同意刘老师解答

(1)by air  乘飞机，航空邮寄（信件）;

(2)in the air 悬而未决; （谣言等）传开， 流行开来; 在空中; 空际;

(3)on the air 广播; 在播送中;

(4)in the open air 在户外;

(1)by air

Casualties had to be brought to hospital by air.  

伤者不得不用飞机送往医院。

The pilot was given clearance to land by air traffic control.  

飞行员得到空中交通管制站发出的着陆许可。

 to send goods by air freight  

空运货物

 The transportation of goods by air costs a lot.  

航空运输货物花费很高.

(2)in the air

A thick haze of acrid smoke hung in the air.  

空气中弥漫着刺鼻的浓烟。 

He threw the baby high in the air and it stopped crying.  

他把孩子向空中高高抛起，孩子就停止了哭闹。

There had been stiffness and long silences and tension in the air.  

生硬的措辞，长时间的沉默，空气中弥漫着紧张的气息。

There was a bite in the air, a smell perhaps of snow.  

空气非常寒冷，似乎有雪的味道。

His breath was hanging in the air before him.  

他的哈气悬浮在自己面前。

(3)on the air

Rockwell hopes the program can be on the air within a year.  

罗克韦尔希望该节目能在一年内播出。

She is going on the air as presenter of a new show.  

她即将上电视主持一档新节目。

We went on the air, live, at 7.30.  

我们上了7点30分进行的现场直播。

We shall be on the air in 5 minutes.  

过五分钟我们开始广播.

Two battalions were disposed for an attack on the air base.  

配置两个营的兵力进攻空军基地.

She is singing on the air.  

她正在广播歌曲.

(4)in the open air

He likes to cook in the open air.  

他喜欢在外面做饭。

He is now taking a walk in the open air.  

他正在户外散步.

 The film will be shown in the open air tonight.  

今晚电影在露天演.

Take exercise in the open air.  

在户外做运动.

We sleep out under the stars, and eat our meals in the open air.  

我们在星空下露宿，在野外用餐。

 In the open air their rapture broke into speech.  

来到户外,他们如醉如痴的狂喜迸发成语言.

 No, I don't like eating in the open air.  

不好.我不喜欢在户外吃东西.

 Then we shall have a meal in the open air.  

然后我们在户外吃饭.

 He also tried just drying the teeth in the open air.  

他还尝试着把牙齿放在空气中风干.

It's good to have exercise in the open air.  

在户外做操是有益的.