## 5 Conclusions  
With further application of a previously developed empirical model of BVOC emissions at a subtropical conifer plantation, China, an empirical model of GPP in this plantation was developed for a better understanding of the processes of GPP and the relationships in GPP and its driving factors. In model development using 2013-2014 observations, 3-factor and 2-factor EMGPP models generally performed good simulations for hourly, daily, monthly and annual GPP values. For example, when using a 3-factor model, the hourly GPP overestimated the observed GPP by 9.96% and 15.52%, respectively, for S/Q \(<\) 0.5 and S/Q \(\geq\) 0.5 situations, and their corresponding RMSE values were 0.07 mg CO\({}_{2}\) m\({}^{-2}\) s\({}^{-1}\) and 11.33% and 0.15 mg CO\({}_{2}\) m\({}^{-2}\) s\({}^{-1}\) and 18.74%. Evaluations were conducted for hourly, daily, monthly and annual GPP against observations in 2013-2016 under all skies (i.e., all S/Q conditions), and reasonable agreement was found between simulated and observed GPP. Generally, the modeled GPP values were still larger than the measured values, and their standard deviations were relatively close. The GPP in 2013-2016 under all sky conditions was computed using 3-factor and 2-factor EMGPP models, and the simulated annual amount of GPP overestimated these observed values by 41% and 29%, respectively. The GPP empirical model captured some characteristics, such as diurnal and seasonal variation patterns. The 2-factor empirical model indicated similar performance as the 3-factor model and can be used when diffuse or direct solar radiation is not available. Sensitivity analysis demonstrated that the responses of GPP to its driving factors (PAR, water vapor and S/Q) evidently changed at different atmospheric GLP loads (i.e., S/Q! 0.5 and S/Q! 0.5, the latter condition being a predominant state of the atmosphere), namely, GPP was more sensitive to changes in PAR than that in E and S/Q at low S/Q, while GPP was most sensitive to the changes in E than to PAR and S/Q at high S/Q. Therefore, atmospheric column GLP contents should be taken into consideration in the processes of GPP and the interactions in PAR-CO\({}_{2}\)--other GLPs. Some inherent relations between GPP and scattered PAR, water vapor, and atmospheric scattered substances were investigated extensively, which were confirmed by other observations and model studies. The contributions of BVOC emissions to NEE (BVOCs/NEE) were small (3.5%) in this coniferous forest and other forests in the world and exhibited a clear latitudinal variation. It is assumed that BVOCs/NEE was the largest in the tropical region and lowest in the Arctic/subarctic region, which was partially confirmed by laboratory measurements and sensitivity analyses. BVOCs/GPP was 0.8% in this coniferous forest. BVOCs, due to their very high reactivity, produce large quantities of new atmospheric GLPs, and they are also a necessary part of carbon in the atmosphere and vegetation; thus, they should be considered in regional and global carbon balance, as well as climate change and global warming. Reducing both carbon emissions and air pollutants has beneficial effects on nature.