VOA慢速英语:北京希望国庆节天空晴朗空气清新

smog – n. fog mixed with smoke: a cloud of dirty air from cars, factories, etc., that is usually found in cities

certain – adj. used to refer to something or someone that is not named specifically

alleviate – v. to reduce the pain or trouble of (something): to make (something) less painful, difficult, or severe

extent – n. the range, distance, or space that is covered or affected by something or included in something