今天早晨，烟雾笼罩着城市。

The ___smog hung heavily over the city this morning.

需要更严格的排放控制来减少城市地区的烟雾。

Stricter emission controls are needed to reduce ___smog in urban areas.

浓重的烟雾导致呼吸困难，并引发了健康警告。

The thick ___smog made it difficult to breathe and caused health warnings.