Corona discharges take shape during storms, the researchers said, because clouds build up strong negative charges that attract the opposite positive charge on the ground below. Opposites attract and this positive electrical ground charge rises up through the trees to the highest point, causing an electric field on the tiny, hair-like tips of leaves that is great enough to create the weak corona glow in both visible and UV form. This UV from the corona breaks apart water vapor, producing hydroxyl.
Hydroxyl is the atmosphere’s main oxidizer. Oxidizers clean the air by reacting with chemicals emitted into the air, making other chemicals that are easier to remove. These chemicals include volatile organic compounds emitted by trees or human activities and the greenhouse gas methane. The team’s prior research found corona discharges to be a substantial source of atmospheric cleansers in the forest canopy.
The chemical conversion is what researchers keyed in on. Several years ago, the team applied high-voltage, low-current electrical impulses to tree branches and found a strong correlation between the UV emissions from corona discharges and the creation of hydroxyl compounds. In that project and the more recent observations, researchers noted leaf damage at the point corona was emitted.
To capture the phenomena in nature and make use of this correlation, the team developed the Corona Observing Telescope System, a Newtonian telescope that feeds into a UV camera. It’s geolocated, equipped with a device for measuring atmospheric electricity and calibrated for UV emissions using a mercury lamp. The solar UV wavelength band is completely blocked, leaving corona, lightning and fire as the only sources of UV in the field.
In North Carolina, this system captured 859 coronae events on the sweetgum tree and 93 on the loblolly pine. Events ranged from a blink to several seconds, McFarland said. During the field campaign, researchers observed coronae in four additional thunderstorms and on four additional tree species.
“It’s nearly invisible to the naked eye but our instruments give rise to a vision of swaths of scintillating corona glowing as thunderstorms pass overhead,” McFarland said. “Such widespread coronae have implications for the removal of hydrocarbons emitted by trees, subtle tree leaf damage and could have broader implications for the health of trees, forests and the atmosphere.”
While the researchers have confirmed the phenomena, they said they still don’t know much about the potential impacts of these corona discharges and have more questions, such as: Are trees harmed during this process? Or do they benefit in some way? Have they evolved to withstand it? Does the atmospheric cleansing have a benefit to the forest? The researchers are beginning collaborations with interested tree ecologists and biologists to answer these questions, thus blazing new paths of discovery into the natural world around us.
This work was supported by the U.S. National Science Foundation. Brune, Jenkins and Miller were co-authors on the research.