Sensitivity of Heterogeneous Chlorine Chemistry on Sulfate Aerosols in the Tropical Stratosphere

Abstract:

Heterogeneous chemical reactions on liquid sulfuric acid particles convert stratospheric chlorine from reservoir species into reactive ones that destroy ozone through gas-phase catalytic cycles. Radiosonde observations and simulations with the WACCM model show that, in the tropical lower stratosphere, transport processes such as monsoonal circulations and Rossby waves can enhance chlorine reservoir levels enough for significant activation to take place when temperatures drop below about 195 K. Sensitivity analysis has shown that the rate of chlorine activation from reactions of ClONO₂ with HCl and H₂O and HOCl with HCl on liquid sulfuric acid aerosols in these regions is most sensitive to perturbations of temperature, with water vapor levels of secondary importance. Increased stratospheric aerosol loading from volcanic eruptions also enhances ozone depletion by providing greater surface area for heterogeneous reactions, with noticeable impacts in years with moderate sized volcanoes such as 2011, although some chlorine activation is present even in volcanically clean years. Comparison of radiosonde and model data has also shown that, due to the strong nonlinearity of these reactions with temperature, accurate measurements are essential, and calculations using low resolution temperature input will frequently underestimate reaction rates. These findings have expanded the known latitude range over which such chemistry can act, and indicate that heterogeneous chemistry on sulfuric aerosols may be a significant contributor to decadal ozone trends in the tropics.