Re-processing TOMS UV Measurements to Retrieve SO2 Emissions From Volcanic Eruptions

Abstract:

The SO₂ Monitoring Group at the NASA Goddard Space Flight Center is producing a new multi-satellite long term data set of volcanic SO₂ column amounts and heights (MSVOLSO2L4) as part of the NASA MEaSUREs Program. Here we present re-analysis of the UV measurements (BUV) from the NASA Nimbus 7 Total Ozone Mapping Spectrometer (N7 TOMS: 1978-1993). Ozone is the dominant atmospheric absorber in the BUV spectrum, but volcanic eruptions can produce enough SO₂ to be distinguished from ozone background. Quantitative retrieval of volcanic SO₂ requires:
1) Separation of the O₃ and SO² absorption in BUV radiances;
2) Close to zero mean SO₂ background;
3) RT forward model that accounts for the presence of volcanic ash in the plume;
4) A priori knowledge of the ozone and SO₂ vertical profiles.
Our iterative retrieval algorithm returns O₃ and SO₂ column amounts, effective reflectivity and its spectral slope. The retrieval model also generates a 4 x 4 gain matrix for the SO₂ free regions that is used to soft calibrate the measured 340 nm BUV radiance. The spectral slope implicitly accounts for the interference of volcanic ash, but more explicit ash treatment is required to better quantify SO2 errors in volcanic plumes heavily loaded with ash. This presentation will discuss the methods used to characterize the error sources and assess the quality of this unique long-term SO₂ data set.