| 14 Jan 2026
Global transport of upper-tropospheric tropical tracers: multi-year insights from idealized simulations
Abstract. Recent studies suggest that significant aerosol formation occurs in the tropical upper troposphere (UT). However, the impact of these particles at lower levels remains poorly understood. We present results from multi-year global EMAC simulations investigating the downward transport of UT tracers and their resulting spatial distribution. Nineteen idealized tracers were released in the tropical UT and subjected to resolved-scale advection, parameterized convection, turbulent mixing, and wet/dry deposition. Transport timescales are highly sensitive to source extent: the age of air at 500 hPa is ≈ 45 days for tropical-wide tracers, compared to over 250 days for regional continental sources, reflecting the importance of mixing and dilution. A complementary time-to-threshold diagnostic reveals faster transport pathways, with all source regions exhibiting descent times shorter than 7 days to reach 10 % of the source average. Advection dominates vertical transport, with convective and vertical diffusion parameterizations contributing marginally. Injection height exerts a stronger influence on descent time than parameterized transport or particle size in the 20–100 nm range. Tracer maxima are typically advected east of their source centers, resulting in significant concentrations (~10–15 % of source values) in the mid-troposphere. Offline calculations show that, for initial particle numbers below ~6 × 107 kg-1, the mid-troposphere values predicted by the model are reduced by less than 10 % when coagulation is considered, but substantial deviations occur at higher concentrations. These results provide quantitative constraints on particle transport efficiency and inform expectations for aerosol distributions following UT nucleation events.
Competing interests: Two co-authors of the manuscript are topical editors of the journal.
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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.