A team of international researchers, including Dr. Kaley Walker, an atmospheric physicist at the University of Toronto, has reported an unexpected increase in hydrogen chloride, a chemical responsible for damaging the ozone layer in the atmosphere.
The researchers recently published their findings in the journal Nature, providing evidence of a trend towards a higher concentration of hydrogen chloride in the northern hemisphere since 2007.
Based on measurements and observations from independent satellite data sets, ground-based observations, and simulations from various models over the past three decades, the study reveals that hydrogen chloride levels had been declining after 1997, in line with what was predicted, but then reversed in 2007.
This decline was predicted after the signing of the Montreal Protocol, an international effort to reduce the ozone hole over Antarctica, in 1987. The Protocol aimed to control the production of ozone depleting substances like chlorofluorocarbons (CFCs) that produce hydrogen chloride as a by-product of consumption.
However, despite the decrease in CFC production, Walker explains that the recent increase in hydrogen chloride levels is a result of slower circulation of “old air,” which has been carrying the ozone-depleting chemical into the Northern Hemisphere’s lower stratosphere since around 2005 or 2006.
“It’s the change in how fast the air is moving in the atmosphere,” Walker says, “It’s not [that there is] more production of CFCs, because we looked at what the possible source [there is] and based on what we measure on the ground, we couldn’t see anything that could be contributing.”
While CFCs and related chlorine and bromine halogens are produced in comparably lower quantities today, when present, one atom of chlorine can destroy over 100,000 ozone molecules. According to the study, stratospheric circulation variability, as observed here could lead to an unpredictable increase or redistribution of hydrogen chloride; therefore, there is a need for careful monitoring in the path of ozone recovery.
“We are on the right track because of the Montreal protocol… we are seeing small variations in how that recovery is happening, because of the atmosphere being a very complex thing,” says Walker.
Walker adds that we should continue to monitor the atmosphere because of these smaller variations that occur at various altitudes, and to ensure that the models that are used to predict ozone layer recovery are representative of what is seen in the atmosphere.