Science in Poetry | Why is the air so fresh after rain?

Autumn Dusk in the Mountain

Wang Wei (Tang Dynasty)

After the rain in the empty mountains, autumn begins late.

The bright moon shines among the pines, and the clear spring flows over the rocks.

The bamboos rustle with the sound of returning washerwomen, and the lotus leaves sway with the sound of fishing boats.

Let the spring flowers fade as they please, and the prince can stay.

This poem by Wang Wei, a great Tang Dynasty poet, is a landscape painting that exudes the freshness of nature. After the rain, especially after a thunderstorm, we always feel that the air is particularly fresh. Have you ever wondered why this is?

The rain washed away the dust in the air

On rainy days, the rain drops down and brings the dust floating in the air to the ground, just like giving the air a bath. We know that dust in the air may cause various respiratory diseases. When the air "takes a bath" and becomes clean, it will naturally give people a refreshing feeling.

A small amount of ozone is generated

There is another fresh smell in the air after the rain, which comes from ozone, which usually appears after a thunderstorm. Thunder and lightning will turn some of the oxygen in the air into ozone. High concentrations of ozone smell very bad, but trace amounts of ozone have a special fresh smell that makes people feel refreshed.

Earthy smell after rain linked to a bacteria

The fresh earthy smell floating in the air after the rain comes from geosmin.

In the soil beneath our feet, there are various kinds of actinomycetes. These are bacteria that have long hyphae extending in all directions like an octopus, and they are named after the actinomycetes in their colonies. Actinomycetes extend their long hyphae to obtain nutrients from as far away as possible.

Actinomycetes

When actinomycetes grow to a certain size, some of the long hyphae will grow new hyphae like new branches growing from a tree branch. These hyphae are called aerial hyphae. Aerial hyphae continue to grow, and when they grow to a certain size, they will produce spore threads, which contain many small spherical spores. These are the "children" of actinomycetes. Spores grow in spore threads, and when they mature, they drill out of the spore threads and spread into the soil or drift into the air, growing new actinomycetes as they go.

Different types of actinomycetes prepare "children's rooms" for babies - the shapes, sizes and colors of the spores are different, including straight, wavy, hooked, and spiral, but the most common is the spiral. The pigments inside the spores are also different. Therefore, the "children's rooms" of actinomycetes are colorful. The above characteristics are also one of the bases for scientists to classify actinomycetes.

Most actinomycetes prefer warm and moist soil. At this time, they will reproduce, speed up the process of spore maturation and dissemination, and grow their own family. When the soil is dry, the spores stay in the spore threads and wait for the opportunity. The longer the dry time, the more spores will accumulate waiting in line in the spore threads. Once it rains, the spores can't wait to drill out of the spore threads, be carried by the wind, and drift around, and seize the time to develop into new actinomycetes. When the spores are about to develop into actinomycetes, the metabolism in the cells begins to change, and a lot of geosmin will be produced.

Cows infected with actinomycetes have lumps under their mouths

Soil is often sparse and porous, and geosmin is mixed in the air and exists in these small pores in the form of small bubbles. When rain falls on the soil surface, these small bubbles rise to the surface of the soil under the action of the buoyancy of water and are gently ejected by droplets to form aerosols with a particle size of 0.01 to 10 microns, just like air freshener sprays, which can be easily inhaled into our noses.

Therefore, after the rain, we can often smell the refreshing "smell of earth". Researchers from the Massachusetts Institute of Technology in the United States used ultra-high-speed cameras to record the amazing process of geosmin forming aerosols, and also found that the amount of hydrosols produced by the impact of light and moderate rain is much greater than the amount of hydrosols produced by heavy rain. Therefore, in light rainy weather, the smell of earth in the air is often stronger.