This paper investigates the timescales and pathways of downward transport from tropical upper troposphere to the middle troposphere. Using simulations of 19 idealized upper tropospheric tracers, the timescales and spatial distributions of the downward transport are characterized. It is found that the downward transport timescales are sensitive to the size and altitude of source region. The downward transport is dominated by resolved large-scale advection with minor roles from parameterized convection and diffusion.

Overall, the paper is easy to read. The method is appropriate and the results contribute to our understanding of downward transport. However, more explanations of the model results are needed. Also, results of downward transport to the lower troposphere should be presented.

Comments:

The motivation of the paper is to understand whether the downward transport of the tropical upper troposphere aerosols is a source of boundary layer aerosols. Therefore, I am surprised that the authors only show results in the middle troposphere (500 hPa). I strongly suggest showing results in the boundary layer, e.g., repeating all the figures for the lower troposphere.

Lines 171-172: The “staggered” tracer are used to produce the age spectrum, right? I would suggest showing and explaining the age spectrum, for example, over the entire tropics and the three chosen land regions.

Line 215: Explain why young air is predominately near the Equator. It appears that the large-scale sink motion is dominated by the subsidence of the Walker Circulation. But how about meridional transport of the Hadley Cell? I would expect that the sinking branches of the Hadley Cell bring young air to the subtropics. Why are the effects of Hadley Cell not seen in the timescales?

The authors attribute the much longer transport timescales for the regional emission sources to mixing and dilution. This explanation is not complete. I think an important implication is that for a given region, downward transport directly from the upper troposphere of that region is not an important pathway. In other words, most of the downward transport comes from outside of that region. Therefore, I suspect that the 3 regional upper tropospheric emissions would have stronger downward transport influences on areas downstream of the emission regions than areas directly below the emissions. To test this, I suggest showing mean age over the entire tropics for the regional emissions. This will show what region is mostly sensitive to the downward transport for each of the regional emissions.

This study examines the downward transport of upper-tropospheric tracers in the tropics using idealized tracers in a global model. The manuscript is concise and generally well written. The method is valid and the results are generally clearly presented. The study is within the scope of Atmospheric Chemistry and Physics. I have the following minor comments for the authors to consider before publication.

Why do the authors choose 500 hPa, instead of lower troposphere, as the target receptor region?

Could the authors comment on how uncertainties in the related EMAC parameterizations (e.g., deposition, turbulent mixing, etc.) may affect their results?

Title: This study mainly examines the transport from upper troposphere (200-300 hPa and 150-250 hPa) to 500 hPa. Therefore, “Downward transport” may be more suitable than “Global transport” for the title.

L84, L152, and others: Is sedimentation part of dry deposition?

L102: Since it is mentioned that the simulations start from a specific date (1/1/1998), are the meteorology of the two simulations driven by or nudged to reanalysis data? Or do the authors just use the initial conditions from this date (L112)?

L172-173: It is not clear how N_stgrd is set. My understanding is that the staggered tracers are initialized twelve times on the first day of each month in 1999. If so, the description that “t₀ is the first day of each month …” is not accurate. According to Eq. 3, this sounds more like N_stgrd is initialized after the first day of each month (when t > t₀). Also, one may need to assume that t only ranges from the beginning to the end of the month.

L195-197: The τ for regional tracers is five times older than the tropical tracer. This comparison may be misleading. Does it indicate the regional tracers have five times weaker downward transport than the tropical tracers? Also, this difference is ascribed to stronger dilution in the manuscript. Is this right? I think the larger τ of the regional tracers is possibly because the regional source is smaller compared to the tropical tracer, and the corresponding receptor at 500 hPa also receives aerosols from other tropical sources.

Fonts are too small for Figures 5 and 6.