What happens to gases above the Amazon rainforest after sunset? Typically, turbulent air movement causes the gases emitted from the atmosphere to mix freely with the surrounding air, as well as with air masses transported by winds from afar. But there are certain atmospheric conditions that might prevent this mixing. A new study at the Amazon Tall Tower Observatory (ATTO) shows how a nighttime atmospheric layer can limit atmospheric movement, helping explain why gases accumulate at some heights while remaining less abundant at others.
Understanding this process matters because the concentration of a gas depends on more than its sources. It also depends on how the air moves. Without knowing how efficiently the atmosphere mixes, scientists may misinterpret where gases come from or how they are transported.
During the day, heating from the sun creates convection, which helps mix the air above the forest. After sunset, the surface cools, and this mixing weakens. Air closer to the forest can become partly separated from the air above it. The depth of this lower layer, known as the nocturnal boundary layer, influences how heat and gases move through the atmosphere.
Carla M. A. Souza and her colleagues investigated how this nocturnal boundary layer changes throughout the night and between wet and dry seasons. Using measurements at different heights along the ATTO towers, they compared conditions in 2022, a La Niña year, and 2023, an El Niño year. They also examined the distribution of carbon monoxide and methane to understand how the nighttime layer affected gas transport.
The researchers found substantial differences. The layer reached approximately 270 meters during the wet season of 2022, compared with only around 100 meters during the dry season of 2023. It also generally became shallower as the night progressed. Stronger cooling and more stable air favoured shallower layers, while wind-driven mixing could help connect the air near the forest with higher levels.
The gas measurements revealed the consequences of this changing structure. On a night with a shallow nocturnal boundary layer, air containing elevated concentrations of carbon monoxide and methane passed above it without mixing efficiently down toward the treetops. The layer acted like an invisible barrier, limiting exchange between the air below and above. On a night with a deeper layer, concentrations were more similar across heights, indicating stronger mixing. Gases already present within the lower layer could also take longer to disperse.
The central message is simple: an increase in gas concentration does not necessarily mean that more gas has been released. It may also mean that atmospheric conditions are allowing gases to accumulate or bringing in air from elsewhere. Likewise, measurements above the forest may capture a passing air mass that does not reach the canopy below.
These findings help researchers interpret atmospheric observations and understand how gases move over the Amazon. They also provide evidence that can guide improvements in weather and climate models, which need to represent the often weak and uneven mixing that occurs at night.
The comparison covers two years, so longer observations are needed to establish how consistently these patterns occur during El Niño and La Niña. Nevertheless, the study demonstrates the value of observing the atmosphere at several heights: what happens above the forest cannot always be understood from measurements at just one level.
Souza et al. published the study “Estimation of nocturnal boundary layer height in the central Amazon, supported by gas concentration profiles” Open Access in Atmospheric Chemistry and Physics.
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