Petrichor, the scent that occurs after light rainfall, is created by bacteria in the soil that becomes airborne during the rain. While this has been known for decades, there hasn’t been an exact mechanism known. A new study used a high-speed camera and found that aerosols are generated when raindrops land on porous surfaces, and could explain how petrichor occurs. Understanding this phenomenon could also help explain how E. coli and other pathogens are lifted from the soil where they can be transported in the air. The research was conducted by Young Soo Joung and Cullen Buie from MIT, and their paper was published in Nature Communications.
“Until now, people didn’t know that aerosols could be generated from raindrops on soil,” Joung said in a press release. “This finding should be a good reference for future work, illuminating microbes and chemicals existing inside soil and other natural materials, and how they can be delivered in the environment, and possibly to humans.”
Joung and Buie used the high-speed camera to analyze hundreds of raindrops as they fell onto 16 kinds of soil found in Massachusetts and 12 manufactured surfaces. They found that upon impact, the raindrop spreads out and traps bubbles of air. As the raindrop begins to retract from the fall, the bubbles rise up off the surface and escape as aerosols.
Sequence of raindrop hitting a porous surface and trapping the air bubbles, which then escape as aerosol droplets. Image courtesy of Youngsoo Joung
Under ideal conditions, hundreds of aerosol droplets can form in mere microseconds. The rate at which they are generated is dependent upon the velocity of the raindrop at impact, as well as the composition of the surface they hit. Heavy rain, which travels relatively quickly, hits many surfaces too quickly to generate the bubbles.
When a light air current was applied during the study, the researchers noticed the droplets taking flight. They suspected that this could be a contributing mechanism to the oils and bacteria that become airborne. Once those particles enter our noses and interact with our olfactory bulb, we are able to experience the sweet smell of petrichor. While it has been known that bacteria is kicked up by the rain, an exact mechanism of how this happens has never before been investigated. This paper does not definitively state that aerosol formed in this manner is that mechanism, but it does offer a plausible explanation.
“Aerosols in the air certainly could be resulting from this phenomenon,” Buie explained. “Maybe it’s not rain, but just a sprinkler system that could lead to dispersal of contaminants in the soil, for perhaps a wider area than you’d normally expect.”
By extension, this could also be how soil pathogens, such as E. coli and certain fungi, are able to be inhaled and lead to infections. The researchers explored possible pathogen transmission by adding fluorescent dye to the raindrops. They were able to confirm that the dye was present in the droplets of aerosol, which indicates it may be possible for pathogens to be carried in this manner as well.
“To prevent transmission of microorganisms from nature to humans, we need to know the exact mechanism. In this work, we provide one possible way of transmission,” Joung concluded.