Trends in tropospheric ozone, an important air pollutant and short-lived climate forcer (SLCF), are estimated using available surface and ozonesonde profile data for 1993-2019. Using a coherent methodology, observed trends are compared to modeled trends (1995-2015) from the Arctic Monitoring Assessment Programme SLCF 2021 assessment. Statistically significant increases in observed surface ozone at Arctic coastal sites, notably during winter, and concurrent decreasing trends in surface carbon monoxide, are generally captured by multi-model median (MMM) trends. Wintertime increases are also estimated in the free troposphere at most Arctic sites, but tend to be overestimated by the MMMs. Springtime surface ozone increases in northern coastal Alaska are not simulated while negative springtime trends in northern Scandinavia are not always reproduced. Possible reasons for observed changes and model behavior are discussed, including decreasing precursor emissions, changing ozone sinks, and variability in large-scale meteorology.

In August 2017, a smoke plume from wildfires in British Columbia and the Northwest Territories recirculated and persisted over northern Canada for over two weeks. We compared a full-factorial set of NASA Goddard Institute for Space Studies ModelE simulations of the plume to satellite retrievals of aerosol optical depth and carbon monoxide, finding that ModelE performance was dependent on the model configuration, and more so on the choice of injection height approach, aerosol scheme and biomass burning emissions estimates than to the choice of horizontal winds for nudging. In particular, ModelE simulations with free-tropospheric smoke injection, a mass-based aerosol scheme and high fire NOx emissions led to unrealistically high aerosol optical depth. Using paired simulations with fire emissions excluded, we estimated that for 16 days over an 850 000 km2 region, the smoke decreased planetary boundary layer heights by between 253 m and 547 m, decreased downward shortwave radiation by between 52 Wm-2 and 172 Wm-2, and decreased surface temperature by between 1.5 oC and 4.9 oC, the latter spanning an independent estimate from operational weather forecasts of a 3.7 oC cooling. The strongest surface climate effects were for ModelE configurations with more detailed aerosol microphysics that led to a stronger first indirect effect.

We assess the effective radiative forcing due to ozone-depleting substances using models participating in the Aerosols and Chemistry Model Intercomparison Project (AerChemMIP). A large inter-model spread in this globally averaged quantity necessitates an “emergent constraint” approach whereby we link the radiative forcing to the amount of ozone depletion simulated during 1979-2000, excluding two volcanically perturbed periods. During this period ozone-depleting substances were increasing, and several merged satellite-based climatologies document the ensuing decline of total-column ozone. We use these analyses to come up with effective radiative forcing magnitudes. For all of these satellite climatologies we find an effective radiative forcing outside or on the edge of the previously published “likely” range given by the 5th Assessment Report of IPCC, implying an offsetting effect of ozone depletion and/or other atmospheric feedbacks of -0.4 to -0.25 Wm-2, which is in absolute terms is larger than the previous best estimate of -0.15 Wm-2.

Acetone is an abundant volatile organic compound with important influence on ozone and atmospheric self-cleaning processes. The budget of acetone is influenced by various sources and sinks. Direct sources include anthropogenic, terrestrial vegetation, oceanic, and biomass burning emissions, while chemistry forms acetone from other compounds. Sinks include deposition onto the land and ocean surfaces, as well as chemical loss. The GISS Earth System Model, ModelE, is capable of simulating a variety of Earth system interactions. Previously, acetone had a very simplistic representation in the ModelE chemical scheme. This study assesses a greatly improved acetone tracer scheme, in which acetone's sources, sinks and atmospheric transport are now tracked in 3 dimensions. Extensive research was conducted to assess how well past literature supported the new global acetone budget. Anthropogenic, vegetation, biomass burning, and deposition schemes fit well with previous studies. While their net fluxes were well-supported, source and sink terms for chemistry and the ocean were overestimated and underestimated, respectively. In iterations of the chemistry scheme, it was found that the production of acetone from hydrocarbon oxidation is a strong leverage to the overall chemical source. Spatial distributions reveal that ocean uptake of acetone dominates northern latitudes, while production is mainly in mid-southern latitudes. Ocean surface conditions influence ocean-acetone interactions and will be considered when modifying the ocean scheme in future work. The seasonality of acetone-related processes was also studied in conjunction with field measurements around the world. These comparisons show promising results, but have shortcomings at urban locations, since the model's resolution is too coarse to capture high-emission areas. Overall, an analysis of the acetone budget aids the development of the tracer in the GISS ModelE, a crucial step to parameterizing the role of acetone in the atmosphere.

