Teresa Campos

Closed-to-open cell mixed-phase cloud transitions within marine cold air outbreaks remain poorly understood despite their importance to high-latitude climate. The Cold-Air outbreak Experiment in the Sub-Arctic Region (CAESAR) research aircraft sampled a cold and polluted closed-to-open cell transition with cloud-top temperatures below −20 °C and upstream cloud condensation nuclei concentrations surpassing 600 cm ⁻³ . The aerosol likely originates from Siberian industrial emissions. As the boundary layer deepens prior to and across the transition, driven by strong turbulent surface fluxes and aided by a weakening cloud-top inversion, aerosol number concentrations decrease and supercooled droplet sizes increase, causing ice particles to shift from primarily vapor depositional growth towards riming. Precipitation shafts within open-cells contain abundant large graupel (>5 mm diameter) with liquid-equivalent precipitation intensities reaching 3 mm h ⁻¹ . Ice precipitation co-exists with lidar-observed surface cold pools, modulated by entrainment events, juxtaposed with surface-based plumes of warm moist air. Updrafts support liquid water production along convergence boundaries at the upwind cold pool edge. Nonetheless, strong buoyancy fluxes exceeding 200 W m ⁻² prevent sub-cloud decoupling. Ice particle number concentrations increase tenfold across the transition, surpassing 10 L ⁻¹ within open-cells, more than 1000× ice nucleating particle number concentrations. Given that cloud temperatures are too cold for Hallett-Mossop rime splintering, this indicates the importance of other secondary ice production mechanisms. This case expands the observations needed to better understand mixed-phase Arctic cloud processes.

Clouds change the chemical composition of aerosols, yet observations characterizing these processes are quite limited. Ground-based in-cloud measurements in coastal Southern California show that the highest nitrate to sulfate ratio (2.5 ± 1.7) in droplet residuals is near cloud base where cloud liquid water content and droplet number concentration are below the observed 20th percentile. The number size distribution of nitrate-rich particles peaks at a larger diameter than sulfate-rich particles, supporting growth to droplets of nitrate-rich particles at lower supersaturations than for sulfate-rich particles. Nitrate mass concentrations are up to three times higher in-cloud compared to pre-cloud and correlated with statistically significant reductions in ozone concentrations, consistent with contributions from in-cloud nitrate formation by N ₂ O ₅ hydrolysis. This study provides quantitative relationships of cloud properties to nitrate production in droplet residuals and associated ozone reduction, illustrating the impacts of aqueous reactions in clouds.

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.

Methane in the upper troposphere and lower stratosphere (UTLS) plays a critical role in atmospheric composition and radiative forcing, yet the processes governing its vertical structure and variability remain insufficiently understood. Here, we use high-quality satellite observations from the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS), together with in situ measurements, to characterize the vertical structure and global seasonal behavior of methane in the UTLS, with particular emphasis on the role of Asian Summer Monsoon (ASM). Methane exhibits a relative maximum that peaks near the tropopause inside the ASM anticyclone. The largest relative enhancement – defined as the difference between air inside and outside the anticyclone–occurs above the tropopause and extends poleward to the north. This behavior contrasts sharply with carbon monoxide, which is confined within the anticyclone. The seasonal cycle of methane near the tropical tropopause reaches maximum in boreal autumn, following the summer peak in surface fluxes and deep convection. This phase lag reflects the combined effects of ASM transport and subsequent large-scale redistribution following the breakdown of the anticyclone. The enhanced methane signal propagates upward into the lower stratosphere, consistent with the upwelling branch of the Brewer–Dobson circulation (BDC). Beyond the seasonal cycle, ACE-FTS data show a pronounced hemispheric asymmetry in lower stratospheric methane trends, consistent with previously reported changes in other long-lived trace constituents. Together, these results provide new observational evidence that ASM transport imprints a distinct methane structure above the tropopause and plays a key role in shaping UTLS variability on a global scale.

Methane in the upper troposphere and lower stratosphere (UTLS) plays a critical role in atmospheric composition and radiative forcing, yet the processes governing its vertical structure and variability remain insufficiently understood. Here, we use high-quality satellite observations from the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS), together with in situ measurements, to characterize the vertical structure and global seasonal behavior of methane in the UTLS, with particular emphasis on the role of Asian Summer Monsoon (ASM). Methane exhibits a relative maximum that peaks near the tropopause inside the ASM anticyclone. The largest relative enhancement – defined as the difference between air inside and outside the anticyclone–occurs above the tropopause and extends poleward to the north. This behavior contrasts sharply with carbon monoxide, which is confined within the anticyclone. The seasonal cycle of methane near the tropical tropopause reaches maximum in boreal autumn, following the summer peak in surface fluxes and deep convection. This phase lag reflects the combined effects of ASM transport and subsequent large-scale redistribution following the breakdown of the anticyclone. The enhanced methane signal propagates upward into the lower stratosphere, consistent with the upwelling branch of the Brewer–Dobson circulation (BDC). Beyond the seasonal cycle, ACE-FTS data show a pronounced hemispheric asymmetry in lower stratospheric methane trends, consistent with previously reported changes in other long-lived trace constituents. Together, these results provide new observational evidence that ASM transport imprints a distinct methane structure above the tropopause and plays a key role in shaping UTLS variability on a global scale.

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.