Tropical Ozone Trends (1998 to 2023): A Synthesis from SHADOZ, IAGOS and OMI/MLS Observations

Abstract. Trends in tropical tropospheric ozone over the past ~20–30 years have been reported using ozonesonde profiles from five SHADOZ sites (Thompson et al., 2021, “T21”; Stauffer et al., 2024, “S24”) and a combination of satellite, SHADOZ and IAGOS aircraft measurements (Gaudel et al., 2024). Selected tropical sonde and aircraft trends also appear in Van Malderen et al. (2024a). We have extended T21 for monthly-averaged five-station SHADOZ data with a Multiple Linear Regression (MLR) model, covering 1998 to 2023. We report: (1) trends in two free tropospheric (FT) ozone layers, lowermost stratosphere (LMS) ozone, total tropospheric column (TrCO_sonde) and tropopause height; (2) trends for 2000–2023 (no 1997–1998 ENSO) and 1998–2019 (no COVID-19). (3) TrCO_sonde trends, 2005–2023, compared to OMI/MLS TrCO_satellite. The findings: (1) Extending SHADOZ trends four years does not change the T21 results: annual trends negligible except in one FT layer (Natal-Ascension) and for tropopause-referenced LMS. A slight reduction in FT trends may reflect a moderating effect of COVID-19. (2) Adding thousands of IAGOS profiles to SHADOZ data similarly showed near-zero MLR trends (p<0.05) in a pressure-defined lower FT; SHADOZ sampling is sufficient. (3) With the TrCO_sonde adding 0–5 km ozone, trends are only detected over SE Asia and Natal-Ascension at 2–3 %/decade, p<0.05; comparison to trends from the TOAR II/HEGIFTOM activity (a 0–300 hPa TrOC) gives similar results. For 2005–2023 MLR annually averaged trends for TrCO_sonde and OMI/MLS TrCO_satellite agree within uncertainties at four of 5 SHADOZ sites.