We quantify the impacts of halogenated ozone-depleting substances (ODSs), methane, N2O, CO2, and short-lived ozone precursors on total and partial ozone column changes between 1850 and 2014 using CMIP6 Aerosol and Chemistry Model Intercomparison Project (AerChemMIP) simulations. We find that whilst substantial ODS-induced ozone loss dominates the stratospheric ozone changes since the 1970s, the increases in short-lived ozone precursors and methane lead to increases in tropospheric ozone since the 1950s that make increasingly important contributions to total column ozone (TCO) changes. Our results show that methane impacts stratospheric ozone changes through its reaction with atomic chlorine leading to ozone increases, but this impact will decrease with declining ODSs. The N2O increases mainly impact ozone through NOx-induced ozone destruction in the stratosphere, having an overall small negative impact on TCO. CO2 increases lead to increased global stratospheric ozone due to stratospheric cooling. However, importantly CO2 increases cause TCO to decrease in the tropics. Large interannual variability obscures the responses of stratospheric ozone to N2O and CO2 changes. Substantial inter-model differences originate in the models’ representations of ODS-induced ozone depletion. We find that, although the tropospheric ozone trend is driven by the increase in its precursors, the stratospheric changes significantly impact the upper tropospheric ozone trend through modified stratospheric circulation and stratospheric ozone depletion. The speed-up of stratospheric overturning (i.e. decreasing age of air) is driven mainly by ODS and CO2; increases. Changes in methane and ozone precursors also modulate the cross-tropopause ozone flux.

Biases in aerosol optical depths (AOD) and land surface albedos in the AeroCom models are manifested in the top-of-atmosphere (TOA) clear-sky reflected shortwave (SW) fluxes. Biases in the SW fluxes from AeroCom models are quantitatively related to biases in AOD and land surface albedo by using their radiative kernels. Over ocean, AOD contributes about 25% to the 60°S-60°N mean SW flux bias for the multi-model mean (MMM) result. Over land, AOD and land surface albedo contribute about 40% and 30%, respectively, to the 60°S-60°N mean SW flux bias for the MMM result. Furthermore, the spatial patterns of the SW flux biases derived from the radiative kernels are very similar to those between models and CERES observation, with the correlation coefficient of 0.6 over ocean and 0.76 over land for MMM using data of 2010. Satellite data used in this evaluation are derived independently from each other, consistencies in their bias patterns when compared with model simulations suggest that these patterns are robust. This highlights the importance of evaluating related variables in a synergistic manner to provide an unambiguous assessment of the models, as results from single parameter assessments are often confounded by measurement uncertainty. We also compare the AOD trend from three models with the observation-based counterpart. These models reproduce all notable trends in AOD (i.e. decreasing trend over eastern United States and increasing trend over India) except the decreasing trend over eastern China and the adjacent oceanic regions due to limitations in the emission dataset.

For decades the scientific community has been trying to reconcile abundant evidence for fluvial activity on Noachian and early Hesperian Mars with the faint young Sun and reasonable constraints on ancient atmospheric pressure and composition. Recently, the investigation of H2-CO2 collision induced absorption has opened up a new avenue to warm Noachian Mars. We use the ROCKE-3D global climate model to simulate plausible states of the ancient Martian climate with this absorptive warming and reasonable constraints on surface paleopressure. We find that 1.5-2 bar CO2-dominated atmospheres with 3% H2 can produce global mean surface temperatures above freezing, while also providing sufficient warming to avoid surface atmospheric CO2 condensation at 0°-45° obliquity. Simulations conducted with both modern topography and a paleotopography, before Tharsis formed, highlight the importance of Tharsis as a cold trap for water on the planet. Additionally, we find that low obliquity (modern and 0°) is more conducive to rainfall over valley network locations than high (45°) obliquity.