Colm O'Regan: Nose dive into nature's smells

Warm weather in autumn and spring, feels strange. Because the days are not of summer-length, it seems to get darker early. It’s like being on holiday somewhere closer to the equator. By Thursday I was getting a fierce urge to “do something cultural because we can’t sit around drinking for the whole holiday”.

It’s cooled down since but for now let’s bask in the afterglow of basking in the glow of last week. It smelled differently. For a few days, the morning was a delicious process of heating up to a summer day after an autumn night. And it had a warm smell on top of a cool smell. And it got me thinking about the smell of nothing.

Or at least the smell of air. Because even when there’s no smell, even when Yer Man Down The Road isn’t illegally burning rubbish or someone hasn’t doused themselves or their surroundings in Lynx Africa or the takeaway hasn’t pumped Eau De MSG out of their vents, there is smell. And a reason for that smell.

Someone has definitely smelled petrichor these days. The smell of dry earth after rain. A mixture of geosmin (blue-green algae and
bacteria, a modified sesquiterpenoid if you must know) and volatile oils in plants. If there’s thunder and lightning, the electricity can force three oxygen atoms together to form ozone which has a sharp fresh smell.

That sea breeze? Next time someone brings that up in conversation and says it’s ‘salty’, smile kindly and announce it’s dimethyl sulfide, released by marine algae. It’s actually the smell of sea-life coping with saltwater. Not the smell of the salt itself. You will be the toast of the wedding afters with that info. 

While they’re still reeling, tell them pine forest smell is from the release of terpenes in trees. If they press you for more just mutter they are carbon chains up to 20 carbons long and that should scare them off any deep dives. I love the smell of freshly cut grass say the poets. That’ll be the green leaf volatiles released because the grass is damaged by the mower. There’s all sorts in there: aldehydes (I’m not sure whether it’s the informal or formal kind) alcohols and esters.

And it’s not your imagination. Warm air and cold air smell differently. Warm air makes molecules more volatile, so odours are more
obvious. What’s more, when it warms up after a cool night, there’s a sudden release of all the smells trapped in molecules overnight.

Cold is more the absence of smell. The molecules are less volatile. They keep their secrets If there’s snow, the air is trapped in the molecules under the snow. Like a fart in a bath. And by the way, bath farting is proper science. That’s how they analysed the chemicals in farts. Dutch gastroenterologist Albert Tangerman had volunteers sit in a bath as soon as they felt they were going have an airborne toxic event and they trapped the farts in inverted beakers. 

If that gives you the ick, at least it’s better than the 19th century when they collected gas from recently executed prisoners. Put that on your LinkedIn. I know all this as I bought one of the greatest book ever written. Nose Dive A Field Guide to the World’s Smells by Harold McGee. 600 pages. And I went straight to the farts page.

So if you’ve ever walked in the door of your home after being away for the day and you think “Oh God, is this what the house smells like to visitors”, don’t fret. Educate yourself. Wake up and smell the coffee.

(Or rather, the furans, furanones, mercaptomethylbutyl formates, and the hydroxyphenyl butanone.)

• Colm plays the fragrant towns of Kenmare, Co Kerry, on September 28; Skerries, Co Dublin 29; and
  Portlaoise, Co Laois on October 18. Tickets at linktree/colmoregan