[Study on critical loads of sulfur and nitrogen in the Pearl River Delta]

Supported by the geographical information system (ArcGIS), critical loads and exceedances of critical loads of sulfur and nitrogen in the Pearl River Delta (PRD) were calculated using Steady-state Mass Balance method with current deposition data, vegetation data and soil data obtained by field sampling and laboratory analysis. Results showed that the present critical loads of sulfur were high in the eastern PRD and low in the west. Higher critical loads occurred in most of Huizhou, north-central Guangzhou, Dongguan and south Zhongshan. The critical loads of these regions were mostly larger than 15 keq x (hm2 x a)(-1). Regions with lower critical loads included most of Jiangmen, most of Zhaoqing and part of Shenzhen with critical loads less than 2 keq x (hm2 x a)(-1). Critical loads of nitrogen were mainly in the range of 1.0-2.5 keq x (hm2 x a)(-1) while values lower than 1.0 keq x (hm2 x a)(-1) were found in Zhaoqing. According to the results of critical load exceedances, in several regions the sulfur deposition exceeded the critical loads whereas in most regions the nitrogen deposition exceeded the critical loads. With the reduction of particulate concentrations in atmosphere in the future, critical loads of sulfur would decrease and sulfur depositions in most regions would exceed their critical loads. Therefore, the control over nitrogen deposition should be strengthened in the present situation and special attention should be paid to the control of sulfur deposition with the reduction of particulate concentrations in the future.