Drive by any large city in the world and you will see it. Sometimes it looks like a big gray or brown cloud hanging in the air. It's called haze（薄雾） or smog or ground level ozone, and it's a genuine health problem for millions of people around the world.
The term smog was first used by H.A. Des Voex in 1905 to describe the foggy conditions in urban areas resulting from the emission of sulfur（硫） dioxide（二氧化物） from the newly created smokestacks（烟窗） of the industrial revolution.
Much of the smog problem today is what is known as photochemical （光化作用的）smog. Basically, the term refers to a complex chemical reaction whereby sunlight breaks down chemical compounds such as volatile（反复无常的） organic compounds (VOCs) and nitrogen（氮） oxides (NOX) into their constituent parts. Once broken down, the now free oxygen molecules can link up with the oxygen in the air (O2) and create ozone, which is a variation of oxygen, with one additional oxygen molecule（分子） attached to it. Hence, the chemical name of ozone is O3.
Both VOCs and NOx occur naturally, and can contribute to ground level ozone problems. However, anthropogenic（人类起源的） (human initiated) sources remain the focal point of policy analysis.
