The space industry is currently expanding rapidly, especially for commercial use . As a result, new spaceports and launch vehicle companies are being established in historically aeronautically active nations such as the United States and Russia, and in nations with emerging space sectors such as China and India. In 2021, commercial space flights by Virgin Galactic, Blue Origin, and SpaceX demonstrated that space tourism is plausible , though the future scale of this nascent industry is uncertain. This rapid growth needs to be accompanied by a detailed understanding of the potential environmental impact of the space industry, including the impact of launch-rocket emissions on the ozone layer and climate.

Stratospheric ozone depletion has been a major environmental issue of the past few decades, since the suggestion in the 1970s that chlorine from chlorofluorocarbons (CFCs) could deplete ozone at 40 km , and especially since the discovery of the Antarctic ozone hole in the 1980s . The Montreal Protocol on ozone-depleting substances (ODSs) was signed in 1987 and ratified 2 years later. With several subsequent amendments, the Protocol now limits the production and consumption of all major long-lived ODSs ultimately emitted to the atmosphere . This has resulted in a turnround in stratospheric chlorine (and bromine) levels and signs of ozone recovery have been detected in the upper and polar lower stratosphere. Based on the projected decrease in halogen loadings, models predict that the date when stratospheric ozone values will return to 1980 levels will occur around the mid-late 21st century . However, other pollution sources, including rocket launches, may affect the rate and extent of this recovery. As the number of national and private space organizations continues to grow, and as the cost of space travel declines, the frequency of rocket launches is expected to rise significantly. This increased activity could make ozone depletion from space launches a serious environmental challenge, as highlighted by . Understanding the impact of rocket engine combustion emissions and exhaust on ozone depletion is essential, as these emissions are injected directly into the atmosphere, including the altitudes of the stratospheric ozone layer.

Rockets can be fuelled by a range of different propellants, which may affect the ozone layer in distinct ways. The pollutants produced by rocket emissions depend on the type of fuel used. For decades, solid propellants have played a fundamental role and have been widely used in the aerospace industry. Solid rocket motors (SRMs), consist of solid aluminium fuel with an ammonium perchlorate (NH₄ClO₄) oxidiser with some also containing hydrocarbons. The benefits of SRMs include easy storability, high reliability, and design simplicity. However, once ignited, they cannot be turned off and are therefore typically used only in the first stage of launch vehicles. The exhaust from SRMs contains a number of compounds of environmental concern, in particular hydrochloric acid (HCl) and alumina particles. As a pollutant emitted by rockets, reactive chlorine could enhance the ozone-depleting catalytic cycles active in the upper stratosphere and polar regions.

Many studies have examined the effects of rocket fuel emissions or single-species impacts, often focusing on emissions from a limited number of injection sites . Both reactive chlorine and alumina particulates are emitted together in SRM exhaust wakes, compounding the ozone losses that occur . used information on 2019 rocket launches and re-entry events to investigate the impacts of multiple rocket-emitted pollutants on stratospheric ozone, including NO_x. They found that ablative NO_x production during re-entry can have a significant effect on stratospheric ozone, while NO_x emissions from launches were relatively less important. The ozone impacts reported in their study were most pronounced in the upper stratosphere. For launch scenarios considering re‐entry NO_x production, a 0.5 % loss of global average column ozone has been estimated, with polar losses exceeding 2 % . In a recent modelling study, persistent levels of black carbon were calculated after 4–6 years of rocket launch activity, with year‐round ozone loss of 5–15 DU in the Northern Hemisphere (NH) and a potentially more severe Antarctic ozone hole . To support such impact studies, rocket emission inventories have been developed, encompassing a wide range of pollutants – including chlorine, NO_x, H₂O, CO₂, black carbon, and alumina – resulting from worldwide rocket launches .

