Asian monsoon lofts ozone-depleting substances to stratosphere

Powerful monsoon winds, strengthened by a warming climate, are lofting unexpectedly large quantities of ozone-depleting substances high into the atmosphere over East Asia, new research shows.

The study, led by the U.S. National Science Foundation National Center for Atmospheric Research (NSF NCAR) and NASA, found that the East Asian Monsoon delivers more than twice the concentration of very short-lived ozone-depleting substances into the upper troposphere and lower stratosphere than previously reported.

The research team drew on airborne observations taken during a major 2022 Asian field campaign: the Asian Summer Monsoon Chemistry and Climate Impact Project (ACCLIP). The findings raise questions about the pace of the recovery of the ozone layer, which shields Earth from the Sun's harmful ultraviolet radiation.

"It was a real surprise to fly through a plume with all those very short-lived ozone-depleting substances," said NSF NCAR scientist Laura Pan, the lead author of the study. "These chemicals may have a significant impact on what will happen with the ozone layer, and it's critical to quantify them."

The study was published in the Proceedings of the National Academy of Sciences. It was funded by NSF, NASA, and NOAA, and co-authored by a large team of international scientists.

The role of monsoons

For thousands of years, people have viewed the Asian summer monsoon as important because of its impacts on local communities. Recently, however, scientists analyzing satellite observations have begun discovering that monsoon storms and winds play an additional role: carrying pollutants high in the atmosphere, where they can influence the world's climate system.

ACCLIP investigated the chemical content of air that was borne by the two primary monsoons in the region -- the South and the East Asian Monsoon -- from Earth's surface to as high up as the stratosphere. Once at that altitude, the chemicals can have far-reaching climate impacts because air in the stratosphere spreads out globally and remains for months to years, unlike the lower atmosphere where air masses turn over weekly.

The ACCLIP observations revealed that the East Asian Monsoon delivered higher levels of pollutants to the upper atmosphere than the South Asian Monsoon during 2022. The scientists measured carbon monoxide levels of up to 320 parts per billion -- a remarkably high level to be found at an altitude of 15 kilometers (about 9 miles). Carbon monoxide is often a sign of industrial pollution, and the measurements indicated that the East Asian Monsoon was closely aligned with emissions of pollutants at the surface.

Pan, Elliot Atlas of the University of Miami, and their co-authors looked into a class of chemicals known as very short-lived organic chlorine compounds, which can destroy ozone but persist only for a relatively short time in the atmosphere (months to years). In contrast, ozone-depleting chlorofluorocarbons (CFCs) remain in the atmosphere for decades to centuries or more and are therefore viewed as a far more significant threat to the ozone layer.

For that reason, the landmark 1987 Montreal Protocol on Substances that Deplete the Ozone Layer focused on phasing out CFCs and other long-lived substances. The international treaty and subsequent revisions have enabled stratospheric ozone to begin recovering. A 2022 United Nations assessment concluded that the ozone layer, including an ozone hole over the Antarctic, will be largely restored over the next several decades.

The Montreal Protocol, however, did not limit the continued manufacture and use of very short-lived ozone-depleting substances. Emissions of these chemicals have soared in South and East Asia, including highly industrialized regions of East China.

In an unfortunate coincidence, those regions lie directly under the East Asian Monsoon, which, of the world's eight regional monsoons, is the one that is predicted to strengthen the most with global warming.

The combination of the monsoon's powerful updrafts occurring in the same region as the increasing emissions of short-lived chlorine compounds has resulted in the unexpectedly high quantity of the chemicals being swept into the stratosphere.

The analysis of the aircraft measurements by Pan and her co-authors revealed high levels of five short-lived chlorine compounds: dichloromethane (CH2Cl2), chloroform (CHCl3), 1,2-dichloroethane (C2H4Cl2), tetrachloroethene (C2Cl4), and 1,2-dichloropropane (C3H6Cl2).

Pan said more research is needed to analyze the potential implications for ozone recovery. The paper also notes that scientists will need to incorporate the new findings into climate models, as stratospheric ozone has complex effects on Earth's temperature.

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Ozone depletion over North Pole produces weather anomalies

Many people are familiar with the hole in the ozone layer over Antarctica, but what is less well known is that occasionally, the protective ozone in the stratosphere over the Arctic is destroyed as well, thinning the ozone layer there. This last happened in the spring months of 2020, and before that, in the spring of 2011.

Each time the ozone layer has been thinned out, climate researchers subsequently observed weather anomalies across the entire northern hemisphere. In central and northern Europe, Russia and especially in Siberia, those spring seasons were exceptionally warm and dry. In other areas, such as polar regions, however, wet conditions prevailed. These weather anomalies were particularly pronounced in 2020. Switzerland was also unusually warm and dry that spring.

Whether there is a causal relationship between stratospheric ozone destruction and the observed weather anomalies is a matter of debate in climate research. The polar vortex in the stratosphere, which forms in winter and decays in spring, also plays a role. Scientists who have studied the phenomenon so far have arrived at contradictory results and different conclusions.

New findings are now shedding light on the situation, thanks to doctoral student Marina Friedel and Swiss National Science Foundation Ambizione Fellow Gabriel Chiodo. Both are members of the group headed by Thomas Peter, Professor of Atmospheric Chemistry at ETH Zurich, and are collaborating with Princeton University and other institutions.

Simulations reveal correlation

To uncover a possible causal relationship, the researchers ran simulations that integrated ozone depletion into two different climate models. Most climate models consider only physical factors, not variations in stratospheric ozone levels, in part because this would require much more computing power.

But the new calculations make it clear: the cause of the weather anomalies observed in the northern hemisphere in 2011 and 2020 is mostly ozone depletion over the Arctic. The simulations the researchers ran with the two models largely coincided with observational data from those two years, as well as eight other such events that were used for comparison purposes. However, when the scientists "turned off" ozone destruction in the models, they could not reproduce those results.

"What surprised us most from a scientific point of view is that, even though the models we were using for the simulation are utterly different, they produced similar results," says co-author Gabriel Chiodo, SNSF Ambizione Fellow at the Institute for Atmospheric and Climate Science.

The mechanism explained

The phenomenon as the researchers have now studied it begins with ozone depletion in the stratosphere. For ozone to be broken down there, temperatures in the Arctic must be very low. "Ozone destruction occurs only when it is cold enough and the polar vortex is strong in the stratosphere, about 30 to 50 kilometres above the ground," Friedel points out.

Normally, ozone absorbs UV radiation emitted by the sun, thereby warming the stratosphere and helping to break down the polar vortex in spring. But if there is less ozone, the stratosphere cools and the vortex becomes stronger. "A strong polar vortex then produces the effects observed at the Earth's surface," Chiodo says. Ozone thus plays a major role in temperature and circulation changes around the North Pole.

Greater accuracy possible for long-term forecasts

The new findings could help climate researchers make more accurate seasonal weather and climate forecasts in future. This allows for better prediction of heat and temperature changes, "which is important for agriculture," Chiodo says.

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