Sentinel-5P TROPOMI SO2 Analysis
End-to-end atmospheric monitoring of sulfur dioxide over the Philippines, combining satellite remote sensing with ground validation and public health integration.
Data Pipeline
Automated ingestion, quality-controlled processing, and multi-stage scientific analysis of Sentinel-5P Level 2 SO2 column density retrievals.
- DOAS Retrieval
- The TROPOMI SO2 product uses Differential Optical Absorption Spectroscopy (DOAS) in the UV (312–326 nm). The 1 km box air mass factor (AMF) assumes SO2 within the lowest 1 km of the atmosphere — appropriate for anthropogenic and passively degassing volcanic sources, but may underestimate column density for elevated eruption plumes.
- QA Filtering
- Only pixels with qa_value > 0.5 are retained, removing cloud-contaminated and low-quality retrievals. This threshold balances data coverage against noise. Notably, valid volcanic signals can be lost during heavy ash loading, as the DOAS fit quality degrades.
- Negative Pixels
- Negative SO2 retrievals (~40% of clear-sky pixels) are retained in all statistics. These represent instrument noise around zero background and are essential for unbiased mean estimates.
Data Quality & Coverage
Daily temporal coverage and SO2 column density over the Philippines for 2025. The dual-axis time series reveals data gaps from orbit geometry and the inverse relationship between coverage and mean SO2.
Spatial Climatology
Annual mean SO2 column density on a 0.1° grid, showing persistent enhancement near active volcanoes and the low background across open ocean areas.
Volcano Monitoring
Systematic monitoring of 24 Philippine volcanoes using TROPOMI SO2 column density within 50 km radii. Ranking by maximum observed SO2 highlights the most active sources.
Eruption Forensics: Kanlaon Case Study
Kanlaon volcano (Negros Island) produced sustained SO2 emissions throughout 2025. Event-level analysis captures plume spatial extent, mass loading, and decay behavior.
Urban & Anthropogenic SO2
Regional SO2 enhancement above ocean background for Metro Manila, Cebu Metro, Batangas, Iligan City, and Cagayan de Oro. Detection signal-to-noise ratio (SNR) indicates statistical significance of excess SO2.
Public Health Integration
Preliminary framework for integrating satellite SO2 observations with population exposure assessment, WHO guideline thresholds, and ground-based air quality monitoring.
WHO Guideline Thresholds
Population Exposure
| Region | Population | Mean SO2 (DU) | Annual DU-days |
|---|
DU-days = mean column SO2 (DU) × 365 days. This is a satellite-derived exposure proxy, not a health risk metric.
Ground Validation
Methodology & Limitations
Transparency in analytical choices and known limitations is essential for scientific credibility. The following issues are documented and tracked.
| Issue | Impact | Mitigation |
|---|---|---|
| SO2 column product (1 km AMF) | Underestimates elevated volcanic plumes | Consider 7/15 km AMF variants for eruptions |
| Enhancement threshold (1 DU) vs. noise floor | TROPOMI noise ~0.5–1.5 DU; potential false positives | Statistical detection via SNR-averaged metric |
| Fixed pixel area (19.25 km²) | Actual 19–56 km²; up to 4× mass loading error at swath edges | Report mass loading as lower bound |
| QA > 0.5 for volcanic events | Dense aerosol may invalidate otherwise valid pixels | Evaluate lower QA thresholds during eruptions |
| Ocean background representativeness | Single 2×2° box; potential ship/volcanic contamination | Multi-region background or latitudinal stratification |
| Parameter | Value | Rationale |
|---|---|---|
| QA threshold | > 0.5 | Standard TROPOMI L2 recommendation |
| Enhancement threshold | 1.0 DU | Above noise floor for aggregated statistics |
| Strong volcanic threshold | 5.0 DU | Clear volcanic signal above all noise sources |
| Volcano monitoring radius | 50 km | Captures near-source plumes within ~1 TROPOMI swath width |
| Minimum reliable pixels | 5 | Statistical minimum for daily mean estimate |
| DU conversion | 4.4615 × 10-4 mol/m² | Standard Dobson Unit definition |