| 05 Oct 2025
AIRTRAC v2.0: a Lagrangian aerosol tagging submodel for the analysis of aviation SO4 transport patterns
Abstract. Aviation-induced aerosols, particularly composed of sulfate (SO4), can interact with liquid clouds by enhancing their reflectivity and lifetime, thereby exerting a cooling effect. The magnitude of these interactions, however, remains highly uncertain and may even offset the combined warming from aviation’s other climate forcers depending on spatiotemporal factors such as emission altitude and season. Here, we introduce AIRTRAC v2.0, the latest advancement of the Lagrangian tagging submodel within the Modular Earth Submodel System (MESSy), and the first submodel to provide aviation-specific sulfate tagging in this framework. AIRTRAC contributes to lowering uncertainty by tracking global contributions of aviation-emitted sulfur dioxide (SO2) and sulfuric acid (H2SO4) to SO4 formation. Using a sulfur-species tagging approach for SO2, H2SO4 and SO4, it enables the characterization of transport patterns and highlights atmospheric regions with enhanced potential for aerosol–cloud interactions. In contrast to some of the existing sulfate tagging models, AIRTRAC considers a full range of microphysical processes along trajectories. To investigate sulfate transport from aviation, two global simulations were performed for January–March and July–September 2015, using pulse emissions of SO2 and H2SO4 distributed across a cruise altitude of 240 hPa (~10.6 km) based on the aviation SO2 inventory of the Coupled Model Intercomparison Project Phase 6 (CMIP6). Comparisons of AIRTRAC-derived SO4 distributions with perturbation-based simulations under analogous conditions show reasonable agreement. Using AIRTRAC v2.0, we estimate median SO2 and SO4 lifetimes of 22 d and 2.1 months, respectively, in northern winter, and 14 d and 2.2 months in summer, consistent with volcanic eruption modeling and observational benchmarks involving high-altitude SO2 injection. The median SO4 production efficiency during summer was found to be statistically significantly larger by 144 % compared to winter, due to a more efficient oxidation of SO2. Large-scale circulation patterns may contribute to enhancing SO4 lifetimes, especially when injected in the Tropics, where emissions could ascend into the stratosphere, past 100 hPa (~16 km). AIRTRAC v2.0 currently excludes SO2 oxidation from aviation nitrogen oxides (NOx) and does not tag other species such as black carbon. Owing to its flexible design, however, the approach can be readily extended to additional aerosols. Overall, AIRTRAC v2.0 offers the novel capability to track the atmospheric transport of aviation-emitted SO2, H2SO4 and SO4, providing critical insights into one of aviation’s most uncertain climate impacts.
Competing interests: Volker Grewe and Patrick Jöckel are topic editors for GMD.
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Review
This manuscript introduces a Lagrangian aerosol tagging submodel (AIRTRAC v2.0), which can help identify the aviation-emitted SO2 and H2SO4 and track their contributions to SO4 formation. AIRTRAC could be a useful tool for us to better evaluate sulfate emissions from aviation and estimate aviation’s climatic impacts. I enjoyed reading the manuscript. It is well written and provides a thorough description, such as the detailed explanation of all terms contributing to the aerosol mass tendency in Eq. 2. I recommend a minor revision for the authors to address my comments below.
Major comments:
Lines 552-553: The first-order Maclaurin polynomial is the linear approximation of the function f(x) near x=0, which cannot be used especially if x>k. To use the first-order Maclaurin polynomial, the authors at least need to show that x (avi term) is smaller than k (rem term).
Line 574: Why is there faster downward transport at point 10? My understanding is that because tropopause height is higher near tropics (point 10) than higher latitudes (point 8) [see Figure 2b in Sun et al. (2023)], so emitted SO2 (at 240 hPa/10.6 km) at Point 8 may be located in the lower stratosphere, while emitted SO2 at Point 10 is definitely in the troposphere. I think this is worth mentioning, which can help to explain why “SO2 emitted at point 8 remains at higher altitudes for longer” (Line 580).
FYI: Sun, H., Bourguet, S., Eastham, S., & Keith, D. (2023). Optimizing injection locations relaxes altitude-lifetime trade-off for stratospheric aerosol injection. Geophysical Research Letters, 50, e2023GL105371. https://doi.org/10.1029/2023GL105371 (already in your references).
Section 4.3: All the analyses of seasonal effects (Figure 7) are based on a one-year emission scenario (2015). Without using the climatological mean, it is possible that the difference between winter and summer is caused by other perturbations rather than the seasonal cycle. The possible perturbation includes internal (e.g., 2015 is a strong El Niño year) or external (e.g., volcano eruptions) forcing.
Line 723: Besides the Northern Atlantic and Southern Tropics, there are several other stratocumulus decks, such as the Northeast Pacific, as shown in Figure D1 (b) and (d). Therefore, aviation sulfate from Point 8 is able to interact with liquid clouds in the Northeast Pacific. See Figure 2 in Muhlbauer et al. (2014).
FYI: Muhlbauer, A., McCoy, I. L., and Wood, R.: Climatology of stratocumulus cloud morphologies: microphysical properties and radiative effects, Atmos. Chem. Phys., 14, 6695–6716, https://doi.org/10.5194/acp-14-6695-2014, 2014.
Minor comments:
Line 203-204: How many vertical layers are in the upper troposphere and low stratosphere? I think you should have more vertical levels in the free troposphere than in the boundary layer.
Line 210-211: Because 28 emission points are at different latitudes, why are they all at the same altitude, especially if we consider the fact that tropopause height varies largely at different latitudes? Would consider height variation make the emissions points more realistic (Line 222-223).
Line 226-228: the altitude is found by the zonal-mean max of SO2 mass flux, which should be a function of latitude.
Line 505: definition of A, A’, and A’’ needs more explanation. What’s the meaning of all terms on the right-hand side of the equations (e.g., f1,4)? Are they all constants?
Line 553: typo: “McLaurin” should be “Maclaurin”.