There are three different ways that the MEGAN model framework has been implemented:

1) INTEGRATED: MEGAN has been integrated directly into coupled physics/chemistry/biology 1D canopy and boundary layer models (e.g., SOSA), regional air quality models (e.g., WRF-chem) and global climate/earth system models (e.g., CESM, GEOS-chem, and E3SM). In each model time step, the MEGAN driving variables are generated and the MEGAN emissions impact atmospheric chemistry enabling direct representation of BVOC interactions and feedbacks. These models include various components of the different versions of the MEGAN framework (e.g., version 2, version 2.1, version 3.2) and may include a mix of components from different versions.  

2) SITE: These MEGAN codes are driven by a time series of inputs at a single site. The codes have been written in Visual Basic, Matlab and Python. They are primarily used to investigate BVOC flux and concentration observations at individual field locations, to assess the MEGAN model at these sites, and to examine the sensitivity of the MEGAN model to various model inputs and processes.  These models include different versions of the MEGAN framework (e.g., version 2.1, version 3). 

3) UNCOUPLED: MEGAN FORTRAN code has been developed to estimate off-line MEGAN emissions for regional air quality models (e.g., CMAQ and CAMx) using WRF model weather outputs.  The publicly available code, MEGAN2.1, is described by Guenther et al. 2012 (see https://gmd.copernicus.org/articles/5/1471/2012/) and has been widely used for regional to global modeling in the past decade. It provides the user with global maps of emission factors and landcover distributions. The MEGAN2.1  User guide, FORTRAN code, input files, global BVOC emissions output and a test case are all available at doi.org/10.5281/zenodo.10525127.  A parallelized version of the code that can be requested by contacting shengting.cui@wsu.edu.  

An updated version of the uncoupled MEGAN3 FORTRAN code (under development) is more transparent (e.g., the user can view and modify the underlying data that determines the emission factors at any given location) and more customizable (e.g., the user can modify the emission factors, landcover and other input variables) to provide domain specific inputs. This code is especially intended to be useful for regional air quality model simulations of urban areas that cannot be accurately characterized using the global data sets provided with MEGANv2.1. Inputs for urban areas of a domain are based on ultra high resolution (e.g., 0.5 to 10 m) landcover and ground surveys of landcover and urban vegetation emission factors.  The updated model also includes other improvements (e.g., canopy model, light and temperature response algorithms, emission response to canopy depth, drought stress response).  The emission factors are based on the data synthesis described in the MEGAN BVOC Emission Factor Encyclopedia that considers available BVOC emission data (e.g., previous emission factor reviews, model databases, individual emission measurement studies). This version is still under development but a beta version is available for testing (contact alex.guenther@uci.edu).  We expect to finalize the new version of uncoupled MEGAN and make the code, inputs and User Guide available soon.