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., f_1,4)? Are they all constants?

Line 553: typo: “McLaurin” should be “Maclaurin”.

This study introduced a Lagrangian aerosol tagging submodel for sulfate aerosols into the atmospheric chemistry modelling framework. Compared to the old version, an important advancement is the incorporation of aerosol microphysical processes in the tagging module, which can enable better quantification of sulfate transport patterns. Overall, this manuscript is well-written and -organized. I recommend some revisions before the publication. 

First, I do suggest the authors to shorten the length the manuscript. Some content can be moved into Supplementary Materials. For example, you don't need to give too detailed background on aerosol effects on cloud albedo, as this is not the key focus of your study. And please refine the methods so that the core parts can be kept in the main text and shown to readers. Describing the new development of your model is interesting enough. 

In Section 4.1, please explain why the emission points 8 and 10 are selected for analyzing. How the result could be changed regarding other emission points, e.g., over Pacific Ocean?

Please indicate a and b in Figure 8 panel. 

I would like to give further comments on a revised version.