VOA慢速英语:研究称新德里再次成为世界上污染最严重的首府城市

particulate – n. of or relating to minute separate particles

concentration – n. the amount of an ingredient in a mixture

monitor – v. to watch, observe, listen to, or check (something) for a special purpose over a period of time

challenge – n. a difficult task or problem : something that is hard to do