a mixture of smoke, gases, and chemicals, especially in cities, that makes the atmosphere difficult to breathe and harmful for health

Smog is a major problem in Athens. 雾霾是雅典的一个主要问题。

As we flew into the airport, we could see a murky yellow smog hovering over the city. 我们飞进机场时，可以看到一层黄色的雾霾笼罩在城市上空。