Dynamics of the Nocturnal Boundary Layer Height in the Central Amazon: Atmospheric Stability and Its Effects on Trace Gas Dispersion

The height of the Atmospheric Boundary Layer (zi) is a crucial parameter in various scientific fields, such as atmospheric chemistry and micrometeorology, as it defines the "box volume" where scalars are dispersed and indicates the magnitude of the most energetic vortices. In recent decades, there have been significant advances in the study of zi, particularly under daytime conditions. However, the behavior of zi during the night is still poorly understood, especially in remote regions like the Amazon rainforest. This knowledge gap is due to both instrumental limitations and the complex interaction between the forest and the atmosphere that governs the Nocturnal Boundary Layer (NBL). Nevertheless, with measurements taken by the towers of the ATTO (Amazon Tall Tower Observatory) project, it is possible to obtain high-frequency data of turbulent fluxes at various heights, up to 325 meters, which enables the estimation of the NBL height. In this context, this study analyzes the temporal evolution of the NBL through sensible heat flux profiles. Two case studies were investigated for two nights with markable differences in atmospheric stability. Additionally, the estimated NBL heights were compared with vertical concentration profiles of carbon monoxide (CO) and methane (CH4), measured at the same towers. The results indicate that under more stable conditions, when the NBL height is very shallow (below 100 meters) and the wind direction within the NBL differs from the wind direction above the NBL, CH4 and CO concentrations above and inside the NBL differ strongly.In contrast, when the NBL height exceeds 150 meters and the wind direction remains consistent with height, these trace gases tend to maintain approximately constant values at all levels.