A poisonous brown gas, NO2, often found in smog and automobile exhaust fumes and synthesized for use as a nitrating agent, a catalyst, and an oxidizing agent.

• 二氧化氮：一种有毒的红棕色气体，NO2，常在烟雾和汽车排放的气体中发现，被合成用来作硝化剂、催化剂以及氧化剂