What controls tropospheric carbon monoxide in the remote Southern Hemisphere?

Abstract. The remote extratropical Southern Hemisphere (SH) has the cleanest atmosphere on Earth, but is subject to rapid changes regarding the emissions landscape and a warming climate, which strongly impact local atmospheric composition. Carbon monoxide (CO) is a product of methane (CH₄) oxidation and a modulator of the tropospheric oxidizing capacity. In this study we investigate if the rapid increase in tropospheric CH₄ levels of ∼7 % between 2008 and 2022 has driven a positive response in CO in the remote SH, based on multi-platform observations and a global chemistry transport model. We find inconsistencies in observations from aircraft, surface and satellite. CSIRO surface flask observations, AGAGE in-situ measurements (2008–2022) and airborne observations from the HIPPO (2009–2011) and ATom (2016–2018) campaigns show CO increases of 5–10 %. CO columns inferred from IASI (2014–2023) show more moderate increases of 3–4 % (statistically insignificant). NOAA surface flask observations and CO columns from MOPITT (2008–2022) do not show changes over time. GEOS-Chem simulations are consistent with the observed positive trend, exhibiting a significant CO increase of ∼7 % throughout the southern extratropical troposphere (2008–2022). These CO increases are partly driven by enhanced production from rising CH₄ (∼50 %) and further affected by increases in biogenic non-methane volatile organic compounds (NMVOC) (∼45 %) and the direct effects of rising temperatures (∼5 %). We further explore how simulated CO levels in the remote SH could be impacted by future enhancements in primary and precursor emissions, changes in the oxidizing capacity, and temperature.