| 19 Nov 2025
The Impact of Rocket-Emitted Chlorine on Stratospheric Ozone
Abstract. Although stratospheric ozone is recovering under the Montreal Protocol, the rapidly expanding space industry may influence the pace of this recovery. We assess the potential for rocket-emitted chlorine, under various launch growth scenarios, to offset the decline in chlorine from regulated Ozone Depleting Substances (ODSs). We used the Whole Atmosphere Community Climate Model (WACCM6) nudged to meteorological reanalyses to simulate realistic atmospheric variability. A modest (times ten) increase in chlorine emissions from rocket launches relative to 2019 causes a near-global column ozone loss of less than 0.1 Dobson Unit (DU) (0.04 %), while a large (times 52) increase leads to 0.6 DU (0.23 %) depletion. Local ozone decreases reach 0.4 % and 2 % in the upper stratosphere for these scenarios. Column losses peak at high latitudes, with strong seasonality and meteorology-driven variability in the Arctic. The impact peaks in October in the Antarctic (0.5 DU and 3 DU depletion for ×10 and ×52 cases), and in April in the Arctic (generally up to 2 DU for the ×52 case, or greater than 8 DU in cold years with meteorology such as 2010/11). Ozone depletion throughout the stratosphere scales linearly with chlorine enhancement. Overall, while the effects of rocket-emitted chlorine under plausible growth scenarios are small, they could partially offset the gains achieved by the Montreal Protocol and should be considered in future assessments of rocket propulsion systems and ozone layer recovery projections.
Competing interests: John M. C. Plane is a member of the editorial board of ACP.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.
Review of Yuwen Li et al., 2025, The Impact of Rocket-Emitted Chlorine on Stratospheric Ozone
This is an interesting, timely and well written article. As rocket launch rates increase, understanding the role of rocket exhaust on stratospheric composition is crucial, since the stratospheric ozone layer is a vulnerable, recovering part of the planet. You report on stratospheric ozone changes under different chlorine emission scenarios from rocket launches. The conclusions drawn are supported by the simulations they present, but much more context and confidence could be provided by model evaluation. I also think the methods section requires clarification. I have compiled my thoughts below, comprising major revisions I think should be addressed before publication.
Major queries:
Minor queries and suggestions:
Line 38: “the hydrocarbon” probably should be “hydrocarbons”
Line 46 onwards: The discussion of Ryan et al (2022) needs some refinement: that paper examined all rocket pollutants, not just NOx, and in addition, it would be fair to say they found launch NOx “relatively less important”, but not “not important” as you state. It is worth pointing out here that the ozone impacts in that paper were most significant in the upper stratosphere.
Line 81: It is worth putting the longitude of Korou too.
Line 81: Link to table 1 here as this is where you outline the emission inventory scaling year on year.
Table 1: if you’re keeping the focus as Clx emissions only, it would be helpful to add a column to the table detailing the magnitude of the Clx emissions (i.e. Tg Clx). This would help the reader understand quickly the extra Clx in each simulation.
Section 3.1: You show Cly increases. Could you elaborate briefly (in the text is fine) on which species dominate the Cly changes in your rocket scenarios?
Line 137: “These are the regions where chlorine chemistry is expected to have an impact on ozone” - this sentence (and possibly the one after it too) sounds like it needs a reference.
Section 3.2: in your discussion of ozone decreases (e.g. of a certain amount of ppb, or DU), it would provide great context to readers slightly less familiar with stratospheric ozone if you included what these changes amounted to as a percentage. (You do this at some points but it could be more widespread.)
Figure 8: one advantage of your simulation period is that there were some significant ozone holes in there. When you talk about interannual variability, you have the opportunity to contextualize the significance of the ozone depletion due to rockets (and each year’s meteorology) against the size of the each year’s actual ozone hole. This could either be as a percentage in the text, or incorporated as a timeseries into Figure 8.
Line 250 onwards: could you hypothesize further on why you think you model a smaller Cly concentration change than Revell et al (2025)? What differences in deposition, chemical scheme or other model factors might give rise to this? This seems like a pretty significant difference in conversion to reactive chlorine, which it would be good to understand (and again make me wonder how well your model performs relative to observations at converting other ODS to reactive chlorine and simulating past ozone holes).