Numerous satellite operators are building megaconstellations in Low Earth Orbit (LEO) with hundreds of satellites, placing new satellites and spent rocket stages in orbit. Once these objects fail, they are often removed from LEO via atmospheric reentry, producing metallic particles that can interact with ozone chemistry and Earth’s radiative balance. The extent of these interactions remains poorly understood despite their importance to current space governance and policymaking. Helping to address this gap, this paper estimates the distribution, lifetime and direct radiative forcing of reentry-ablated alumina using an Earth system model. We consider a future scenario where all megaconstellations publicly filed at the Federal Communications Commission as of 2022 are operating, amounting to 2.52 Gg/yr of reentry-ablated alumina emissions. As a conservative approximation, we find that reentry-ablated alumina particles have an atmospheric lifetime between one and two years, leading to a cooling radiative forcing of approximately -0.378mW/m2. Simulations with fine alumina particles produce between 14% and 36% larger radiative forcings and have lifetimes 1.54 times longer than simulations with coarse alumina emissions. Alumina emitted only in the South Pacific produces an asymmetrical radiative forcing. Furthermore, modeling alumina with time-averaged, constant emissions rather than in discrete reentry plumes in results in 21% to 24% overestimation of alumina’s radiative forcing. These results are sensitive to numerous assumptions on initial particle size, radiative indices and coagulation characteristics of the aerosol. In-situ observation and a sophisticated understanding of reentry-ablated alumina particles are necessary to better predict the atmospheric consequences of reentry-ablated alumina.

Nuclear and coal power use in the United States are projected to decline over the coming decades. Here, we explore how simultaneous phase-outs of these energy sources could affect air pollution and distributional health risk with existing grid infrastructure. We develop an energy grid dispatch model to estimate the emissions of CO2, NOx and SO2 from each U.S. electricity generating unit. We couple the emissions from this model with a chemical transport model to calculate impacts on ground-level ozone and fine particulate matter (PM2.5). Our yearlong scenario removing nuclear power results in compensation by coal, gas and oil, leading to increased emissions that impact climate and air quality nationwide. We estimate that changes in PM2.5 and ozone lead to an additional 9,200 yearly mortalities, and that changes in CO2 emissions over that period lead to an order of magnitude higher mortalities throughout the 21st century. Together, air quality and climate impacts incur between \$80.7-\$126.1 billion of annual costs. In a scenario where nuclear and coal power are shut down simultaneously, air quality impacts due to PM2.5 are larger and those due to ozone are smaller, because of more reliance on high emitting gas and oil, and climate impacts are substantially smaller than that of nuclear power shutdowns. With current reliance on non-coal fossil fuels, closures of nuclear and coal plants shift the distribution of health risks, exemplifying the importance of multi-system analysis and unit-level regulations when making energy decisions.

The NASA Goddard Earth Observing System (GEOS) Composition Forecast (GEOS-CF) provides recent estimates and five-day forecasts of atmospheric composition to the public in near-real time. To do this, the GEOS Earth system model is coupled with the GEOS-Chem tropospheric-stratospheric unified chemistry extension (UCX) to represent composition from the surface to the top of the GEOS atmosphere (0.01 hPa). The GEOS-CF system is described, including updates made to the GEOS-Chem UCX mechanism within GEOS-CF for improved representation of stratospheric chemistry. Comparisons are made against balloon, lidar and satellite observations for stratospheric composition, including measurements of ozone (O3) and important nitrogen and chlorine species related to stratospheric O3 recovery. The GEOS-CF nudges the stratospheric O3 towards the GEOS Forward Processing (GEOS FP) assimilated O3 product; as a result the stratospheric O3 in the GEOS-CF historical estimate agrees well with observations. During abnormal dynamical and chemical environments such as the 2020 polar vortexes, the GEOS-CF O3 forecasts are more realistic than GEOS FP O3 forecasts because of the inclusion of the complex GEOS-Chem UCX chemistry. Overall, the spatial pattern of the GEOS-CF simulated concentrations of stratospheric composition agrees well with satellite observations. However, there are notable biases – such as low NOx and HNO3 in the polar regions and generally low HCl throughout the stratosphere – and future improvements to the chemistry mechanism and emissions are discussed. GEOS-CF is a new tool for the research community and instrument teams observing trace gases in the stratosphere and troposphere, providing near-real-time three-dimensional gridded information on atmospheric composition.

Detailed examination of the impact of modern space launches on the Earth’s atmosphere is crucial, given booming investment in the space industry and an anticipated space tourism era. We develop air pollutant emissions inventories for rocket launches and re-entry of reusable components and debris in 2019 and for a speculative space tourism scenario based on the recent billionaire space race. This we include in the global GEOS-Chem model coupled to a radiative transfer model to determine the influence on stratospheric ozone (O3) and climate. Due to recent surge in re-entering debris and reusable components, nitrogen oxides from ablation and chlorine from solid fuels contribute equally to all stratospheric O3 depletion by contemporary rockets. Decline in global stratospheric O3 is small (0.01%), but reaches 0.15% in the upper stratosphere (~5 hPa, 40 km) in spring at 60-90°N after a decade of sustained 5.6% a-1 growth in 2019 launches and re-entries. This increases to 0.22% with a decade of emissions from space tourism rockets, undermining O3 recovery achieved with the Montreal Protocol. Rocket emissions of black carbon (BC) produce substantial global mean warming of 8 mW m-2 after just 3 years of routine space tourism launches. This is a much greater contribution to global radiative forcing (6%) than emissions (0.02%) of all other BC sources, as warming per unit mass emitted is ~500 times more than surface and aviation sources. The O3 damage and warming we estimate should motivate regulation of an industry poised for rapid growth.