07 Aug 2023
 | 07 Aug 2023

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

Imran A. Girach, Narendra Ojha, Prabha R. Nair, Kandula V. Subrahmanyam, Neelakantan Koushik, Mohammed M. Nazeer, Nadimpally Kiran Kumar, Surendran Nair Suresh Babu, Jos Lelieveld, and Andrea Pozzer

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.

Imran A. Girach et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2023-1524', Anonymous Referee #1, 13 Sep 2023
    • AC1: 'Reply on RC1', Imran A. Girach, 05 Dec 2023
  • RC2: 'Comment on egusphere-2023-1524', Anonymous Referee #2, 26 Sep 2023
    • AC2: 'Reply on RC2', Imran A. Girach, 05 Dec 2023

Imran A. Girach et al.

Imran A. Girach et al.


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Short summary
We investigated surface ozone variability at East Antarctica based on the measurements and EMAC global model simulations during austral summer. Nearly half of the surface ozone is found to be of stratospheric origin. The east coast of Antarctica acts as a stronger sink of ozone than surrounding regions. Photochemical loss of ozone is counterbalanced by downward transport of ozone. Study highlights intertwined role of chemistry and dynamics in governing the ozone variations over East Antarctica.