## 4 Discussion  
### EMGPP/CO\({}_{2}\), PAR and Related Mechanisms  
The EMGPP model expresses PAR transfer and usage in atmosphere-vegetation-land associated with the PAR attenuation of the equivalent CO\({}_{2}\) for producing GPP, absorption and usage of PAR due to GLPs (except CO\({}_{2}\)), and GLPs scattering.  
In brief, PAR absorption and indirect utilization by GLPs through chemical and photochemical reactions mainly include NO\({}_{2}\) + hv (\(\lambda\) = 420 nm) \(\rightarrow\) NO\({}_{2}\)*, NO\({}_{2}\)* + H\({}_{2}\)O \(\rightarrow\) HONO + OH [89], and BVOCs + OH + O\({}_{3}\). The photochemical term represents the total PAR utilization by all atmospheric GLPs (except CO\({}_{2}\)) associated with NO\({}_{2}\)* and OH radicals. In Equations (1) and (2), the photochemical term, the scattering term and the complex interactions between PAR and the three terms (absorbing, scattering and BVOC terms, which are replaced by the GPP term in this study) are fully explained in previous papers [27; 40; 70]. In more specific, the chemical compositions (O\({}_{3}\), NOx, SO\({}_{2}\), HCHO, etc.) react with BVOCs and anthropogenic VOCs (AVOCs) through OH radicals and H\({}_{2}\)O; their roles are implicitly described in the photochemical term in EMBE and EMGPP. If the individual roles of these species in GPP and BVOCs need to be studied quantitatively, the specific constituents (e.g., O\({}_{3}\)) can be added and explicitly described in EMGPP and EMBE, following the previous successful method [40; 90]. The fact that high O\({}_{3}\) results in GPP decline in general [91; 92] is an important issue, and the authors suggest studying it by adding an O\({}_{3}\) term to EMGPP in the future.  
The EMBE model can reasonably simulate BVOC emission fluxes over different time scales (diurnal to annual) and in different situations (e.g., single measurement and hourly average in the measurement campaign) [27]. Although the BVOC concentrations (usually in ppb or ppt) were very low in the atmosphere, their roles in PAR and interactions with other factors (e.g., photochemical and scattering) can be readily captured and represented in photochemical and scattering terms. Similarly, CO\({}_{2}\) concentration (several hundred ppm) is much higher than BVOCs in the atmosphere and its role can also be captured by using the PAR balance method and expressed in their related terms.  
PAR balance and EMGPP (Equations (1) and (2)) express the multiple interactions of PAR, CO\({}_{2}\) and other GLPs associated with the main energy processes of CO\({}_{2}\), absorbing and scattering GLPs, i.e., solar energy-atmospheric substances. Thus, the objective energy relationships were quantified and then applied to compute GPP.  
The signs of coefficients A\({}_{i}\) are \(-\), \(+\), \(+\), \(-\) in Equation (1) (corresponding to B\({}_{i}\) +, \(+\), \(-\), \(+\) in Equation (2)), demonstrating that GPP resulting from the presence of PAR is attenuated by the equivalent CO\({}_{2}\) in the atmosphere. The negative sign of A\({}_{1}\) reveals that this part of the energy is captured from the photochemical term/system through the interactions between CO\({}_{2}\) molecules and PAR photons, total PAR direct absorption and indirect consumption by all atmospheric GLPs (excluding CO\({}_{2}\)). This is similar to the roles of BVOCs in this coniferous forest [27].