科学家们用“空洞”一词表示臭氧浓度低于220个多布森单位（多布森单位是用来度量大气中臭氧柱尺度的单位。
Scientists use "hole" as a metaphor for the area in which ozone concentrations drop below the historical threshold of 220 Dobson Units.
这里所谓的“空洞”不是字面上含义，并不存在任何一处平流层完全没有臭氧，科学家们用空洞这一说法来描述这些臭氧浓度低于历史读数220多布森单位的区域。
The hole isn't literal; no part of the stratosphere is empty of ozone. But scientists use the hole metaphor to describe the area where ozone drops below a historical concentration of 220 Dobson Units.
这里所谓的“空洞”不是字面上含义，并不存在任何一处平流层完全没有臭氧，科学家们用空洞这一说法来描述这些臭氧浓度低于历史读数220多布森单位的区域。
The hole isn't literal; no part of the stratosphere is empty of ozone. But scientists use the hole metaphor to describe the area where ozone drops below a historical concentration of 220 Dobson Units.
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