A Study of the Low-Ozone Episode over Scandinavia and Northwestern Russia in March 2025

Following a very cold first half of the Arctic stratosphere winter of 2024–2025, the stratospheric polar vortex weakened from late February. The increase in the polar lower stratosphere temperature led to a decrease in the polar stratospheric cloud (PSC) type I (NAT) volume from ~80 million km3 to zero. The polar vortex weakening and temperature increase continued in early March, when major sudden stratospheric warming occurred. Although the polar cap total column ozone (TCO) significantly increased during this period, an ozone mini-hole formed over Scandinavia and northwestern Russia, with TCO values as low as 220–240 Dobson units, according to satellite observations and ground-based measurements over St. Petersburg and Moscow on 5–6 March 2025. Chemistry-transport model calculations using MERRA2 reanalysis data were performed to investigate the role of chemical ozone depletion and dynamical processes in the low TCO values in early March. Model experiments show that dynamical processes played a predominant role in the formation of low TCO values, but the role of chemical processes was not negligible. Associated with the TCO anomaly, the difference relative to the standard ozone level in the UV indices over Moscow, St. Petersburg and Helsinki reached up to 60–100%.