Tropospheric ozone in western Antarctica driven by synoptic-scale transport over 25 years at Belgrano II station

Tropospheric ozone (O₃) is a key oxidant and secondary pollutant that influences air quality and radiative forcing. Understanding its variability is crucial in regions highly sensitive to climate change, such as Antarctica, where complex interactions among stratosphere–troposphere exchange, air mass origin and local meteorology govern O₃ dynamics. Detailed characterisations of ozone vertical profiles under specific airflow regimes remain limited, especially for western Antarctica. This study analyses a comprehensive 25-year dataset (1999–2023) of ozone and meteorological profiles (754 in total) collected at Belgrano II station (77.87° S, 34.62° W). The objective is to characterise the vertical distribution of tropospheric ozone in western Antarctica and identify the main drivers of its variability.

Atmospheric transport and synoptic conditions were assessed using seasonal 850 hPa geopotential height maps and HYSPLIT back trajectories. A homogenisation procedure enabled the computation of seasonal and monthly means and long-term trends. The region is influenced by the Antarctic Polar Anticyclone, semi-permanent cyclones over the Weddell and Amundsen–Bellingshausen Seas, and persistent katabatic winds from the Antarctic Plateau. Four distinct transport regimes were identified: strong marine influence from the Weddell Sea, continental flows from northern and southern sectors, and mixed marine–continental influence over the Antarctic Peninsula.

Seasonal analysis of tropospheric ozone revealed increasing concentrations with altitude, ranging from ~20–30 ppb near the surface to ~45–55 ppb in the upper troposphere. O₃ concentrations peaked in winter (~25–35 ppb at low levels, ~45–50 ppb aloft) and early spring (~28–38 ppb at low levels, ~50–55 ppb aloft), while lower values were observed in summer (~20–25 ppb at low levels, ~40–45 ppb aloft) and autumn (~22–28 ppb at low levels, ~42–48 ppb aloft). These variations reflect the interplay of reduced photochemical destruction, enhanced stratosphere–troposphere exchange under the polar vortex, and increasing solar radiation during spring and summer.

Lower-tropospheric O₃ profiles (950–700 hPa) were modulated by transport regime. Highest mean concentrations occurred under purely marine flows from the Weddell Sea (29.8 ± 1.2 ppb), while Weddell–Antarctic Peninsula flows showed the lowest values (23.5 ± 1.7 ppb) due to topographic effects and halogen-driven ozone depletion. Continental flows exhibited intermediate levels (Northern: 26.9 ± 1.5 ppb; Southern: 24.3 ± 1.4 ppb). Finally, analysis of monthly mean profiles over the past two decades revealed a modest increase throughout the troposphere, below 1 ppb dec⁻¹.

These results highlight the combined influence of large-scale circulation, local dynamics, and seasonal processes on Antarctic tropospheric ozone and provide a baseline for evaluating future changes in the western Antarctic troposphere.