Name: 【科學小常識】為什麼下雨後空氣格外清新？ (Why Does Rain Smell So Good?)
Uploaded: 2017-08-24T21:59:59.000Z
Duration: 3 min 30 s

Petrichor is the smell of dust after rain. It's a combination of plant oils, bacteria

「潮土油」是泥土雨後散發的氣味，它是由植物油脂、細菌

and ozone. But this doesn't tell the whole story. Petrichor smells, well dusty. How does

和臭氧所組成，但這並非事情的全部。潮土油聞起來像，嗯...像土

the smell get from the ground to our noses?

這股氣味是怎麼從土裡傳到我們鼻子裡呢？

A recent study from researchers at MIT suggests an aerosol effect is a main cause of the smell

麻省理工學院研究員的近期研究提出「氣溶膠效應」是造成這種

We all know the smell. On a summer day when the air has been stale and hot for days and suddenly

我們都知道，在炎炎夏日，好幾天空氣不流通又熱，突然

a summer shower provides some relief. When we venture out to greet the world anew, we're met

降下的陣雨帶來救贖。當我們踏出戶外迎接全新的世界，我們

with one of the most unique smells on the planet. It's a smell as old as time itself,

碰到了地球上最特別的氣味之一。這氣味和時間一樣古老

but it went without a name for a surprisingly long time. Petrichor was coined by two scientists,

但它好長一段時間沒有名字。「潮土油」是在1964年，由兩位科學家，

Isabel Joy Bear and R. G. Thomas of Australia's Commonwealth Scientific and Industrial Research

澳洲聯邦科學與工業研究組織的Isabel Joy Bear和R. G. Thomas

Organization, in 1964. The word uses Greek term, Petra-meaning stone, and ichor, the

所命名。這個字是希臘語，petra-是石頭的意思，而ichor是

special substance that flows in the veins of gods.

But the mechanism behind Petrichor was a bit of a mystery until MIT scientists captured

但是潮土油背後的機制一直是個謎，直到麻省理工學院的科學家拍攝到

slow motion images of falling water drops. The videos revealed that rain drops trap air

雨滴落下的慢動作影像。這支影片揭露雨滴會困住氣泡

bubbles. When they hit the ground, the bubbles capture small molecules from the soil. These

當雨滴到達地面，氣泡會抓住土壤中的小分子

molecules bubble up and release aerosols, not unlike the bubbles in a glass of champagne.

這些分子變成氣泡並釋出氣溶膠，有點像又不太像香檳裡的氣泡

Capturing this common occurrence took a little while. The scientists performed over 600 experiments

捕捉這常見的現象花了一點時間，科學家進行超過600次實驗

dropping water on 28 different surfaces. 12 artificial surfaces and 16 soil samples. To

將水滴在28種不同表面上，12種是人工表面，16種是土壤樣本

recreate the different types of rainfall, light and heavy and everything in between,

為了重現雨滴的不同種類，輕的、重的和之間所有形式

they varied the height. The higher the drop, the faster it fell. They found a Goldilocks

他們改變高度，水滴在越高處，下降速度越快，他們發現有金髮女孩效應

effect. The most aerosols were produced when the porous surface wasn't too wet or too

最多氣溶膠產生的時機在於，當多孔表面不是太潮濕或太乾燥

dry and when the rain wasn't too fast or too slow. If the rain falls too fast, there's

以及當雨下得不是太急或太緩，若雨下得太急

not enough time for the bubbles to form. So that's how the smell gets in the air, but

便沒有足夠時間讓氣泡成形。這就是這氣味飄散到空中的方法，然而

Previous studies suggest that the smell comes from oil given off by plants during dry periods

先前的研究認為該氣味來自植物在乾季時所釋放

Speaking of dirt, another part of petrichor comes from bacteria from the genus

說到土壤，潮土油的另一部份來自放線菌屬的

actinomycetes. The same bacteria that are responsible for a recent break in

細菌，就是最近在抗生素的突破有功的同一種細菌

antibiotics. When these bacteria die, they release an organic compound called Geosmin,

當這些細菌死亡，它們會釋出一種稱為「土霉味(geosmin)」的有機化合物

which comes from the greek "earth smell". This is the damp, musty, strong well, earthy

在希臘語中是「土壤的氣味」的意思。這是一種在暴雨後潮濕、發霉、強烈、嗯...土壤的

smell after a storm. Human noses are extremely sensitive to the compound, we sniff it out

味道。人類的鼻子對這種化合物非常敏感，只要有

it just 5 parts per TRILLION. It's a harmless chemical, while we love it outside, it can

5ppt（兆分之五）我們就能聞到。它是無害的化學物質，我們喜歡在戶外聞到它

reduce the quality of drinking water and wine.

但是它會降低飲用水和酒的品質

The smell of storms comes from ozone. A word which also has it's roots in greek for "smell".

暴雨的氣味來自於臭氧(ozone)，這個字的字根在希臘語是「嗅」的意思

Ozone is an alternative form of oxygen. While ground level Ozone is an icky, harmful smog,

臭氧是氧氣的另一種形式，雖然地面的臭氧是討人厭、有害的煙霧

atmospheric Ozone helps block some of the sun's most damaging rays. Storm-related

但大氣層中的臭氧幫助屏蔽太陽光中一些最具破壞性的輻射。和暴雨相關的

ozone is created when an electrical charge splits atmospheric nitrogen and oxygen molecules

臭氧產生於當一個電荷將大氣中的氮氣和氧氣分子

into separate atoms. A few chemical reactions later, the oxygen reorganizes itself. Most

拆散成分開的原子。經過一些化學反應，氧原子重新排列

atmospheric oxygen is made up of two atoms. the typical O2. Yet during a storm, the reorganized

大氣中大多的氧是由兩個氧原子所組成，也就是常見的O2。但在暴雨中，重新排列的

oxygen forms O3. The O3 is pushed down from the atmosphere by downdrafts, allowing us

氧原子組成O3，O3被下沉氣流從大氣層帶下來，使我們

Scientist still haven't found a way to bottle Petrichor. They've tried. How many candles

科學家尚未找到將潮土油裝瓶的方法，他們已經盡力了。多少蠟燭

and laundry detergents have names like "spring rain". Most artificial scents haven't

和洗衣精都有「春雨」之類的名字。多數人造香氣都

come close because Petrichor is so complex and some compounds exist only in small traces

仿造不來，因為潮土油很複雜而且有些化合物只有微量

that can't be detected by some machines. I wish they'd hurry up though, I'm tired

一些儀器偵測不到。但我希望他們能快點，我已經厭倦

So do you love the smell of rain? Let me know in the comments below …

你們喜歡下雨的味道嗎？在下面的留言告訴我吧...