## 3 Results
### Sensitivity Analysis of GPP  
The sensitivity/response of GPP to changes in its influencing factors (i.e., PAR, E and S/Q changed by \(-\)80%, \(-\)40%, \(-\)20%, \(+\)20%, \(+\)40%, \(+\)80%, \(+\)160%) was calculated using the 3-factor EMGPP model. Under realistic atmospheric conditions, the change in GPP due to a change in each factor was studied while keeping all other factors at their original levels. In this sensitivity test, the data in developing the EMGPP models were used for the conditions of S/Q \(<\) 0.5 and S/Q \(\geq\) 0.5. Their samples were 91 and 985, respectively. The mean changes in GPP (%) in response to the change in each factor (%) are shown in Figure S4 (in Supplementary Material) and Table S6 (in Supplementary Material) (taking the change rates of 20% in each influencing factor as an example). It is evident that GPP at this conifer plantation was more sensitive to PAR than to water vapor and S/Q and more sensitive to water vapor than to S/Q at low S/Q. corresponding to better/cleaner atmospheric conditions (high PAR and air temperature and low GLP loads, mainly aerosols, clouds and smog). At high S/Q corresponding to the worse/smoggy atmospheric conditions (low PAR and air temperature, and large GLP loads), GPP was most sensitive to E (meaning that the water supply/drought is a very important and predominant factor in controlling/influencing GPP under most atmospheric conditions) than to PAR and S/Q and was slightly more sensitive to PAR than to S/Q. This result reveals that GPP would decrease significantly with only the increase in PAR at low S/Q (e.g., under the conditions of stomatal closure at noon in the summer) and a much smaller GPP decrease at high S/Q (Table S5, in Supplementary Material). The increase or decrease in GPP with the decrease or increase in PAR reveals a mechanism by which the PAR energy is absorbed and utilized by plants, while all other factors remain at their original levels. GPP will increase similarly to the increase in water vapor under both S/Q conditions, meaning that sufficient water supplies are beneficial to plant growth and ecosystem carbon uptake. GPP will increase with a decrease in S/Q, implying that the decrease in scattering GLPs in the atmosphere results in an increase in diffuse PAR arriving at the canopy, followed by an increase in GPP, which corresponds to enhanced diffuse radiation promoting carbon assimilation [80; 81; 82; 83; 84; 85; 86], and increased cloudiness reduces total radiation, which decreases GPP [87]. The mechanism of and relationship between scattered PAR (or scattered GLPs, S/Q) and GPP are also manifested when higher GLP loads (i.e., S/Q) cause a larger decrease in GPP (compared to lower GLP loads) when S/Q changes at the same rate, and vice versa (Table S5, in Supplementary Material). The responses of GPP to its influencing factors strongly depend on the levels of influencing factors, e.g., larger responses of GPP to PAR at high PAR (4.57 umol m\({}^{-2}\) s\({}^{-1}\), S/Q < 0.5) and to S/Q for high S/Q (0.76, S/Q \(\geq\) 0.5), along with closer responses of GPP to E at close E (\(\sim\)28.0 hPa) under S/Q < 0.5 and S/Q \(\geq\) 0.5 conditions. The average S/Q value of 0.85 under all sky conditions in 2013-2016 indicated that the atmosphere in the Qianyanzhou region was dominated by high GLP loads, i.e., S/Q \(\geq\) 0.5 (Section 2.1). Under these atmospheric conditions (common situations), (1) GPP will increase with climate wetting and decrease with climate drought, and climate drought will lead to a larger influence on GPP increase (i.e., enhancement in carbon fixation capacity) than on GPP loss caused by climate wetting; (2) GPP would increase with the cleaner atmosphere (decrease in S/Q), meaning that the carbon fixation capacity of the forest would benefit from the cleaner air and improved air quality (low GLP concentrations). It should be noted that the response of GPP to the decrease of each factor was slightly larger than to the increase of each factor.