The contribution of natural emissions to tropospheric composition above the Amazon

Natural emissions, such as BVOCs and soil NOx, from tropical forests can affect tropospheric composition, reliant on the magnitude of emission and on what escapes from the canopy. In-canopy processes are not typically resolved in 3D regional or global models. Here, we investigate the diurnal variability of emissions, deposition and chemistry occurring at the ATTO site in the Amazon using the FORCAsT column model, which resolves the canopy space in 17 levels and extends above the boundary layer. Comparison to observations of meteorology and chemical species made at 8 heights confirms satisfactory performance of the model for the ATTO site. We explore the contribution of soil NOx to the atmospheric oxidative capacity above the canopy including its role in O₃ formation. In combination with canopy level deposition, we identify substantial deposition of NOx at the soil surface due to slow mixing at this height. In addition, at this dark level within the forest, NO reaction with O₃ is an important driver of chemistry. Finally, our model suggests buildup and net removal of NOx within the canopy overnight as mixing decreases, followed by release of NO in the morning. These findings reveal the role of in-canopy chemistry and deposition on above-canopy composition under pristine conditions, providing a baseline for comparison to polluted conditions and to global models without a resolved canopy.