Observations of Corona Discharges on Trees Under Thunderstorms
Restricted (Penn State Only)
- Author:
- Mc Farland, Patrick
- Graduate Program:
- Meteorology and Atmospheric Science
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- March 14, 2023
- Committee Members:
- Paul Markowski, Professor in Charge/Director of Graduate Studies
William Henry Brune, Thesis Advisor/Co-Advisor
Eugene Edmund Clothiaux, Committee Member
Matthew Robert Kumjian, Committee Member - Keywords:
- Electrical Discharges
Corona
Atmospheric Chemistry
Hydroxyl
Forest Ecology - Abstract:
- Weak electrical discharges known as corona discharges form on pointy objects such as lightning rods and tree leaves in the presence of a strong electric field, usually supplied by an overhead thunderstorm. Corona discharges produce prodigious amounts of the hydroxyl (OH) radical, the atmosphere’s primary cleaning agent, which has important implications for climate change, air quality, and human health. However, the prevalence and frequency of corona discharges remains unknown, as existing literature often claims these discharges form on trees under thunderstorms, but without reference or quantification. Thus, it is currently impossible to quantify the significance or impacts that corona-generated OH has on the surrounding environment. This project answers some of the unknowns regarding corona formation by proving that corona do form on trees under thunderstorms, identifying some of the environmental conditions necessary for corona formation, and estimating the amount of OH they generate. A new instrument designed to detect corona discharges, the corona observing telescope system (COTS), is developed and calibrated. COTS consists of two telescopes: one that detects solar visible radiation, and one that detects the 240-280 nm wavelength ultraviolet (UV) radiation emitted by corona. The COTS UV camera is a black and white, on and off camera, with black pixels signifying no detected UV radiation, and illuminated white pixels signifying a detection of UV radiation. A two-step calibration involving the OFiL camera, a smaller commercial corona detecting instrument, as a transfer standard relates the COTS UV signals (illuminated white pixels s-1) to production rates of OH (molecules s-1) calculated from OH measurements by the Penn State Ground-based Tropospheric Hydrogen Oxides Sensor (GTHOS). COTS UV signals have a logarithmic relationship to OH production rates, with an estimated uncertainty of ±50% at 95% confidence. UV signals detected by the COTS and OFiL camera decrease according to the square of the distance from the corona discharge. Additionally, attenuation from rainfall has little impact on UV signals, with transmissivities above 90% 100 m from the corona source for rainfall rates below ~10 mm hr-1. A few case studies demonstrate that corona do form on trees under thunderstorms and are quantifiable. From these case studies, corona are identified to form most readily directly beneath the thunderstorm core on pointy leaves when those points are oriented parallel to the electric field lines. The strongest ambient electric fields are found under the thunderstorm core, and leaves containing fine points can locally enhance the strong electric field, increasing the likelihood that corona will form. When the fine points of a leaf are oriented parallel to the electric field lines by the wind, that leaf maximizes its ability to locally enhance the ambient electric field. Corona are also observed to form in environments with steady-state electric fields of at least 3 kV m-1, in agreement with other studies. Using the calibrated relationship between the COTS UV signal and OH production rates, corona are estimated to produce 2 × 1014 OH molecules s-1 with a ±50% uncertainty at 95% confidence. Many more cases observing corona on trees under thunderstorms are required to answer lingering questions regarding the prevalence of corona under thunderstorms as well as potential impacts this additional source of OH has on trees and the surrounding environment.