# Effects of Aerosols on Gross Primary Production from Ecosystems to the Globe
## 4 Discussion
### Differences in the Impact of Aerosol Diffuse Fertilization Effect on GPP in Different Regions  
According to our modeling, aerosol-induced changes in PAR have a strong impact on GPP depending on aerosol loading and cloud thickness. Moderate aerosol increases the photosynthesis of plants by increasing the \(\text{D}_{\text{f}}\) of total PAR, while the further increase in aerosol loading can have the opposite effect due to the strong attenuation of total PAR. Moderate aerosol loading would increase plant photosynthesis and the total partition of PAR to \(\text{D}_{\text{f}}\), while further increases in aerosol loading could have an opposite effect due to a strong attenuation of total PAR. The key mechanism of aerosols affecting GPP is its effect on total PAR and partitioning of total PAR into direct and diffuse fractions. Diffuse PAR is more effective than direct PAR in penetrating vegetation canopy [11; 30; 42] and therefore has a higher LUE coefficient (3.61 g C m\({}^{-2}\) MJ\({}^{-1}\)) than direct PAR (1.68 g C m\({}^{-2}\) MJ\({}^{-1}\)). In addition, the aerosol diffuse fertilization effect (DFE) was very closely related to cloud cover [12; 59], which caused the \(\text{D}_{\text{f}}\) of the area to easily reach saturation and reduce the availability of light [60]. Therefore, the overall impact of aerosol DFE on GPP may be positive in areas with low cloud cover and aerosol loading, and may be negative in areas with higher cloud cover and aerosol loading.  
The negative impact of aerosols on GPP is the greatest in the tropics (\(\Delta\text{GPP}<-4\%\)). This is mainly because tropical regions with strong evapotranspiration have limited potential for aerosol DFE due to thick cloud cover [12; 61], which cannot compensate for the reduction of total PAR and direct PAR (Figure 3b,d). However, our results showed that \(\Delta\text{GPP}>2\%\) is common in subtropical areas with low cloud cover, especially in some arid and semi-arid regions (\(\Delta\text{GPP}>3\%\)) (Figure 6d). The cloudiness in these areas is generally low and does not inhibit the aerosol DFE [25; 61] likely because the vegetation photosynthesis in these areas does not saturate [8; 62] and the change of aerosol to diffuse PAR increases the potential LUE. The mean value of diffuse PAR in these areas was relatively high and exhibited an increasing trend.  
However, compared with subtropical regions with moderate aerosols, southeastern China did not exhibit increases in GPP due to aerosol DFE despite a heavy aerosol loading (AOD > 0.5). Previously, [PERSON] and [PERSON] [63] considered the impact of aerosols and ozone pollutants on net primary productivity (NPP) and found that the aerosol DFE would increase NPP by 0.2 Pg C (5%) in eastern China. The reason why our research results are different from them is likely because we have considered that the increase of \(\text{D}_{\text{f}}\) is limited by aerosol loading and the corresponding offsetting effect of direct PAR attenuation [25].  
Although aerosols have positive and negative effects on GPP, they are even offset by positive and negative effects in the global effect. What cannot be ignored, is the change of scattered radiation caused by aerosols. Although the relationship between scattered radiation and aerosol coincidence leads to a small change in GPP, it can be compensated by the increase of LUE [10].