Extreme Change Events of Stratospheric HCl and N2O in the Mid-Latitude Region of the Northern Hemisphere

Hydrogen chloride (HCl) is the main reservoir species of chlorine and chemical decomposition of nitrous oxide (N2O) is the primary source of NOx (=NO + NO2) in the stratosphere. Changes in stratospheric HCl and N2O play a critical role in modulating variations in stratospheric ozone. Thus, long-term trends in stratospheric HCl and N2O have been investigated in many studies, whereas short-term changes have not received enough attention. Here, using satellite observations and a chemical transport model, we found that two extreme change events for HCl and N2O in the Northern Hemisphere mid-latitude middle and lower stratosphere have occurred over past decades, which are characterized by a sharp increase in HCl and a decrease in N2O over several months; for example, HCl increased (and N2O decreased) by 0.135 ppbv (−33.352 ppbv) in 1987/1988 and by 0.196 ppbv (−28.553 ppbv) in 2010/2011. Further analysis shows that the extreme change events of stratospheric HCl and N2O in these two periods are closely related to anomalous residual circulation caused by the joint effects of the strong easterly phase of the semi-annual oscillation and the strong polar vortex.