This is the 995th most frequent Korean word.
많고
"많고" means "there is/are many" or "plentiful."
Here, '많고' is used to describe the abundance of '좋은 일' (good things) and contrasts it with '나쁜 일' (bad things).
세상에는 좋은 일이 많고 나쁜 일도 많습니다.
There are many good things in the world, and there are also many bad things.
In this sentence, '많고' highlights the plentiful resources in the library and transitions to mentioning the students present.
도서관에는 자료가 많고 공부하는 사람들도 많이 있습니다.
There are many resources in the library, and there are also many people studying.
The word '많고' links the idea of many trees with the quality of clear air, showcasing its use in transitioning between related ideas.