Downward transport of tropical upper-tropospheric aerosols: multi-year insights from idealized simulations

Kurzfassung

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 diagnostics are highly sensitive to source extent: the age of air at 500 hPa is ≈ 45 d for tropical-wide tracers, compared to over 250 d for regional continental sources, reflecting the importance of mixing, dilution, and source-receptor geometry. A complementary time-to-threshold diagnostic reveals faster transport pathways, with all source regions exhibiting descent times shorter than 7 d 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.