Recently, used the SOCOL coupled chemistry-climate model (CCM) in free-running mode in a series of 25-year timeslice experiments (following 10 years of spin-up) to study the possible impact of near-future rocket launches on the ozone layer. They investigated two scenarios for the year 2030 of increased emissions relative to their benchmark 2019 inventory of 97 launches per year: an “ambitious” scenario with 2040 launches per year (× 21 increase on 2019), and a “conservative” one with 884 launches per year (× 9). They found decreases of 0.17 % and 0.29 % in the near-global annual mean ozone abundance under the conservative and ambitious scenarios, respectively. The ambitious scenario produced up to 3 % depletion in the upper stratosphere. Ozone changes in the lower stratosphere were largely not statistically significant compared to the CCM variability, with the notable exception of a large 3.9 % additional depletion in the Antarctic (60–90° S) spring (September–October–November mean). diagnosed that the rocket-induced ozone depletion was mainly due to chemical loss from the emitted chlorine, and circulation changes due to heating by black carbon particles. For reference, their ambitious scenario of 884 launches per year resulted in annual emissions of 10.13 Gg chlorine, thereby increasing the modelled upper stratospheric volume mixing ratio of inorganic chlorine (Cl_y) by around 0.2 parts per billion (ppbv).

In this paper, we use the Whole Atmosphere Community Climate Model Version 6 (WACCM6) to assess and quantify the impact on ozone from the potential injection of Cl_x from a rocket emission inventory . We aim to extend on the work of by investigating the impact of rocket-emitted Cl_x under a range of scenarios and in a model constrained (by nudging) to have realistic polar meteorology. This is important for quantifying polar ozone depletion and its interannual variability, and reduces the limitations when comparing model simulations with small differences in forcing. Other species, including NO_x, have different sources, chemistry, and spatial–temporal patterns of impact. Adding them to the same sensitivity experiments would make it harder to attribute ozone changes and to isolate the effect of SRM chlorine. The roles of Cl_x and NO_x also vary with altitude, latitude, and season. This study focuses on the polar lower stratosphere, where chlorine activation on polar stratospheric clouds (PSCs) drives ozone loss and where SRM chlorine will likely have a stronger impact. Section 2 describes our setup of WACCM6. Section 3 presents our results for long-term global mean impacts of the rocket emissions and the dependence on variability in polar meteorology. Our conclusions are summarised in Sect. 4.

We use the rocket emission inventory of , which is based on 97 successful rocket launches in the year 2019 from 18 active launch sites located at latitudes between 30° S and 62.9° N. The inventory is compiled from reports from governments and companies and provides the emission mass of H₂O, CO₂, NO_x, Cl_x, BC and Al₂O₃. Here we consider only Cl_x emissions from solid rocket motors, and do not include other rocket-emitted species. The majority of launches with this fuel in 2019 occurred at low latitudes, in particular from Kourou (5.2° N, 307.3° E). In our model simulations we scale the 2019 inventory emissions and apply the emissions each year during the model run (see Table 1).

We have performed a series of experiments with WACCM6 which extends from the Earth's surface to around 140 km. All simulations use 88 vertical levels within this domain and a horizontal resolution of 1.9° latitude × 2.5° longtitude. The model includes chemical mechanisms for the simplified tropospheric chemistry with detailed middle atmospheric chemistry (WACCM6 – MA), including heterogeneous reactions on sulfate aerosols and PSCs.

WACCM is very well established and widely used in the international community e.g.. Several previous studies have shown that WACCM6-SD can well characterise the spatial distribution, vertical profile, and interannual variability driven by dynamical and chemical processes in the Arctic and Antarctic stratosphere, and these studies have evaluated model performance by using satellite observations e.g..

The chemical species within WACCM6 include the extended O_x, NO_y, HO_x, Cl_y, and Br_y chemical families, along with CH₄ and its degradation products. In addition to CH₄, we also include N₂O (major source of NO_x), H₂O (major source of HO_x), plus various natural and anthropogenic precursors of the Cl_y and Br_y families. This mechanism also includes primary non-methane hydrocarbons and related oxygenated organic compounds. The chemical mechanism has evolved from previous versions e.g, and now includes a total of 100 species, 312 chemical reactions including 91 photolysis reactions, 19 odd-oxygen reactions, 17 odd-hydrogen reactions, 27 odd-nitrogen reactions, 36 odd-chlorine reactions, 21 odd-bromine reactions, 6 odd-fluorine reactions, 17 organic-halogens reactions, 17 sulfur reactions, 12 C1 reactions, 4 tropospheric aerosol and 17 stratospheric aerosol heterogeneous reactions and 28 ion reactions. The source gases (CO₂, CH₄, N₂O, ODSs) are from the CMIP6 forcing dataset (see Supplement, Fig. S1) which is constructed from a combination of direct atmospheric measurements, ice core reconstructions, and industrial emission records. The time-varying sea surface temperature and sea ice coverage (to match the forcing meteorology) are from and .

