentinel-5 Precursor". Remote Sensing of Environment, 120, pp. 70–83.
Theys, N., De Smedt, I., Yu, H. et al. (2017). "Sulfur dioxide retrievals from TROPOMI onboard Sentinel-5 Precursor: algorithm theoretical basis". Atmospheric Measurement Techniques, 10, pp. 119–153.
Carn, S.A., Fioletov, V.E., McLinden, C.A. et al. (2017). "A decade of global volcanic SO₂ emissions measured from space". Scientific Reports, 7, 44095.

Data Sources & References

Sources

WRD_S5SO2_99

Methodology

Il biossido di zolfo (SO2) viene mappato utilizzando i dati Sentinel-5P TROPOMI, prodotto OFFL L3_SO2 disponibile su Google Earth Engine, con risoluzione nativa di circa 1113 metri (~0.01° a nadir). L'SO2 entra nell'atmosfera attraverso processi naturali (~30%) e antropici (~70%): le fonti antropiche principali sono la combustione di combustibili fossili (centrali a carbone), raffinazione del petrolio e fusione dei metalli; le fonti naturali includono eruzioni vulcaniche e sorgenti geotermiche. Le emissioni di SO2 hanno impatti negativi sulla salute umana e sulla qualità dell'aria, e contribuiscono ai cambiamenti climatici attraverso la formazione di aerosol di solfato che riflettono la radiazione solare. La banda utilizzata è 'SO2_column_number_density', che misura la densità colonnare verticale di SO2 al livello del suolo in mol/m². È disponibile anche la banda a 15km per rilevare SO2 stratosferica di origine vulcanica. Le acquisizioni vengono filtrate per qualità (qa_value > 0.5). Viene applicato un ricampionamento bilineare a 500m (risoluzione più bassa rispetto agli altri S5P per la maggiore incertezza del retrieval SO2), quindi calcolata la media pixel-wise sul periodo.