Linking In-Canopy Chemistry to Above-Canopy O₃, BVOCs, and NO_x Gas Fluxes in the Amazon Rainforest

Abstract. The forest canopy is a distinct chemical and dynamical environment compared to the atmosphere above, characterised by natural emissions, deposition processes, and chemistry that vary with height. However, the role of in-canopy chemistry and its influence on above-canopy concentrations of ozone (O₃) and bi-directional exchange of natural compounds are necessarily simplified within large-scale models. Whilst canopy models have been applied to temperate forests, there are few studies in tropical forests. Here, we apply the FORCAsT canopy column model to an Amazonian site. Simulation of the 2015 El Niño shows that biomass burning enhances O₃ flux into the canopy, increases oxidation chemistry and elevates O₃ deposition to vegetation. Sensitivity tests show sesquiterpenes enhance O₃ chemical loss from approximately 3 % of the total in-canopy losses to 10 %–15 %, but only marginally reduce the total canopy O₃ flux. Sesquiterpene canopy escape efficiency varies by 45 %–55 % across simulations, controlled by O₃ oxidation and vertical turbulence. For other biogenic volatile organic compounds (BVOCs), pool-dependent emissions demonstrate greatest variability in escape efficiency with environmental conditions (monoterpenes 84 %–95 %, isoprene 95 %). Average soil NO_x escape efficiency (40 %–50 %) is higher than many existing models suggest and exhibits a strong diurnal cycle that drives O₃ production, especially in the early morning, which may be important to consider in global atmospheric chemistry models. Overall, we highlight reactive BVOCs by inclusion of sesquiterpene emissions and reactivity as major sources of uncertainty in in-canopy chemistry and emphasise the critical role of turbulence in linking canopy processes to above-canopy atmospheric composition.