The manuscript "The global impact of the transport sectors on aerosol and climate under the Shared Socioeconomic Pathways (SSPs)" present quantification of the impact of the transport sectors (land transport, shipping and aviation) on aerosol and the resulting climate effects for present day and a selection of SSPs and future years. The global chemistry-climate model EMAC is used to quantify the effects.

The manuscript is clearly written, and well structured, and I have only minor comments on the paper.

Regarding the title: "The global impact of the transport sectors on aerosol and climate under the Shared Socioeconomic Pathways (SSPs)", it can be interpreted as the work consider the global impact of transport on climate, but it is only the aerosol impact of the transport sector on climate that is considered. Consider rephrasing, also in the abstract.  

Regarding the above, is it possible to add a discussion regarding the CO2 warming from the sectors in the scenarios? 

In the abstract: "In spite of the recently introduced regulations to limit the fuel sulphur content in the shipping sector, shipping emissions are still responsible for a considerable impact on aerosol sulfate near-surface concentrations", I wonder how well the scenarios take into account the most recent regulations. 

L39: "some" I will say "all", as all of these components will either directly or indirectly (via chemistry) affect climate.

L57: CMIP6 emission inventory (Hoesly et al., 2018; van Marle et al., 2017) Hoesly et al. is the historical anthropogenic emissions used in CMIP6 that end in 2014. 

L171: Related to above: "and future projections until the year 2100" the two references cover the historical period and not the future projections. 

I find the two first paragraphs in section 3 on the emissions a bit unstructured. When reading, I was expecting the matrix structure of the SSPs to appear earlier. The link and harmonization of the historical and the scenarios could be made clearer, as described in Gidden et al. 2019. 2015 emissions in the harmonized emissions are similar in all the scenarios. 

There are a wide range of SSP scenarios, but for climate modelling, only a small selection is gridded and harmonized for use. For SSP1 and SSP3, there are two other forcing targets available. Would be good if the differences related to aerosol emissions in these could be slightly mentioned. 

Figure 2: Add to the figure caption the domain over which the burden is calculated.

All figures: Replace the rainbow color scale with a different color scale.

5 Climate impacts. In the beginning of this section, it would be useful with a clearer definition of the radiative forcing definition. Do you calculate RF or ERF (including the adjustments). Is it both radiative forcing for aerosol cloud interaction (aci) and aerosol radiation interaction (ari)? And more specifically, how to compare the clear sky and all sky RF to separate the role of aci and ari. 

Figure 9: How is the 90% CI calculated?

L533-549: This section conclude the results for present day 2015? Indicate that in the first sentence.

L580: Any studies on ammonia in the transport sector?

GENERAL REMARKS

The manuscript on the "The global impact of the transport sectors on aerosol and climate under the Shared Socioeconomic Pathways (SSPs)" presents a quantification of the impact of the transport sectors (land transport, shipping, and aviation) on the atmospheric aerosol load and discusses the resulting climate effects for present day and a selection of SSPs and future years. The authors use the lobal chemistry-climate model EMAC to quantify the effects.

The study is carefully designed and performed. The presentation of the applied methods and  achieved results is well structured and clear. The manuscript fits well into the scope of the journal and can be accepted for publication, after few minor revisions have been implemented.

SPECIFIC COMMENTS

1. The scope of the study is the assessment of the global transportation sector on the atmospheric aerosol burden and the resulting effects on the climate. The title, however, suggests a slightly different topic. A more specific title could be: “The global impact of the transport sectors on the atmospheric aerosol and resulting climate effects under the Shared Socioeconomic Pathways (SSPs)”.
2. In Section 3, the authors discuss the emission of aerosol and precursors for today’s conditions and future scenarios. Emission scenarios are discussed on the basis of changing activities by scaling the aerosol properties of current sources. However, the aerosol characteristics may change significantly with respect to chemical composition and resulting changes in aerosol-cloud-interactions when new fuels are used ; see, e.g., Petzold et al. (2005) and Moore et al. (2017) for aviation and Petzold et al. (2011) for shipping. These studies report changing aerosol properties when switching to low-sulphur fossil or biogenic fuels. The potential impact of those changing aerosol properties should be mentioned and discussed. This comment refers also to the statement on line 91ff, “that particle number emission factors in low-sulphur fuels are not significantly decreased compared to standard heavy fuel oil”. Although the total number concentrations may not change significantly, the chemical nature of the particles changes and thus the interaction with clouds.
3. In Section 4, the Transport impact on aerosol concentrations is discussed. The section presents the results very clearly, but the potential impact of changing background conditions should at least be mentioned. As an example, how will the relative contribution of the transport sector to the BC burden look like when the total atmospheric BC load changes under heavy forest fire conditions? A short discussion might give more insight on the relevance of the presented results in a changing environment.

MINOR ISSUES

Line 78: Please correct to “… implementations of reduction measures”.

Line 81: Which fleet is referred to here? Please clarify.

Line 149: The authors use ERA-Interim reanalysis data. Would the use of ERA 5 reanalysis data produce significant differences?

Figure 2: The y-axis title of the first row is confusing and might be understood as mass normalized to particle number. Please check.

Line 556: Please correct to “A peculiarity of this scenario is the low climate impact …”

REFERENCES

Moore, R. H., Thornhill, K. L., Weinzierl, B., Sauer, D., D'Ascoli, E., Kim, J., Lichtenstern, M., Scheibe, M., Beaton, B., Beyersdorf, A. J., Barrick, J., Bulzan, D., Corr, C. A., Crosbie, E., Jurkat, T., Martin, R., Riddick, D., Shook, M., Slover, G., Voigt, C., White, R., Winstead, E., Yasky, R., Ziemba, L. D., Brown, A., Schlager, H., and Anderson, B. E.: Biofuel blending reduces particle emissions from aircraft engines at cruise conditions, Nature, 543, 411-+, doi: 10.1038/nature21420, 2017.

Petzold, A., Gysel, M., Vancassel, X., Hitzenberger, R., Puxbaum, H., Vrchoticky, S., Weingartner, E., Baltensperger, U., and Mirabel, P.: On the effects of organic matter and sulphur-containing compounds on the CCN activation of combustion particles, Atmos. Chem. Phys., 5, 3187–3203, 2005.

Petzold, A., Lauer, P., Fritsche, U., Hasselbach, J., Lichtenstern, M., Schlager, H., and Fleischer, F.: Operation of Marine Diesel Engines on Biogenic Fuels: Modification of Emissions and Resulting Climate Effects, Environ. Sci. Technol, 45, 10394-10400, doi: 10.1021/es2021439, 2011.