In this study, WACCM is nudged to MERRA-2 reanalyses to ensure that the model simulations have realistic lower stratospheric meteorology, which is important for chlorine-induced polar ozone loss. The nudging is applied only to winds and temperature ; the chemical species evolve freely. We performed 4 model simulations (see Table 1). The reference case (Rocket1) included all emission sources and is based on the released WACCM6 with a modal aerosol microphysical model (MAM4) of the Community Earth System Model Version 2 (CESM2.1.5) . Simulation Rocket1 also includes annually repeating 2019 rocket Cl_x emissions. The other three sensitivity runs were the same as run Rocket1 but scaled the 2019 rocket Cl_x emissions with different factors of 10, 52 or 120. The × 10, × 52, and × 120 scenarios are sensitivity experiments, not specific projections. They are scaled from the 2019 launch inventory, which provides a consistent reference, before rapid growth, to test ozone response under stronger emissions and to examine the linearity of the impacts. This study does not aim to predict the future of SRMs; rather it focuses on the effect of SRM-derived chlorine when injected into the stratosphere. Polar ozone loss is very sensitive to chlorine under cold conditions, so even a relatively small number of SRM launches may cause a clear signal in the polar lower stratosphere. The largest assumed increases in rocket launches are unrealistic for the near future but allow us to test the sensitivity of model results to different levels of chlorine. As the winds in WACCM are constrained here by nudging, we do not investigate the impact of rocket-emitted black carbon on ozone through changes in circulation.

In each simulation the model was integrated over 22 years using the observed source gas boundary conditions based on CMIP6 (see Fig. S1). The runs were nudged to the MERRA-2 meteorology from 1990–2012 to ensure that the simulations experienced a realistic range of meteorological conditions, especially at the poles. This is essential for simulating chlorine-driven polar ozone loss, particularly in the more variable Arctic. The winter of 2010/11 was included because it was an exceptionally cold Arctic winter with near-record ozone depletion . The first 7 years were treated as spin-up and the model years 1997–2012 were analysed for the impacts of rocket emissions. This length of model run is sufficient for us to diagnose the targetted chlorine impact for a range of polar conditions and reasonable background chlorine loading. Although stratospheric chlorine is decreasing, and will continue to do so, we use differences in our model runs to diagnose the relative impact of additional rocket-based emissions.

A motivation for this study was to further investigate the impact on ozone of rocket-emitted chlorine presented by . We now quantitatively compare our results with that study. For reference, the annual 2019 stratospheric emissions of chlorine in the inventory of , as processed in our WACCM simulations, is 0.5 Gg yr⁻¹. This is 16 % of the overall rocket emissions from 97 launches in that year. Assuming a 5-year stratospheric residence time of these emissions, the mean increase in Cl_y volume mixing ratio from continuous release of these emissions would be around 2.8 parts per trillion (pptv). The peak in stratospheric chlorine loading in the mid-late 1990s was about 3600 pptv, of which around 3000 pptv was contributed by ODSs .

reported decreases of 0.17 % and 0.29 % in the near-global (60° S–60° N) annual mean column ozone abundance under their conservative (884 launches per year) and ambitious (2040 launches per year) scenarios, respectively. They also reported 3.9 % depletion in Antarctic (60–90° S) springtime (September–October–November mean) column ozone. These reductions can be compared with observations of ozone changes over the past few decades. reported that the current (2017–2020) total column ozone was about 2.3 % below the 1964–1980 reference mean in the near-global average, about 1.1 % below the reference mean in the tropics (20° S–20° N), and about 3.6 % and 4.7 % below the reference means in NH and SH mid-latitudes (35–60° latitude), respectively.

