A new discrete wavelength BUV algorithm for consistent volcanic SO 2 retrievals from multiple satellite missions

Abstract. This paper describes a new discrete wavelength algorithm developed for retrieving volcanic sulfur dioxide(SO 2 ) vertical column density (VCD) from UV observing satellites. The Multi-Satellite SO 2 algorithm (MS_SO2) simultaneously retrieves column densities of sulfur dioxide, ozone, Lambertian Effective Reflectivity (LER) and its spectral dependence. It is used operationally to process measurements from the heritage Total Ozone Mapping Spectrometer(TOMS) on board NASA's Nimbus-7 satellite (N7/TOMS: 1978–1993) and from the current Earth Polychromatic Imaging Camera (EPIC) on board Deep Space Climate Observatory (DSCOVR: 2015–) from the Earth-Sun Lagrange (L1) orbit. Results from MS_SO2 algorithm for several volcaniccases were validated using the more sensitive principal component analysis(PCA) algorithm. The PCA is an operational algorithm used by NASA to retrieve SO 2 from hyperspectral UV spectrometers, such as Ozone Monitoring Instrument(OMI) on board NASA’s Earth Observing System Aura satellite and Ozone Mapping and Profiling Suite (OMPS) on board NASA-NOAA Suomi National Polar Partnership (S-NPP) satellite. For this comparative study, the PCA algorithm was modified to use the discrete wavelengths of the Nimbus7/TOMS instrument, described in S1 of the paper supplement. Our results demonstrate good agreement between the two retrievals for the largest volcaniceruptions of the satellite era, such as 1991 Pinatubo eruption. To estimate SO 2 retrieval uncertainties we use radiative transfer simulations explicitly accounting for volcanicsulfate and ash aerosols. Our results suggest that the discrete-wavelength MS_SO2 algorithm, although less sensitive than hyperspectral PCA algorithm, can be adapted to retrieve volcanicSO 2 VCDs from contemporary hyperspectral UV instruments, such as OMI and OMPS, to create consistent, multi-satellite, long-term volcanicSO 2 climate data records.