High-resolution mapping of SO2 using airborne observations from the GeoTASO instrument during the KORUS-AQ field study: PCA-based vertical column retrievals

The Geostationary Trace gas and Aerosol Sensor Optimization (GeoTASO) instrument is an airborne hyperspectral spectrometer measuring backscattered solar radiation in the ultraviolet (290–400 nm) and visible (415–695 nm) wavelength regions. This paper presents high-resolution sulfur dioxide (SO₂) maps over the Korean Peninsula, produced by SO₂ retrievals from GeoTASO measurements during the Korea–United States Air Quality Field Study (KORUS-AQ) from May to June 2016. The highly sensitive GeoTASO instrument with a spatial resolution of ~250 m × 250 m can detect point emission sources of SO₂ within its fields of view, even without merging multiple overlapping observations. To retrieve SO₂ vertical columns from the GeoTASO measurements, we apply an algorithm based on principal component analysis (PCA), which is effective in suppressing noise and biases in SO₂ retrievals. The retrievals successfully capture SO₂ plumes and various point sources such as power plants, a petrochemical complex, and a steel mill, located in South Chungcheong Province, some of which are not detected by a ground-based in situ measurement network. Spatial distributions of SO₂ from GeoTASO observations in source areas are consistent with those from the Stack Tele-Monitoring System reports and airborne in situ SO₂ measurements. Comparisons of SO₂ retrievals from GeoTASO and existing satellite sensors demonstrate the significance of high-resolution SO₂ observations, by indicating that GeoTASO detects small SO₂ emission sources that are not precisely resolved by single overpasses of satellites. To assess future geostationary SO₂ observations having a higher spatial resolution, we upscale the GeoTASO SO₂ retrievals to a spatial resolution of the Geostationary Environment Monitoring Spectrometer (GEMS). Since the upscaled GeoTASO retrievals also detect SO₂ plumes clearly, we expect from GEMS to identify even small SO₂ emission sources over Asia.