| 19 Mar 2026
Developing tracer interrelationships to derive stratospheric age of air from satellite observations of nitrous oxide
Abstract. Chemistry-climate models predict a strengthening of the Brewer-Dobson Circulation (BDC) in response to climate change, which has implications for global atmospheric composition, radiation, and climate. This predicted acceleration has not been confirmed with observations, and models also disagree about the mean stratospheric circulation and mixing strength. The BDC impacts the distribution of long-lived tracers and their empirical relationships with one another. Age of air is an important diagnostic for changes in the BDC, and it can be derived from long-lived trace gases, such as sulfur hexafluoride (SF6) and nitrous oxide (N2O). We introduce an updated technique to calculate age of air using satellite observations of N2O. We (1) compute tracer interrelationships of age of air and N2O (Age:N2O) and demonstrate that they vary with latitude, and then (2) use these relationships to calculate a new N2O-derived age timeseries that takes this latitude variability into account from 2005 to 2012. The tracer interrelationships and their variability with latitude provide a better understanding of the structure and seasonality of the BDC. In particular, latitudinally-resolved Age:N2O relationships reflect the relative importance of photochemical loss of N2O in different regions and enable hemispheric structural comparisons. The N2O-age product has more extensive spatial coverage than previous counterparts. Additionally, N2O and SF6-age compare well, showing that Age:N2O relationships are robust on seasonal and interannual time scales. While this timeseries is only 7 years long, this manuscript lays the groundwork for calculating a longer record of N2O-age to understand long-term variability and shifts of the BDC.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics.
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Review for Castillo et al., entitled "Developing tracer interrelationships to derive stratospheric age of air from satellite observations of nitrous oxide", submitted to ACPD.
This manuscript presents a methodology for calculating age of air (AoA) using nitrous oxide observations from MLS and ACE-FTS, supplemented by SF6-derived relationships from MIPAS. While the study provides a valuable framework for extending the stratospheric age record, several methodological choices regarding data presentation and normalisation require further clarification and sensitivity testing. Overall, I am happy to accept the manuscript with minor corrections if authors can address the technical points raised below.
The study successfully demonstrates that tracer interrelationships between sink-corrected SF6-age and normalised nitrous oxide are highly compact and display a clear meridional dependence. A significant finding is the identification of a distinct inflection point at a normalised nitrous oxide value of 0.7, where photochemical loss begins to dominate over transport, particularly in the deep tropics. Below this threshold, typically corresponding to ages between 3.0 and 3.5 years, the nitrous oxide abundance decreases rapidly with only minimal further aging. Furthermore, the authors confirm interhemispheric asymmetries in AoA seen in chemical models, noting that at a constant normalised nitrous oxide abundance, mean ages in the Southern Hemisphere are older than those in the Northern Hemisphere between values of 0.7 and 0.3.
The comparison between the newly derived MLS and ACE-FTS nitrous oxide-AoA products and existing SF6-age records confirms the broad structure of the Brewer-Dobson circulation while highlighting specific instrumental biases. For instance, MIPAS SF6-age remains older than the MLS nitrous oxide based AoA estimates in the lower stratosphere, likely due to known high biases in tropical MIPAS retrievals. These latitudinally-resolved relationships provide a more subtle comparison between the hemispheres and highlight the relative importance of mixing versus chemical loss in different regions. To further strengthen the validation of these products, especially considering the reported instrumental drifts in MLS after 2010, I would suggest comparing the results with the TCOM-N2O dataset (Dhomse et al., https://zenodo.org/records/18197444). This record utilises machine learning and TOMCAT simulations to provide an observationally constrained, gap-free nitrous oxide profile that could serve as a robust independent reference.
Regarding the chemical mechanisms, it is important to elaborate on the pathways of nitrous oxide destruction to provide clearer context for the observed interrelationships. The loss occurs via two primary pathways: one that results in the formation of inert molecular nitrogen (N2) and another that produces reactive species within the NOy family. Adding specific chemical branching ratios and the fact that these yields are heavily influenced by the incoming UV radiation — which is itself regulated by the thickness of the overhead ozone column — are details from broader stratospheric science which would be useful to the readers.
The significance of the inflection points at a normalised value of 0.7 deserves further emphasis in the text as a marker of shifting dynamics. At values above 0.7, which correspond to the lower stratosphere, the tracer distribution is primarily governed by the horizontal mixing of young tropical air with more aged air from the extratropic. Once air parcels reach the 0.7 threshold, they have transitioned into a region where local photochemical sinks become the dominant influence, explaining why the tropical relationship exhibits a rapid decrease in nitrous oxide for a relatively small increase in age beyond this point. The emergence of hemispheric asymmetry at this same threshold suggests it is a robust indicator for the onset of significant chemical depletion within the stratospheric circulation.
Minor Comments:
Figure 1: The caption should contain all the relevant information required for reproducibility. Specifically, it should define what is meant by the lower stratosphere in this context and explain how the data has been screened for quality. Furthermore, it is unclear why ACE-FTS data is not plotted alongside MIPAS and MLS in this figure, as it would provide a useful third-party validation for the tropical entry values.
Line 207: The normalisation procedure is somewhat confusing. Using the maximum VMR at each time step as the denominator may inadvertently skew the distribution. I note that Supplementary Figure S6 shows the time variation of maximum nitrous oxide concentrations in absolute terms, but it does not include the equivalent normalised values. To better assess the impact of the normalisation choice, I suggest modifying S6 to be a two-panel figure: one panel showing the absolute VMR (as currently shown) and a second panel showing the corresponding normalised VMR values. Each panel should include two lines for mean and maximum VMR (solid/dashed) for different pressure levels and an extra line NOAA MBL measurements at the surface), and second panel should show normalised value estimates using both the mean VMR and the maximum VMR as the reference.Â
Finally, I would like to recommend that authors use the TCOM-N2O dataset (Dhomse and Chipperfield, 2023; 2026) to assist in quantifying the impacts of MLS instrumental drifts.