3 facts you did not know about ozone

Ozone is an air pollutant well known for its harmful effects on human health and vegetation. It is formed in the lower layers of the atmosphere, mainly during the summer and under the influence of solar radiation and high temperatures. 

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Summary

The smell of rain comes from the ozone in the air 

When a rainstorm is coming, people can tell that they smell it arriving, and they’re right. 

A storm always follows the same steps each producing distinctive odors. Before the rain starts to fall, a fresh and bleach-like smell can be perceived; this is the pungent scent of ozone.

💡 Ozone comes from ozein meaning ” to smell” in Ancient Greek.

Ozone can be found as a pollutant in lower layers of our atmosphere mainly because of anthropogenic activities, but also during a storm where lightnings break the naturally present dinitrogen and dioxygen into ozone. If it is usually present in higher altitudes, the storm downdrafts carry it to the ground level and to our nose which can pick out the smell at very small concentrations.

During the rain, the smell of ozone is masked by two other odors which are direct consequences of the rain hitting the ground: During the rain, the smell of ozone is masked by two other odors which are direct consequences of the rain hitting the ground: the pétrichor and the geosmin.

💡 Petrichor comes from the word petri meaning  “rock” and ichor  “divine blood” in Ancien Greek.

💡 Geosmin means “smell of the Earth” en Grec Ancien

Both are due to an accumulation of molecules produced by plants and bacteria, during dry period, into soils and at the surface of rocks, concrete, and asphalt.

In contact with rainwater, these molecules release a distinctive earthy smell to which we are much more sensitive than ozone. The longer the dry period, the stronger these smells will hit; therefore, we often consider it as a summer storm scent.

After the rain, the smell of petrichor and geosmin fade away to give way to the fresh scent of ozone again until it vanishes.

The ozone paradox

Ozone (O₃) Ozone (O3) is a secondary pollutant and is caused by the presence of its precursors, high temperature, and solar radiation. Its main precursor is nitrogen dioxide (NO₂), mostly caused by traffic pollution.

However, ozone concentrations are very often higher in rural areas, where nitrogen dioxide pollution is quite low: this is called the ozone paradox. This can be explained by the fact that ozone is formed from pollution but is also degraded thanks to it, especially thanks to nitrogen monoxide (NO).

💡 Although cities have slightly less ozone pollution than the countryside, this does not mean that they are not polluted at all!

We explain: 

Due to traffic, NO and NO₂ are highly concentrated in cities and almost absent in rural areas.

When sunshine is stronger and temperatures are higher during the day, urban NO2 is transformed into O3. The O3 can then be transported by the wind to rural areas, unlike NO, which is degraded beforehand.

NO, present in cities, reacts back with O3 to form NO₂, which leads to a diminution of O₃ concentration in cities. On the contrary, O₃ continues to accumulate in rural areas where it degrades very slowly due to the absence of NO.

This explains why we often notice higher ozone concentration in rural areas.

This issue also explains the paradox that appears in cities when anti-pollution measures are taken and the weekend effect: During the weekend or when the traffic is limited by anti-pollution measures, less nitrogen oxides are emitted. Ozone is thus less formed but also much less degraded, which leads to a temporary ozone increase in cities before a stable decrease.

The hole in the ozone layer is closing

If the hole in the ozone layer is a fact known by all, its current shrinking is much less known. Indeed, it is a very long process. The stratospheric ozone layer, or “good” ozone, absorbs UV rays which are harmful to health

Because of massive industrialization and the emission of aerosols (CFC), this layer has thinned over the years and especially above Antarctica: this is what we call the « hole » in the ozone layer . Since the Montreal Protocol in 1987 aiming to reduce and eventually eliminate ozone-depleting substances (ODS), around 98% of these substances have been phased out and the ozone protective layer is being replenished.

It should be entirely recovered by 2060’s and even earlier for some regions.

This protocol has had some impressive health and environmental benefits:

👉 By 2030, approximately 2 million people will be prevented from skin cancer every year

👉 The risk of eye issues, like cataract is largely limited

👉 – ODS being potent green-house gases, the current global warming is 0.5 to 1°C lower than it would have been without the protocol

The Montreal Protocol has been updated a few times to deal with new risks. The latest update is the Kigali Amendment (enforcement in 2019) mainly focusing on environmental risks and global warming by targeting the emissions of green-house gases.

This protocol and its powerful results are now one of the best examples of what can be achieved when the whole world is working together, even if it takes some time.

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