Recent Northern Hemisphere stratospheric HCl increase due to atmospheric circulation changes

Area of Science:

• Atmospheric Chemistry
• Stratospheric Science
• Climate Dynamics

Background:

• Chlorine-containing gases, like chlorofluorocarbons, significantly increased atmospheric chlorine from the 1970s-1990s.
• These gases contribute to ozone depletion, leading to the Montreal Protocol and a subsequent decrease in surface chlorine.
• Stratospheric chlorine peaked post-1996 and was expected to decrease, mirroring surface trends.

Purpose of the Study:

• To investigate a recent, unexpected increase in stratospheric hydrogen chloride (HCl).
• To understand the drivers behind this anomaly in contrast to decreasing surface chlorine source gases.
• To assess the implications of this trend for stratospheric ozone layer evolution.

Main Methods:

• Analysis of ground-based and satellite data for hydrogen chloride (HCl) concentrations.
• Utilizing atmospheric model simulations to interpret observed trends.
• Comparing stratospheric HCl trends with near-surface concentrations of chlorine source gases.

Main Results:

• A significant increase in stratospheric hydrogen chloride (HCl) was observed starting around 2007 in the Northern Hemisphere.
• This trend contrasts with the ongoing decrease in near-surface chlorine source gases.
• Model simulations attribute the anomaly to a slowdown in Northern Hemisphere atmospheric circulation.

Conclusions:

• A slowdown in Northern Hemisphere atmospheric circulation is responsible for increased stratospheric HCl.
• This dynamical variability leads to more aged air and greater conversion of source gases to HCl in the lower stratosphere.
• Future studies of the ozone layer must consider this short-term dynamical variability and its impact on stratospheric tracers.