Modeling methane biogenic emissions using different wetland and soil carbon pool maps in Amazonia

The Amazon rainforest has a large area occupied by rivers and wetlands, responsible for the majority of biogenic methane (CH4) emission in the region. Biogenic emissions of CH4, which is a potent greenhouse gas, are a key source of uncertainty for the global budget. Atmospheric transport models can be useful to assess the spatial distribution of CH4 concentrations and the influence of land use and climate change. It is important to use computational models with updated emission data to obtain more accurate results in atmospheric gas transport simulations. Here we present an analysis performed with the Weather Research and Forecasting model coupled with the Greenhouse Gases module (WRF-GHG), using updated maps of wetland and fast carbon pool to calculate biogenic emissions of CH4 in the Amazon region. Three wetland maps (wetmaps) and two fast carbon pool (CPOOL) maps were used in the simulations. The resulting methane concentrations were compared to in situ observations at the ATTO tower (Amazon Tall Tower Observatory). The simulations were performed from 1st to 13th January 2023 (considering the first seven days as spin up), in a single domain with 6 km resolution and grid of 212 x 121 centered at ATTO. Boundary conditions were provided by ERA5 and CAMS. The simulation using the default emission model along with the minimum inundation wetmap and the updated CPOOL map showed results closer to observational data (bias of 15 ppb) than the other simulations (bias in the range 20-320 ppb). Using the default maps resulted in an overestimation of 4.1% in CH4 concentrations at ATTO. The modeled CH4 concentrations time series showed a pronounced diurnal variability, likely driven by boundary layer dynamics and advection. On the other hand, observations showed rather constant concentrations, suggesting that background regional emissions dominate the CH4 signal at ATTO. Considering a model grid cell over a section of the Amazon River near the ATTO site (150 km southeast), simulated emissions ranged between 42 (minimum inundation and updated CPOOL map) and 623 (maximum inundation and default CPOOL map) mg CH4 m^-2 day^-1, while WetCHARTs emission inventories are in the range 153-276 mg CH4 m^-2 day^-1 and the literature reports averages of 18-21 mg CH4 m^-2 day^-1 for the Amazon River, based on field measurements. Overall, the results show a high sensibility of the WRF-GHG model towards the choice of wetland and CPOOL maps in Amazonia. Also, the correct representation of CH4 background concentrations is key to improve the simulations of near surface concentrations in areas less impacted by local wetland emissions, like ATTO.