Balloonsonde SO2 and O3 Profiles from TICOSONDE in Costa Rica for the Validation of OMI and Sentinel-5 Precursor Retrievals

Balloonsonde measurements of ozone use an electrochemical concentration cell (ECC) with the redox reaction between KI and O₃ (Komhyr, 1969). SO₂ causes an interference reaction with the result that for each one molecule of SO₂, the ECC registers one less molecule of O₃. When the sonde passes through a plume of SO₂, the resulting O₃ profile exhibits a sudden decrease and subsequent increase in mixing ratio, distinct from typical O₃ fluctuations. Our regular ozonesonde flights as part of the TICOSONDE project in Costa Rica have frequently through emission plumes from nearby Turrialba Volcano. Since 2006 more than 100 Ticosonde ozone profiles have contained the telltale notches of SO₂ plume interaction. These notches are frequently several kilometers deep and centered near 3.5 km, the elevation of Turrialba. The deployment of a dual ozonesonde instrument that simultaneously measures O₃ and SO₂ (Morris et al., 2010) in February 2012 demonstrated that the notches were due to SO₂. Starting in 2013 TICOSONDE began regularly flying dual ozonesondes in support of OMI SO₂ retrieval validation, with 42 dual ozonesonde flights completed through July 2017. This study reviews the TICOSONDE SO₂ column observations from both the dual sonde measurements and the recently developed inferred SO₂ approach (Morris et al., 2017) and demonstrates the great value of the coincident observations for validation of OMI, OMPS and Copernicus Sentinel 5 Precursor SO₂ retrievals.