The key component of the nitrogen oxide detector is the gas sensor，Gas sensors can be divided into three categories in principle：

A) Gas sensors using physical and chemical properties: such as semiconductor type (surface control type, volume control type, surface potential type), catalytic combustion type, solid thermal conductivity type, etc.

B) Gas sensors using physical properties: such as thermal conductivity type, light interference type, infrared absorption type, etc.

C) Gas sensors using electrochemical properties: such as fixed-potential electrolytic type, galvanic battery type, diaphragm ion electric type, fixed electrolyte type, etc.

Nitrogen oxide detector can realize effective monitoring of nitrogen oxide emissions, thereby reducing accidents. Nitrogen oxides mainly composed of nitrogen oxides and nitrogen dioxide are an important reason for the formation of photochemical smog and acid rain. The toxic smoke formed by the reaction of nitrogen oxides and nitrogen-hydrogen compounds in automobile exhaust through ultraviolet radiation is called photochemical smog. 

Photochemical smog has a special smell, which irritates the eyes, damages plants, and can reduce the visibility of the atmosphere. In addition, nitric acid and nitrous acid generated by the reaction of nitrogen oxides with water in the air are components of acid rain. The nitrogen oxides in the atmosphere mainly originate from The burning of fossil fuels and the burning of plants, as well as the conversion of nitrogenous compounds in farm soils and animal excreta.