Profiling Volcanic SO2 with Balloonsondes in Costa Rica for Validation of TROPOMI OMI, and OMPS

The measurement principle for ozone in the electrochemical concentration cell (ECC) ozonesonde is a redox reaction between KI and O₃; this reaction is short-circuited in the presence of SO₂. Thus coherent plumes of SO₂ from industrial and natural sources such as volcanoes tend to produce negative notches in ozone soundings that intercept such plumes. In Costa Rica, we have had the good fortune to have an ozonesonde site located in the path of emission plumes from Turrialba Volcano, and since 2006 we have obtained over 100 ozone profiles with the telltale notches indicative of SO₂. These notches are up to several kilometers in depth and are typically centered near 3.5 km, the elevation of Turrialba. Conclusive evidence of the presence of SO₂ in these notched profiles was obtained in early 2012 when we launched a payload with two ozonesondes which derives SO₂ using by simple filter differencing technique. The following year we began regularly flying these dual ozonesondes in support of OMI and OMPS SO₂ retrieval validation. Through July 2018, we have launched 55 dual sondes, and now with the release of the operational TROPOMI SO₂ retrievals, there are frequent opportunities for validation. We report here on intercomparisons between sondes with both TROPOMI and OMPS retrievals over the past 9 months. The sonde launched on March 23, 2018 was particularly notable. SO₂ was first encountered at approximately 3.2 km, and a total 3.4 DU of SO₂ were detected in the notch that extended up to 5 km, which is the same value obtained for the co-located TROPOMI pixel. This is a remarkable agreement, but not unexpected. The payload on March 23 also included the new SO₂ sonde under development at the University of Houston. While the mixing ratios obtained by this prototype are very preliminary, the new sonde clearly shows that the depth of the actual SO₂ layer was 0.9 km, whereas hysteresis effects in the dual ozonesonde produced a notch twice that depth.