In the study, scientists looked at how chemicals like CFCs affect the ozone layer in the stratosphere. These chemicals break down chlorine, which can harm the ozone. Between the 1970s and 1990s, there was a lot of chlorine in the air, leading to ozone loss, especially in polar areas. But when countries agreed to the Montreal Protocol in 1987 to reduce these chemicals, chlorine levels started going down. Now, instead of being harmful, chlorine is mostly turning into HCl, a different form that shows how well the Montreal Protocol is working. By tracking HCl, we can see that the Montreal Protocol is helping protect the ozone layer.