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

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 (O 3 ) 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 circle S, 76.2 circle E, similar to 35 m above mean sea level) with EMAC (ECHAM5/MESSy Atmospheric Chemistry) atmospheric chemistry-climate model simulations. The model reproduced the observed surface O 3 level (18.8 +/- 2.3 nmol mol - 1 ) with negligible bias and captured much of the variability ( R = 0.5). Model-simulated tropospheric O 3 profiles were in reasonable agreement with balloon-borne measurements (mean bias: 2-12 nmol mol - 1 ). Our analysis of a stratospheric tracer in the model showed that about 41 %-51 % of surface O 3 over the entire Antarctic region was of stratospheric origin. Events of enhanced O 3 ( similar to 4-10 nmol mol - 1 ) were investigated by combining O 3 vertical profiles and air mass back trajectories, which revealed the rapid descent of O 3 -rich air towards the surface. The photochemical loss of O 3 through its photolysis (followed by H 2 O + O( 1 D)) and reaction with hydroperoxyl radicals (O 3 + HO 2 ) dominated over production from precursor gases (NO + HO 2 and NO + CH 3 O 2 ) resulting in overall net O 3 loss during the austral summer. Interestingly, the east coastal region, including the Bharati station, tends to act as a stronger chemical sink of O 3 ( similar to 190 pmol mol - 1 d - 1 ) than adjacent land and ocean regions (by similar to 100 pmol mol - 1 d - 1 ). This is attributed to reverse latitudinal gradients between H 2 O and O( 1 D), whereby O 3 loss through photolysis (H 2 O + O( 1 D)) reaches a maximum over the east coast. Further, the net photochemical loss at the surface is counterbalanced by downward O 3 fluxes, maintaining the observed O 3 levels. The O 3 diurnal variability of similar to 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 O 3 levels and variability over East Antarctica.