PCA-Based Trace Gas Retrievals from GeoTASO Airborne Remote Sensing Measurements during the KORUS-AQ 2016 Campaign

National Institute of Environmental Research (NIER) of the Ministry of Environment in Korea is planning to launch the Geostationary Environmental Monitoring Spectrometer (GEMS) instrument in 2019. It is one of three missions for air quality monitoring from geostationary orbit together with NASA’s Tropospheric Emissions: Monitoring of POllution (TEMPO) and ESA’s Sentinel-4. The GEMS instrument employs a 2-dimensional charge coupled device (CCD) detector array covering wavelengths from 300 to 500 nm. The North-South field of regard (FOR) of GEMS ranges from 5°S to 45°N latitude, and the East-West FOR ranges from 75°E to 145°E longitude, focusing on Asia.

The Geostationary Trace gas and Aerosol Sensor Optimization (GeoTASO) airborne instrument is a test-bed for the satellite instruments mentioned above. It measures backscattered radiance with a spectrometer covering the spectral range between 290–695 nm. GeoTASO onboard B-200 (UC-12B) – LARC aircraft flew over the Korean Peninsula with the spatial resolution of 250 m x 250 m, during the Korea-United States Air Quality (KORUS-AQ) campaign from May to June 2016. Because of the similarities between characteristics of the upcoming GEMS and the GeoTASO instruments, measurements from GeoTASO in Korea can be employed for preparation of algorithms for GEMS trace gas retrievals.

Principal component analysis (PCA) technique was used to retrieve slant column densities (SCDs) of sulfur dioxide (SO₂), nitrogen dioxide (NO₂), and formaldehyde (HCHO). The fitting windows we used for SO₂, NO₂, and HCHO retrievals are 310–325 nm, 423–451 nm and 328.5–356.5 nm, respectively. The principal components (PCs) of each species were collected from relatively clean areas on the flight paths where are expected to have less SCDs than other areas. SCDs of each trace gas were obtained by fitting the PCs extracted from each cross-track of GeoTASO and the absorption cross section of each species to measured radiance. To convert the obtained SCDs to vertical column densities (VCDs), air mass factors (AMFs) of each species were calculated using atmospheric profiles from chemical transport model outputs. The VCDs of each species well capture point sources on the flight paths and their plumes propagating downwind areas. The retrieved column amounts were compared and/or validated against other measurements, including trace gas amounts retrieved from other GeoTASO algorithms, and ground-based measurements during the campaign.