Air is matter.

空气是物质。

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The thicker stuff has more earth in it and less air, the lighter stuff has more air in it and less earth, but all matter is made up of these four elements.

厚的东西含有更多土更不气，轻的东西含更多气更少土，但所有物质都由这四种要素组成。

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Air quality is represented by annual mean concentration of fine particulate matter (PM10 and PM2.5, i.e. particles smaller than 10 or 2.5 microns).

空气质量用年平均细颗粒物的浓度表示(PM 10至PM 2.5，即粒径小于等于10或2.5微米的细颗粒)。

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The study also noted that when tilled soil particles come in contact with air, soil organic matter is lost through the process of oxidation.

VOA: special.2010.09.28

Speaking without a permit, in the open air, is another matter: Mr Chee has been prosecuted for that.

ECONOMIST: Future trends: On the loose in Singapore | The

Holding a wireless wheel in the air in front of you is goofy no matter how you slice it.

ENGADGET: Sony PlayStation Move Racing Wheel hands-on (video)

The new study looked at how much metal this so-called particulate matter is contained in the air of underground train systems.

BBC: Railway tracks