## 2 Methodology
### Atmospheric Oxygen Emission Assessment  
Photosynthesis accounts for 98% of the world's oxygen output, while the splitting of water molecules by ultraviolet radiation accounts for the other 1%-2% [44]. The process of plants producing and consuming oxygen consists of three stages. The first is photosynthesis. Plants convert inorganic carbon to organic carbon and release oxygen, with the initial carbon referred to as GPP. The second is autotrophic respiration. Plants consume part of the organic carbon produced by photosynthesis and  
Figure 1: Location of the study area.  
absorb some oxygen through respiration. The remaining organic carbon is referred to as NPP [45, 46, 47]. The third is heterotrophic respiration of soil organic matter. Part of the organic carbon in NPP expended by heterotrophic respiration and oxygen consumption of soil organic matter will enter the soil in the form of dead branches, fallen leaves, or other plant material. The organic carbon remaining after deducting the consumption by heterotrophic respiration of soil organic matter from the NPP is referred to as net ecosystem productivity (NEP) [48]. Thus, the oxygen emission of plants through photosynthesis, respiration, and heterotrophic respiration of soil organic matter can be calculated by the mass ratio between NEP and oxygen emission after estimating the NEP.  
In rapidly urbanizing regions like the PRD, large vegetation changes have occurred at different intervals. The traditional calculation method, which uses ground-based data, is time costly and not suitable for a rapidly changing environment. The introduction of remote sensing technology has made it possible for measuring oxygen emissions in regions of rapid urbanization then the empirical value of land use type [49]. The MOD17A2-GPP data set reflects the wide range of spatial and temporal variations of vegetation photosynthesis in real-time. Thus the eight day time elapsed NPP and NEP values have been derived, and oxygen emissions have been estimated from the GPP and temperature data.  
The models used for estimating NPP and NEP can be roughly divided into three categories [50]: traditional meteorology correlation models [51, 52], ecological process models [53, 54, 55], and rational light utilization models [56, 57]. The C-FIX model used in this study is a rational light utilization model, which can determine the calculation of NPP and NEP with a time-space attribute at the regional scale [58]. The model requires few input parameters, but has a high calculation efficiency [58]. In the C-FIX model, GPP, NPP, and NEP have the following relationships, where the subscript \"d\" stands for day:  
\[\mathrm{NPP_{d}=GPP_{d}\left(1-A_{d}\right)} \tag{1}\]  
\[\mathrm{NEP_{d}=NPP_{d}-R_{h,d}} \tag{2}\]  
where \(A_{d}\) is the autotrophic respiration rate of plants, and \(R_{h,d}\) is the heterotrophic decomposition rate of soil organic matter. \(\mathrm{NPP_{d}}\) is the net primary productivity on pixel, \(\mathrm{GPP_{d}}\) is the gross primary productivity on pixel, \(\mathrm{NEP_{d}}\) is the net ecosystem productivity on a pixel. The unit is tonne (t).  
The autotrophic rate of plants and heterotrophic decomposition rate of soil organic matter are primarily related to temperature. The heterotrophic respiration rate will increase 1.5 times when the temperature is increased by 10 \({}^{\circ}\)C [59]. Finally, NEP of every eight days is estimated by entering the GPP remote sensing data and temperature data into the C-FIX model, and the yearly oxygen emission can be estimated by the summary of the eight days' NEP.