Influences of downward transport and photochemistry on surface ozone over East Antarctica during austral summer: in situ observations and model simulations

Abstract. Studies of atmospheric trace gases in remote, pristine environments are critical for assessing the accuracy of climate models and advancing our understanding of natural processes and global changes. We investigated the surface ozone (O₃) variability over East Antarctica during the austral summer of 2015–2017 by combining surface and balloon-borne measurements at the Indian station Bharati (69.4° S, 76.2° E, ~35 m above mean sea level) with EMAC atmospheric chemistry-climate model simulations. The model reproduced the observed surface O₃ level (18.8 ± 2.3 nmol mol^-1) with negligible bias and captured much of the variability (R=0.5). Model simulated tropospheric O₃ profiles were in reasonable agreement with balloon-borne measurements (mean bias: 3–11 nmol mol^-1). Our analysis of a stratospheric tracer in the model showed that about 40–50 % of surface O₃ over the entire Antarctic region was of stratospheric origin. Events of enhanced O₃ (~4–10 nmol mol^-1) were investigated by combining O₃ vertical profiles and air mass back trajectories, which revealed the rapid descent of O₃-rich air towards the surface. The photochemical loss of O₃ through its photolysis (followed by H₂O+O(¹D)) and reaction with hydroperoxyl radicals (O₃+HO₂) dominated over production from precursor gases (NO+HO₂ and NO+CH₃O₂) resulting in overall net O₃ loss during the austral summer. Interestingly, the east coastal region, including the Bharati station, tends to act as a stronger chemical sink of O₃ (~190 pmol mol^-1 d^-1) than adjacent land and ocean regions (by ~100 pmol mol^-1 d^-1). This is attributed to reverse latitudinal gradients between H₂O and O(¹D), whereby O₃ loss through photolysis (H₂O+O(¹D)) reaches a maximum over the east coast. Further, the net photochemical loss at the surface is counterbalanced by downward O₃ fluxes, maintaining the observed O₃ levels. The O₃ diurnal variability of ~1.5 nmol mol^-1 was a manifestation of combined effects of mesoscale wind changes and up- and downdrafts, in addition to the net photochemical loss. The study provides valuable insights into the intertwined dynamical and chemical processes governing the O₃ levels and variability over East Antarctica.