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 (ECHAM5/MESSy Atmospheric Chemistry) 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: 2–12 nmol mol−1). Our analysis of a stratospheric tracer in the model showed that about 41 %–51 % 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.