## 3 Results
### Spatial Variation of Atmospheric Oxygen Consumption and Balance  
The energy and oxygen consumption in the PRD was large and had increased rapidly, as shown in Table 2, Figure 4. The oxygen consumption in 1998 was estimated to be \(1.635\times 10^{8}\) t in the PRD by [PERSON] [27]. Guangdong Province's primary energy consumption in 1998 was about \(7\times 10^{4}\) t of standard coal equivalent, but by 2011 it had increased to be \(2.413\times 10^{5}\) t, three times more than in 1998. The oxygen consumption in 2011 was \(5.581\times 10^{8}\) t, which was also three times more than in 1998. Guangzhou and Shenzhen, the main oxygen sinks in the PRD, consumed the greatest volume of oxygen at \(27.13\%\) and \(22.15\%\), respectively. The primary oxygen consuming activity in the PRD was the burning of fossil fuels, which accounted for \(96.14\%\) of the total oxygen consumption. The oxygen consumed by human and livestock respiration only accounted for \(3.86\%\) of the total. Figure 4: Accumulated atmospheric oxygen emission and consumption in each city of the PRD in 2011. Figure 3: Accumulated atmospheric oxygen emissions in the PRD for each season of 2011. The PRD's oxygen balance was weak, with 9.37 times the current oxygen emissions needed from outside inputs to ensure an adequate balance (Table 3). In the winter, when there is usually a north wind, the PRD can obtain oxygen from the Nanjing Mountains and the Yunnan-Guizhou Plateau. During the summer, there is usually a northeast wind, and the PRD can obtain oxygen from offshore locations [65]. As an economic center, the PRD inevitably has a relatively large oxygen deficit. From the perspective of oxygen security, to maintain a certain oxygen balance is a need to ensure a relatively high concentration of atmospheric oxygen even in a calm environment during a long period. Of the nine cities in the PRD, Dongguan had the weakest oxygen balance index of 85.91. The city had no more than 30% of green area and resulted in low oxygen emissions of 300 t km\({}^{-2}\) a\({}^{-1}\). The density of oxygen consumption was up to 2.58 \(\times\) 10\({}^{4}\) t km\({}^{-2}\) a\({}^{-1}\), which was even higher than that in Guangzhou. The oxygen balance of Foshan, Zhuhai, and Zhongshan was also low, with the index value more than 40. This was because the low oxygen emission rate of green land results in an overall low oxygen emission density. Thus, they were in great need to upgrade industrial facilities, transform \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**City**} & \multirow{2}{*}{**Coal**} & \multirow{2}{*}{**Oil**} & \multirow{2}{*}{**Gas**} & \multirow{2}{*}{**Human**} & \multirow{2}{*}{**Livestock**} & \multirow{2}{*}{**Total Oxygen**} & \multirow{2}{*}{**Oxygen**} \\  & & **Combustion** & & & & & & **Oxygen** \\  & & & & & & & & **Consumption (\%) \(\ast\)** \\ \hline Guangzhou & 7273.9 & 5926.2 & 1492.7 & 372.3 & 77.8 & 15,142.9 & 27.13 \\ Shenzhen & 5965.5 & 4860.2 & 1224.2 & 305.6 & 4.4 & 12,359.9 & 22.15 \\ Zhuhai & 776.3 & 632.5 & 159.3 & 45.8 & 18.8 & 1632.7 & 2.92 \\ Foshan & 3970.3 & 3234.7 & 814.8 & 211.1 & 60.8 & 8291.7 & 14.86 \\ Jiangmen & 1264.2 & 1029.9 & 259.4 & 130.4 & 88.9 & 2772.8 & 4.97 \\ Dongguan & 3181.3 & 2591.9 & 652.9 & 241.0 & 10.3 & 6677.4 & 11.96 \\ Zhongshan & 1280.4 & 1043.2 & 262.8 & 91.8 & 16.1 & 2694.3 & 4.83 \\ Huizhou & 1871.6 & 1524.8 & 384.1 & 135.3 & 74.6 & 3990.4 & 7.15 \\ Zhaoqing & 980.6 & 798.9 & 201.2 & 115.4 & 153.4 & 2249.5 & 4.03 \\ PRD & 26,564.1 & 21,642.3 & 5451.4 & 1648.7 & 505.1 & 55,811.6 & 100.00 \\ \hline \hline \end{tabular} Note: * Proportion of total oxygen consumption = oxygen consumption of the city/oxygen consumption of the PRD.  
\end{table} 
Table 2: Accumulated atmospheric oxygen consumption in each city of the PRD in 2011 (unit: 10\({}^{4}\) t).