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MEGAN

Overview

Implements the Model of Emissions of Gases and Aerosols from Nature (MEGAN) for biogenic isoprene emissions. The scheme computes emission rates using activity factors for leaf area index (LAI), temperature (light-dependent and light-independent pathways), photosynthetically active radiation (PAR via PCEEA), and CO₂ inhibition.

Ported from HEMCO's hcox_megan_mod.F90 with Kokkos optimizations.

Registration Names

  • Native C++: "megan"
  • Fortran bridge: "megan_fortran"

Configuration Parameters

YAML Key Type Default Description
beta double 0.13 Temperature response coefficient (light-independent)
ct1 double 95.0 MEGAN temperature coefficient 1
ceo double 2.0 MEGAN emission coefficient
ldf double 1.0 Light-dependent fraction [0–1]
aef_isop double 1.0e-9 Isoprene activity emission factor
lai_coeff_1 double 0.49 LAI correction coefficient 1
lai_coeff_2 double 0.2 LAI correction coefficient 2
standard_temp double 303.0 Standard reference temperature [K]
gas_constant double 8.3144598e-3 Gas constant [kJ mol⁻¹ K⁻¹]
ct2_const double 200.0 MEGAN temperature coefficient 2
t_opt_coeff_1 double 313.0 Optimal temperature coefficient 1 [K]
t_opt_coeff_2 double 0.6 Optimal temperature coefficient 2
e_opt_coeff double 0.08 Optimal emission coefficient
wm2_to_umolm2s double 4.766 W/m² to μmol/m²/s conversion factor
ptoa_coeff_1 double 3000.0 Top-of-atmosphere PAR coefficient 1
ptoa_coeff_2 double 99.0 Top-of-atmosphere PAR coefficient 2
gamma_p_coeff_1 double 1.0 PAR gamma coefficient 1
gamma_p_coeff_2 double 0.0005 PAR gamma coefficient 2
gamma_p_coeff_3 double 2.46 PAR gamma coefficient 3
gamma_p_coeff_4 double 0.9 PAR gamma coefficient 4
co2_concentration double 400.0 Ambient CO₂ concentration [ppm]
gamma_co2_coeff_1 double 8.9406 CO₂ gamma coefficient 1 (Possell)
gamma_co2_coeff_2 double 0.0024 CO₂ gamma coefficient 2 (Possell)

Import Fields

Field Name Units Description
temperature K Surface air temperature
leaf_area_index m²/m² Leaf area index
par_direct W/m² Direct photosynthetically active radiation
par_diffuse W/m² Diffuse photosynthetically active radiation
solar_cosine dimensionless Cosine of solar zenith angle

Export Fields

Field Name Units Description
isoprene_emissions kg/m²/s Isoprene emission flux

Algorithm

  1. Compute LAI correction: γ_LAI = c1 * LAI / sqrt(1 + c2 * LAI²).
  2. Compute light-independent temperature factor: γ_T_LI = exp(β * (T − T_std)).
  3. Compute light-dependent temperature factor using optimal temperature and emission potential derived from 15-day average temperature.
  4. Compute PAR factor via PCEEA algorithm using direct/diffuse PAR, solar zenith angle, and day of year.
  5. Compute CO₂ inhibition factor (Possell relationship) once during initialization.
  6. Combine: emission = NORM_FAC * AEF * γ_LAI * γ_CO2 * ((1−LDF)*γ_T_LI + LDF*γ_PAR*γ_T_LD).

YAML Configuration Example

physics:
  - name: megan
    config:
      beta: 0.13
      ldf: 1.0
      aef_isop: 1.0e-9
      co2_concentration: 400.0

Implementation Notes

  • Available as both native C++ (Kokkos) and Fortran bridge implementations
  • 15-day average temperature (T_AVG_15) and daily average PAR (PAR_AVG) are currently set to constant placeholders (297 K and 400 W/m², day-of-year 180) inside the kernel
  • CO₂ gamma factor is computed once during Initialize and held constant