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

• Published in: Atmospheric Chemistry and Physics
• Main Authors: Girach, Imran A., Ojha, Narendra, Nair, Prabha R., Subrahmanyam, Kandula V., Koushik, Neelakantan, Nazeer, Mohammed M., Kiran Kumar, Nadimpally, Babu, Surendran Nair Suresh, Lelieveld, Jos, Pozzer, Andrea
• Format: Text
• Language: English
• Published: 2024
• Subjects: Antarc*; Antarctic; Antarctica; East Antarctica
• Online Access: https://doi.org/10.5194/acp-24-1979-2024; https://acp.copernicus.org/articles/24/1979/2024/

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 ∘ 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 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 ( ∼ 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 ( ∼ 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 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 ∼ 1.5 nmol mol −1 was a manifestation of ...