The results of are summarised in Table 2, along with the equivalent quantities from our WACCM sensitivity runs compared to the reference run Rocket1. For near-global column ozone we find decreases of 0.04 %, 0.59 % and 1.3 % from runs Rocket10, Rocket52 and Rocket120, respectively. The decreases in Antarctic SON column ozone from the same runs are 0.20 %, 1.14 % and 2.79 %. Although we did not perform identical simulations to , our Rocket10 simulation is similar to their conservative scenario and we can use the derived sensitivity of our simulations to Cl (see Fig. ) to estimate the WACCM results for their ambitious (× 21) case. Overall we derive substantially smaller ozone depletion than , for example 0.09 % for global ozone and 0.45 % for SON Antarctic ozone from an “ambitious” scenario. It should be noted that we are only deriving the impact of the chlorine emissions, while included the effects of other emissions, notably the impact of black carbon on circulation. also constructed a specific scenario of future rocket launches which caused chlorine emissions at different locations to those assumed in our simplified scaling. Nevertheless, the differences between the model simulations are large since chlorine, which is transported globally by the Brewer-Dobson circulation, appears to be responsible for the majority of the chemical ozone depletion. It seems that the differences between our simulations are at least partly due to the magnitude of the modelled chlorine perturbations from the rocket emissions. For a given rocket scenario we find a smaller increase in Cl_y than . For example, they show increases in stratospheric Cl_y of around 0.2 ppbv from their ambitious run (their Fig. 3c), while we see smaller increases even from our Rocket52 simulation (Fig. ). This is despite their ambitious scenario emitting 10.13 Gg of chlorine into the stratosphere per year, while a × 21 scenario in our WACCM setup would emit 21 × 0.5 = 10.5 Gg yr⁻¹, a similar amount.

The smaller Cl_y response simulated here compared to may reflect differences in detail of the location of the rocket emissions and the stratospheric circulation in the respective models. For example, the spread of the emissions through the stratosphere by the slow Brewer-Dobson circulation, and thus their residence time in the stratosphere, will depend on the circulation which can vary between models. Further diagnosis of this is beyond the scope of this paper and would need formal model-model intercomparisons.

We have used a detailed CCM (WACCM6) to study the potential impact of rocket-emitted chlorine on stratospheric ozone. In particular, we have investigated the potential for future increased numbers of rocket launches, fuelled by solid propellants, to offset recovery of the ozone layer from the effects of ODSs. The CCM is nudged towards meteorological reanalyses from 1990–2012 to achieve realistic lower stratospheric meteorology, in order to allow accurate quantification of small differences in the model experiments. Different scenarios for the number of annual rocket launches are obtained by scaling a reference 2019 inventory .

Lower stratospheric ozone loss and column depletion are largest at high latitudes with a pronounced annual cycle and, in the Arctic, large meteorology-driven variability. The impact on Antarctic ozone peaks in October (additional depletion of 0.5 DU (modest growth in launches) and 3 DU (large growth)), while the impact in the Arctic peaks in April (2 DU for large growth). Although the mean impact in the Arctic is much smaller than for the Antarctic, the ozone loss shows large variability. In very cold years (e.g. exemplified by 2010/11 meteorology), the column loss in the Arctic exceeds the Antarctic for all launch scenarios and can exceed 8 DU for large growth. Ozone depletion in both the polar lower stratosphere and upper stratosphere shows a clear linear dependence on the level of chlorine enhancement.

Overall, the estimated impact of rocket-emitted chlorine for reasonable growth scenarios is small but does have the potential to offset some of the gains of the Montreal Protocol. However, we find a smaller impact than the recent study of , possibly at least in part due to different increases in stratospheric chlorine loading from similar emission scenarios. The potential impact of chlorine needs to be considered when deciding on propulsion systems for future rocket launches and in projections of ozone layer recovery.