Chemical reaction pathways affecting stratospheric and mesospheric ozone

Abstract

Catalytic cycles and other chemical pathways affecting ozone are normally estimated empirically in atmospheric models. In this work we have automatically quantified such processes by applying a newly developed analysis package called the “Pathway Analysis Program” (PAP). It used modeled chemical rates and concentrations as input. These were supplied by the “Module Efficiently Calculating the Chemistry of the Atmosphere” MECCA box model, itself initialized by the Free University of Berlin Climate Middle Atmosphere Model with Chemistry. We analyzed equatorial, midlatitude and high-latitude locations over 24-hour periods during spring in both hemispheres. We present results for locations in the lower stratosphere, upper stratosphere and midmesosphere. Oxygen photolysis dominated (>99%) in situ ozone production in the equatorial lower stratosphere, in the upper stratosphere and in the mesosphere. In the lower stratosphere midlatitudes the “ozone smog cycle” (already established in the troposphere) rivaled oxygen photolysis as an in situ ozone source in both hemispheres. However, absolute ozone production rates in midlatitudes were rather slow compared with at the equator, typically 16–50 ppt ozone/day. In the equatorial lower stratosphere, five catalytic sinks were important (each contributing at least 5% to chemical ozone loss): a HOx cycle, a HOBr cycle and its HOCl analog, a water-HOx cycle and a mixed chlorine-bromine cycle. Important in midlatitudes were the HOx cycle, a NOx cycle, the HOBr cycle and the mixed chlorine-bromine cycle. In lower-stratosphere high latitudes, the chlorine dimer cycle and the mixed chlorine-bromine cycle dominated in both hemispheres. A variant on the latter, involving BrCl formation, also featured. In the upper stratosphere high latitudes (where strong negative ozone trends are observed), a nitrogen cycle, a chlorine cycle, and a mixed chlorine-nitrogen cycle were found. In the mesosphere, three closely related HOx cycles dominated ozone loss.