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Studying showers

The heavens open on Jessica Rowbury as she looks at the science behind the sweet smell of summer rain

With summer in full swing, some may experience the aroma of petrichor — the fresh, earthy smell that often follows the first shower after a dry spell.

Thanks to high-speed and high-resolution imaging, scientists have been able to identify the mechanism that releases this characteristic smell and, in separate research, how rain occurring at warmer temperatures contributes to the spread of bacteria and disease.

The smell of rain

Using high-speed cameras, MIT researchers Cullen R Buie and Youngsoo Joung conducted roughly 600 experiments to identify how the aroma of petrichor is released into the air. Building on the work, Buie, Joung and colleagues used high-resolution imaging to demonstrate how rain spreads bacteria, in a study published in March 2017 in Nature Communications.

The initial study looked at the mechanism of release of the earthy aroma that often follows a rainstorm. The researchers observed that when a raindrop hits a porous surface, it traps tiny air bubbles at the point of contact. As in a glass of champagne, the bubbles then shoot upward, ultimately bursting from the drop in a fizz of aerosols.

The team was also able to predict the amount of aerosol released, based on the velocity of the raindrop and the permeability of the contact surface.

Although scientists have long observed that raindrops can trap and release aerosols when falling on water, this research, published in Nature Communications in 2015, was the first to show this effect on soil.

In the lab, the team deposited single drops of water on 28 types of surfaces — 12 engineered materials and 16 soil samples — simulating various intensities of rainfall by adjusting the height from which the drops were released; the higher the droplet’s release, the faster its ultimate speed.

A system of high-speed cameras was set up to record the drops on impact. The images they produced revealed a mechanism that had not previously been detected: As a raindrop hits a surface, it starts to flatten; simultaneously, tiny bubbles rise up from the surface, and through the droplet, before bursting out into the air. Depending on the speed of the droplet, and the properties of the surface, a cloud of ‘frenzied aerosols’ may be dispersed.

‘Frenzied means you can generate hundreds of aerosol droplets in a short time — a few microseconds,’ Joung explained. ‘And we found you can control the speed of aerosol generation with different porous media and impact conditions.’

From their experiments, the team observed that more aerosols were produced in light and moderate rain falling on sandy or clay soils, while far fewer aerosols were released during heavy rain.

Buie said this mechanism may explain petrichor, a phenomenon first characterised by Australian scientists as the smell released after a light rain.

‘They talked about oils emitted by plants, and certain chemicals from bacteria that lead to this smell you get after a rain following a long dry spell,’ Buie said. ‘Interestingly, they don’t discuss the mechanism for how that smell gets into the air. One hypothesis we have is that that smell comes from this mechanism we’ve discovered.’

How rain spreads bacteria

In the latest study, published in March 2017, the team found that aerosols released from soil carried up to several thousand bacteria, which remained alive for more than an hour after being thrown into the air.

If the airborne bacteria were lofted further by wind, they could travel a good distance before settling back on the ground to colonise a new location, said Buie.

‘Imagine you had a plant infected with a pathogen in a certain area, and that pathogen spread to the local soil,’ Buie continued. ‘We’ve now found that rain could further disperse it. Manmade droplets from sprinkler systems could also lead to this type of dispersal. So this [study] has implications for how you might contain a pathogen.’

Furthermore, the team calculated that precipitation may be responsible for 1 to 25 per cent of the total amount of bacteria released from land worldwide.

In the lab, the team looked at rainfall’s effect on three non-pathogenic species of soil bacteria, which they infused in six types of dry soil, including clay, sandy clay, and sand.

The researchers simulated rainfall by dispensing single drops of water through the hole of a small disc that was placed just above a soil sample to catch any aerosols bursting up from the surface. They varied the surface temperature of the soil, as well as the height at which a droplet was released, to speed up or slow down a droplet’s impact speed, thereby simulating certain intensities of rainfall.

They found droplets produced the highest number of aerosols in soils with temperatures of around 30°C, similar to soils found in tropical regions. Droplets also produced more aerosols when dispensed on sandy clay soils; sand tended to absorb the droplets completely before any bubbles or aerosols could form. With high-speed cameras, the researchers were also able to observe higher aerosol counts when droplets fell at speeds between 1.4 and 1.7 metres per second — about the intensity of a light rain shower.

‘At this just-right speed, water wicks into the soil without splashing, but fast enough to trap air,’ Buie explained. ‘That trapped air gets released as bubbles that burst, releasing the aerosols. We found the relationship between the distribution of aerosol size and the number of bubbles bursting.’

The team collected the aerosols that sprayed up onto the small disc, and transferred them to culture dishes to count the number of bacteria in each aerosol. They found the number of bacteria varied from zero to several thousand from a single raindrop, depending on the type of soil, the density of bacteria within a given soil, the soil temperature, and the raindrop’s impact speed.

Going a step further, the team identified three main parameters necessary for estimating the total number of bacteria or other particles that may be dispersed by a single raindrop hitting a porous surface: the density of both bacteria and air bubbles on a given surface, and a parameter they call aerosolisation efficiency — the ratio of the number of bacteria on a surface to the number of bacteria that are ultimately dispersed from that surface.

Using these three parameters, along with estimates for the world’s total land area and precipitation patterns, the researchers estimated that the total number of bacteria dispersed by raindrops can range from 10,000 trillion to 800,000 trillion cells per year. As a result, global precipitation may contribute to releasing 1.6 to 25 per cent of the total amount of bacteria from land.

‘Further investigation is required to narrow down the range of global emission of bacteria by rain, but aerosol generation by rain could be a major mechanism of bacteria transfer into the environment,’ said Joung. ‘Future work on these findings could provide new clues to trace soil-borne bacteria responsible for infections in humans, animals, and plants, as well as climate impacts due to cloud formation and ice nucleation.’

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