Clara Maria Nussbaumer

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.

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.

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.

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.

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.

Tropospheric ozone (O3) is a greenhouse gas and air pollutant. Despite efforts to control O3 precursor emissions, O3 levels frequently exceed the Swiss air quality standards. We present multi-decadal summertime measurements of O3 and its precursors across Switzerland from 12 NABEL (Nationales Beobachtungsnetz für Luftfremdstoffe) stations, which are representative of traffic, (sub)urban, rural and background conditions. Average O3 levels have decreased at rural and background sites, remained constant at (sub)urban sites and increased under traffic conditions over the past two decades. Traffic, (sub)urban and rural sites exhibited a pronounced weekend effect at the beginning of the century, which has weakened over time and only persists under traffic conditions today, suggesting that O3 formation is becoming more NOx-sensitive. O3 exhibits a strong dependence on temperature (dO3/dT), which has weakened uniformly at all site types over time. At polluted sites, this effect could be associated with the decreasing influence of titration. While reductions of precursor levels have shifted the probability of O3 exceedances to higher temperatures, O3 is still frequently exceeded on hot summer days and the number of days exceeding 30 °C has tripled since 2000. Ozone formation has been suppressed due to the titration by NO in many locations in the past but is dominated by NOx-sensitive O3 chemistry in background, rural, and (sub)urban environments today. Ozone titration remains dominant under traffic conditions, where O3 levels are currently increasing with NOx and will likely increase for several years before emissions reductions will become effective.

The natural landscape has undergone profound transformations due to human activities, with vast areas being converted for agriculture and grazing. This shift has far-reaching consequences for the Earth's system, impacting various components such as surface reflectivity, roughness, evapotranspiration, and atmospheric composition.To better understand the effects of land cover change on atmospheric chemistry, this study employs the chemistry–climate model EMAC to simulate two distinct scenarios. The first scenario represents the current state of land cover, characterized by widespread deforestation for agricultural and grazing purposes, with the potential natural vegetation (PNV) cover simulated by the model. In contrast, the second scenario depicts an extreme reforestation scenario, where grazing land is restored to its natural state.The results of this study reveal that the expansion of agricultural land leads to a decline in global emissions of biogenic volatile organic compounds (BVOCs). This decrease in BVOC emissions, in turn, results in higher surface concentrations of hydroxyl radicals (OH, +5.7%) and lower mixing ratios of carbon monoxide (CO, -6.2%). Notably, this trend persists despite increased CO emissions from agricultural biomass burning.At the same tim, the mixing ratios of nitrogen oxides (NOx) exhibit an increase (+7.8%) due to enhanced anthropogenic and natural soil sources. While regional ozone responses may vary, the global ozone production sensitivity shifts from a NOx- to a VOC-sensitive regime.These changes have significant implications for radiative forcing, with reductions in tropospheric ozone and methane lifetimes contributing to a combined radiative effect of −60 mW m−2 (cooling). This cooling effect partially offsets the warming resulting from reduced BVOC-driven aerosol formation.

Tropospheric ozone (O₃) is a greenhouse gas and air pollutant. Despite efforts to control O₃ precursor emissions, O₃ levels frequently exceed the Swiss air quality standards. We present multi-decadal summertime measurements of O₃ and its precursors across Switzerland from 12 NABEL (Nationales Beobachtungsnetz für Luftfremdstoffe) stations, which are representative of traffic, (sub)urban, rural and background conditions. Average O₃ levels have decreased at rural and background sites, remained constant at (sub)urban sites and increased under traffic conditions over the past two decades. Traffic, (sub)urban and rural sites exhibited a pronounced weekend effect at the beginning of the century, which has weakened over time and only persists under traffic conditions today, suggesting that O₃ formation is becoming more NO_x-sensitive. O₃ exhibits a strong dependence on temperature (dO₃/dT), which has weakened uniformly at all site types over time. At polluted sites, this effect could be associated with the decreasing influence of titration. While reductions of precursor levels have shifted the probability of O₃ exceedances to higher temperatures, O₃ is still frequently exceeded on hot summer days and the number of days exceeding 30 °C has tripled since 2000. Ozone formation has been suppressed due to the titration by NO in many locations in the past but is dominated by NO_x-sensitive O₃ chemistry in background, rural, and (sub)urban environments today. Ozone titration remains dominant under traffic conditions, where O₃ levels are currently increasing with NO_x and will likely increase for several years before emissions reductions will become effective.

Tropospheric ozone (O₃) is a greenhouse gas and air pollutant. Despite efforts to control O₃ precursor emissions, O₃ levels frequently exceed the Swiss air quality standards. We present multi-decadal summertime measurements of O₃ and its precursors across Switzerland from 12 NABEL (Nationales Beobachtungsnetz für Luftfremdstoffe) stations, which are representative of traffic, (sub)urban, rural and background conditions. Average O₃ levels have decreased at rural and background sites, remained constant at (sub)urban sites and increased under traffic conditions over the past two decades. Traffic, (sub)urban and rural sites exhibited a pronounced weekend effect at the beginning of the century, which has weakened over time and only persists under traffic conditions today, suggesting that O₃ formation is becoming more NO_x-sensitive. O₃ exhibits a strong dependence on temperature (dO₃/dT), which has weakened uniformly at all site types over time. At polluted sites, this effect could be associated with the decreasing influence of titration. While reductions of precursor levels have shifted the probability of O₃ exceedances to higher temperatures, O₃ is still frequently exceeded on hot summer days and the number of days exceeding 30 °C has tripled since 2000. Ozone formation has been suppressed due to the titration by NO in many locations in the past but is dominated by NO_x-sensitive O₃ chemistry in background, rural, and (sub)urban environments today. Ozone titration remains dominant under traffic conditions, where O₃ levels are currently increasing with NO_x and will likely increase for several years before emissions reductions will become effective.