Tropospheric ozone trends and drivers at a Southern Hemisphere background site in Chile

Abstract

Tropospheric ozone (O3) is a significant anthropogenic climate forcer with uncertain distribution in the Southern Hemisphere due to sparse observations. This study analyzes 28 years of in situ ozone, methane, carbon monoxide, and meteorological data at Tololo (30.17° S, 70.80° W, 2154 m a.s.l.), Chile, integrating reanalysis and atmospheric chemistry modeling. Here we identify a rising ozone trend of 2.1 ± 0.8 ppbv per decade since 2006, possibly driven by increasing background methane. We quantify contributions from biomass burning and transport of stratospheric and upper tropospheric air to Tololo, each adding approximately 5 ppbv per event during late winter and spring O3 maximum. Events of air of stratospheric and upper tropospheric origin are linked to synoptic-scale troughs and cutoff lows, modulated by El Niño Southern Oscillation phases. These findings enhance understanding of ozone variability in the Southern Hemisphere free troposphere and underscore the importance of sustained observations at Tololo to monitor tropospheric ozone dynamics amid climate change.