Space-based measurements of HCl: Intercomparison and historical context

Abstract

[1] The peak in stratospheric HCl was reached in the late 1990s. Between 1998 and 2004 the stratospheric loading of HCl was relatively constant, with some month to month fluctuation; this was followed by a more pronounced decrease in HCl since 2004. We use probability distribution functions (PDFs) and scatter diagrams for validation and bias characterization of Aura Microwave Limb Sounder (MLS) HCl retrievals. Both these methods allow us to use large statistical samples and do not require correlative measurements to be colocated in space and time. The bias between the Halogen Occultation Experiment (HALOE) and Aura MLS is greatest above the 525 K (~21 km) isentropic surface. The global average mean bias between Aura and the Atmospheric Chemistry Experiment (ACE) for January 2005 was 2% and between Aura MLS and HALOE was 14%. The widths of the PDFs are a measure of the spatial variability and measurement precision. The Aura MLS HCl PDFs are consistently wider than those for ACE and HALOE, this reflects the retrieval uncertainties. The median observation uncertainty for Aura MLS v1.51 HCl is 12%, and the median ACE v2.2 uncertainty is 8%. We also connect Aura MLS HCl with the heritage of HALOE HCl by using neural networks to learn the interinstrument biases and provide a seamless HCl record from the launch of the Upper Atmosphere Research Satellite (UARS) in 1991 to the present.