“…For air pollution monitoring, sensors detecting CO 2 [4], CO [5], NO x (x = 1, 2) [6], SO x (x = 1, 2) [7], H 2 S [8], NH 3 [9], Cl 2 [10], O 3 [11] and particulate matter are frequently used. For water pollution monitoring, sensors monitoring heavy metal [12], pH value [13], conductivity [14], dissolved oxygen (DO) [15], turbidity [16], the content of carbon [17], chlorine [18], the content of phosphorus [19,20], and the content of nitrogen [21] are used. Some contaminants can be detected directly by sensors, especially in air environment, such as CO x , NO x , and SO x , but some sensors can only detect a type of pollutants and cannot measure the content of a specific contaminant.…”