The atmosphere has enhanced its capacity to remove air pollutants
An international research team analysed two long-running datasets of air samples collected from New Zealand and Antarctica, starting in the 1980s. The research was led by New Zealand’s National Institute of Water and Atmospheric Research (NIWA), and included researchers from Finland, Germany, and the USA.
The study reveals that the atmosphere's self-cleansing capability has been strengthening in the Southern Hemisphere since approximately 1997. This suggests that the contribution of methane to global warming may be at least partially compensated, and would have been more dramatic if the atmosphere's ability to clean itself had not strengthened. The findings also indicate that methane emissions may be higher than previously documented.
This study examines the atmosphere's self-cleansing ability, facilitated by the hydroxyl radical (OH). It is a highly reactive chemical species formed in the atmosphere during the daytime when solar ultraviolet radiation interacts with ozone in the presence of water vapour. Although its lifetime is only about one second, OH plays a crucial role in cleansing the troposphere, the lowest layer of the atmosphere, of harmful gases. Notably, OH can remove up to 90% of methane present in the air.
Due to its short lifetime and low concentrations, OH is very difficult to measure accurately. Traditional methods and models for predicting hydroxyl abundance have produced conflicting estimates.
The new research utilizes radiocarbon monoxide (14CO) as a reliable tracer for OH. Since OH effectively removes carbon monoxide, a decrease in carbon monoxide indicates a strengthening of hydroxyl's oxidizing capacity. “The progress in developing a model for radiocarbon production in the atmosphere was very important to this study. Our recent model CRAC (Cosmic Ray Atmospheric Cascade) is able to simulate its production with high accuracy, allowing for robust identification of changes in atmospheric OH”, explains Professor Ilya Usoskin from the University of Oulu.
The results were surprising, revealing highly significant decreases in the 14CO level since 1997: approximately 12% per year in New Zealand and even 43% (± 24%) in Antarctica in mid-summer. These research findings suggest that the atmosphere's oxidizing capacity, driven by hydroxyl, has been strengthening over recent decades.
“The findings regarding the increasing self-cleansing capacity of the atmosphere are intriguing; however, they were obtained in the Southern Hemisphere, making it challenging to project these results to the entire Earth. It would be very important to conduct similar measurements in Finland and the whole Northern Hemisphere, which is more polluted”, notes Usoskin.
Researchers have also identified the key drivers of changes in hydroxyl levels in the atmosphere: nitrogen oxides mainly from motor vehicles, industrial combustion, lightning, and wildfires contribute to an increase in hydroxyl; stratospheric ozone depletion and water vapor, both influenced by global warming, also increase hydroxyl, while methane emission decreases it.
Space Physics and Astronomy Research Unit
