fresh [freʃ]: adj. 新鲜的；清新的

I love the fresh air in the summer morning.

我喜欢夏天早上清新的空气。

Exercise, fresh air, and good posture are all helpful.

体育运动、新鲜空气和良好姿势都很有效果。

A walk in the fresh air will pep you up.

在清新空气中散散步会使你精神振奋。

Let‘s go out for some fresh air.

咱们出去呼吸点新鲜空气。

The fresh air is doing her good.

新鲜空气对她有好处。

那空气中有雾，烟尘时又该怎么描述呢？

thick: hard to see because of dust, smoke, fog or something. 浑浊的，浓厚的

thick [θɪk] 经常用来形容雾， 烟，空气浓厚或浑浊

The air is so thick.

空气好浑浊啊。

The air was thick with dust.

空气由于灰尘弥漫而闷塞。

The fog is thick today.

今天雾很大。

The air was thick with smoke.

空气中烟雾弥漫。

记住常用表达：

The air is thick with... 空气中充满着/弥漫着.……

The air was thick with tention when the host revealed the winner of the competition.

当主持人宣布竞赛赢家时空气中弥漫着紧张的气氛。

invisible [ɪnˈvɪzəbl] adj. 看不见的

Air is invisible but we can feel its motion when there is a wind.

空气是看不见的，但有风时，我们能感觉到风的运动。

The air is invisible, but it is very important to us.

空气是无形的，但它对于我们来说非常重要。

Air is made of invisible gases.

空气是由肉眼看不见的气体组成的。

brisk [brɪsk]：cold but pleasantly fresh adj.清凉的

The air is turning brisk, the leaves are changing colors and the sleeping bags are ready to be packed!

空气渐渐变得清爽，树上的叶子转变了颜色，是时候准备好露营睡袋啦！