Summary of the Article:

Title: Biogenic Volatile Organic Compounds (BVOCs) in the Community Land Model (CLM)

Key Points:

1. BVOC Emissions Model in CLM:
   - The BVOC emissions model in CLM was initially based on Guenther et al. (1995) and was later updated in CLM4 and CLM4.5 based on MEGAN2.1 (Guenther et al., 2012).
   - The MEGAN2.1 model includes four main features: (1) expansion to 147 chemical compounds, (2) treatment of the light-dependent fraction (LDF) for each compound, (3) inclusion of the inhibition of isoprene emission by atmospheric CO2, and (4) emission factors mapped to the specific plant functional types (PFTs) in CLM.

2. Equation for BVOC Emissions:
   - The BVOC emissions from plants to the atmosphere are expressed as a flux (F_i) for each chemical compound (i).
   - The flux is calculated using the equation: F_i = γ_i ρ Σ_j ε_i,j (wt)_j, where γ_i is the emission activity factor, ρ is the canopy loss and production factor, ε_i,j is the emission factor for PFT j, and (wt)_j is the fractional coverage of PFT j.
   - The emission activity factor (γ_i) depends on factors such as plant functional type, temperature, leaf area index (LAI), leaf age, and soil moisture.

3. Light-Dependent Fraction (LDF):
   - In MEGAN2.1, each chemical compound is assigned an LDF value, ranging from 1.0 for isoprene to 0.2 for some monoterpenes, VOCs, and acetone.
   - The activity factor for the light response of emissions is estimated using the equation: γ_P,i = (1-LDF_i) + γ_P_LDF LDF_i, where γ_P_LDF is the LDF activity factor.

4. Emission Factors:
   - The emission factors (ε_i,j) for each chemical compound and PFT are provided in an external file, allowing for easier updates.

In summary, the article describes the BVOC emissions model in the Community Land Model (CLM), which is based on the MEGAN2.1 approach and includes various updates and features to better represent the emissions of a wide range of chemical compounds from different plant functional types.