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Variations in stratospheric inorganic chlorine between 1991 and 2006

A consistent time series of stratospheric inorganic chlorine Cly from 1991 to present is formed using space‐borne observations together with neural networks. A neural network is first used to account for inter‐instrument biases in HCl observations. A second neural network is used to learn the abundance of Cly as a function of HCl and CH4, and to form a time series using available HCl and CH4 measurements. The estimates of Cly are broadly consistent with calculations based on tracer fractional releases and previous estimates of stratospheric age of air. These new estimates of Cly provide a critical test for global models, which exhibit significant differences in predicted Cly and ozone recovery.