Is there more oxygen in a park than in a city centre?

So this morning on Hallam FM I called in to answer a question that the presenter had asked, part of a “find-out-friday” sort of feature. It was kinda of like the I’m a Scientist event I took part in back in June, except with an adult.

The question was, “Is there less oxygen in Fargate than in clumber park?” Not being a climate scientist, or with any REAL knowledge in air quality monitoring, I thought I’d ruminate on the question and give it a go trying to answer.

First off, my answer was “No – this wouldn’t be the case in Sheffield, though there have been instances in very densely populated cities that are very busy with people, traffic, and not with many green spaces, where lower than average oxygen levels have been measured”. So what’s the reasoning behind the difference in Sheffield and somewhere like, say, Mexico city?

First off, some background to this question: Michelle Mustard rightly thought that trees produce oxygen and so if there’s lots of them, surely there would be more oxygen in a place where there’s more trees, and less oxygen in a place where there aren’t as many trees. Trees produce oxygen as a waste product of photosynthesis – the way that they make energy to grow and live – and use carbon dioxide, sunlight and water.

So how comes my answer was no? Well, oxygen is a gas, and gases move about so given that there isn’t anything to hold the oxygen where it is, like a big, air-tight bubble, or biodome, it will just dilute away in to the atmosphere – I liken this to the atmosphere being a massive tank of water, and in one place you drop some green food colouring (this is the oxygen from trees) – but the green colour won’t stay in one place, it’ll mix in with the masses of water and get diluted away, so you won’t be able to see any difference in colour at one part to the next.

What has lead to there being less oxygen recorded in the middle of big cities? Well that’s just it – they’re big. There comes a point when the population density is so high, there’s so many people breathing out carbon dioxide and breathing in oxygen, and there being so many cars that use up some oxygen when burning fossil fuels too that it can make a significant impact. Carbon dioxide actually displaces oxygen, it pushes it out of the way, so on hot sunny days where trees are pumping out oxygen, but in a city with tall buildings and when the air is very still and stagnant, there can be a measurable difference. There are almost 9 million people living in Mexico city though, not to mention it sits in a valley surrounded by mountains and volcanoes in addition to generally weak wind patterns, it’s reknownded for it’s pollution, coming from over 50,000 industries and an estimated 4 million cars operating in the area. Whereas within the city limits of Sheffield there are only about 550,000 inhabitants – plus lets face it, Fargate and the surrounding area isn’t exactly the busiest and most congested city centres in the UK by a long-shot (I was just driving in London city yesterday, so I can personally vouch for this, even if I can’t give you exact numbers just now to prove it).

But the levels of oxygen shouldn’t be your biggest concern. The air around us is on average, made up of 21% oxygen (78% nitrogen, 0.1%argon and 0.03% carbon dioxide) – but in the history of our planet, intrepid scientists have been able to show that the levels of the different gases in air have changed. They do this in a sort of Jurrasic Park manner by drilling in to lumps of amber with pockets of air trapped in them and seeing how much of the gases make up that air. From this they’ve worked out that the levels of oxygen in air have varied quite a lot over hundred of millions of years, back to around 100 million years ago when air was made up by about 35% of oxygen.

Where has it gone then??? Well, some believe that it has to do with industrialisation, but others think it has something to do with a massive forest fire that happened about 10 million years ago. They can see in the levels of charcoal that they find that forest fires became more common as the levels of oxygen increased (oxygen is needed for fires to burn).

Given that we only use about a quarter of the oxygen we breath I’m not sure we need to worry too much – people like the Tibetans have adapted to living at high altitudes where there is less oxygen because the air isn’t as dense, so if the levels steadily decrease we may be able to adapt. I’ve seen a figure of 7% being the lowest we could tolerate.

So oxygen crisis? What oxygen crisis! Yes we have to be vigilant to the amounts of pollution we pump out, that is obvious, but when thinking specifically about the levels of oxygen, it shouldn’t be too much of a worry.

Sheffield, depsite a bad rep from the industrial revolution, has actually got really good clean air standards for a city – not to mention is one of the greenest cities is Europe. With 4 trees per person and an award-winning air quality and monitoring team we can feel happy and healthy. If you’re vaguely interested they actually host a website where you can check on the air quality from over a hundred different monitoring sites. We shouldn’t be complacent, if only for the health and comfort of our fellow city-dwellers.

Sheffield smog

p.s. Has anyone seen a bus shelter with a green roof (ie grass and stuff) in Sheffield?

Fun fact: 2 fully grown, mature trees can in one year produce the equivalent amount of oxygen to support a family of 4.

6 thoughts to “Is there more oxygen in a park than in a city centre?”

  1. The question never gets an answer.
    Q. Is there a 21% level of oxygen in a city centre?
    A. No. And the more trees you cut down, the less oxygen and more carbon dioxide you get

  2. Soooo… what’s the answer? Is there, or is there not less than 21% oxygen in a city centre, like Sheffield, or most interestingly, London?

    Perhaps you could comment and let us know 😉

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