雪花和“坏”臭氧之间的关系

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　　提到臭氧，大家的第一反应是保护地球的一种物质，因为南极上空出现的臭氧空洞非常令人担心。可是臭氧也有“好坏”之分吗？难道不都是好的吗？雪花和它又有什么关系呢？让我们跟随TPO52 Lecture3一起寻找答案吧！

　　Prof首先从我们最熟悉的雪花开始介绍，介绍了雪花是如何形成的：水蒸气在潮湿的环境下凝结成冰晶，形成了六边形的结构。

　　Prof：Anyway, snowflakes have a particular form, there’s a six-sided center with six branches or arms that radiate out from it. But how did they get that way? Well, you start with water vapor. You need a pretty humid atmosphere. And that water vapor condenses directly into ice, into an ice crystal. At this point it looks kind of like a thin dinner plate that rather than being circular, is hexagonal with six flat edges.

　　【学科词表】

　　snowflake [ˈsnoʊfleɪk] n. 雪花

　　radiate [ˈreɪdieɪt] v. 辐射，散发

　　radiate out from 从……中发出

　　crystal [ˈkrɪstl] n. 晶体

　　ice crystal 冰晶

　　hexagonal [hekˈsæɡənl] a. 六边形的

　　可是雪花和臭氧又有什么关系呢？这就涉及到雪花的一个重要特征：类液体薄层(QLL)，它是存在于冰的表面上的一层还未结冰的薄薄的水，即使温度在零度以下，它仍然存在。

　　Now one important characteristic of snowflakes is that they have something called a quasi-liquid layer, the QLL. Our snowflake is an ice crystal, right? Well, we find a quasi-liquid layer on the surface of ice is basically a thin layer of water that’s not completely frozen. And the existed temperature is well below freezing, though thickness varies at different temperatures. Now this quasi-liquid layer, it plays an important role on what we are going to talk about next.

　　【学科词表】

　　quasi-liquid layer [ˈkwɑːzi ˈlɪkwɪd] 类液体薄层

　　听到这里，大家不禁疑问：温度都零下了，怎么还会有液体存在呢？Prof给出了解释：当水结成冰，其原理是水分子们互相固定在一起。但冰的最外层水分子其实只有一面是和其他水分子固定的，所以它们就可以稍微自由移动，而这一层就形成了类液体薄层(QLL)。

　　FEMALE STUDENT

　　How can liquid exist below freezing? Why doesn’t it freeze?

　　MALE PROFESSOR

　　Well… when water becomes ice, the molecules bond together and they get sort of, uh, locked into place. They can’t move around as much anymore. So each molecule is surrounded by other molecules, and they’re all locked together. But what about the exterior of the ice? There’s a layer of water molecules on the surface… they’re attached to molecules only on one side, so they’re a bit freer… they can move around a bit more. …… well, this top layer would be the quasi-liquid layer, and it wouldn’t be completely frozen.

　　【学科词表】

　　bond [bɑːnd] v. 结合

　　exterior [ɪkˈstɪriər] a. 外部的

　　attach [əˈtætʃ] v. 依附

　　be attached to 附属于

　　解释清楚了雪花的QLL才终于到了它和臭氧之间的关系。臭氧既有天然形成来保护地球免受太阳辐射的，所谓“好”臭氧；也有因人类活动形成且会污染大气的，所谓“坏”臭氧。而雪花的QLL会释放某种化学物质来减少“坏”臭氧的含量，从而减少对大气的污染。

　　Prof：Now, there’s the ozone found in the stratosphere, which is the layer of the atmosphere from 6 to 30 miles above the Earth. This is considered “good” ozone, which occurs naturally and helps block harmful radiation from the Sun. But there is also ground-level ozone. It’s exactly the same gas, but it’s found closer to the surface of the Earth. This ground-level ozone results from human activities, and at high concentrations it can be a pollutant.

　　Now, snowflakes’ quasi-liquid layer plays an important role in some complex chemical reactions. Basically, these reactions cause certain chemicals to be released, and these chemicals reduce the amount of ground-level ozone. So the more quasi-liquid layer, the more reactions, and the more chemicals that reduce ground level ozone.

　　【学科词表】

　　ozone [ˈoʊzoʊn] n. 臭氧

　　stratosphere [ˈstrætəsfɪr] n. 平流层

　　release [rɪˈliːs] v. 释放

　　所以，文章一开始的问题就搞清楚了：臭氧确实有“好坏”之分，而雪花的一种特质——类液体薄层(QLL)就是用来对付“坏”臭氧的。下次天空飘雪的时候，你会联想到这篇听力文章吗？

　　作者介绍