VATIS UPDATE Article

A team of researchers from University of Leicester and University of Leeds, the United Kingdom, University of Waterloo, Canada, and Old Dominion University, Jet Propulsion Laboratory, and Hampton University, the United States, have reported the comparison of global distributions and trends of HF measured in the Earth’s atmosphere by the satellite remote-sensing instruments ACE-FTS (Atmospheric Chemistry Experiment Fourier transform spectrometer), which has been recording atmospheric spectra since 2004, and HALOE (HALogen Occultation Experiment), which recorded atmospheric spectra between 1991 and 2005, with the output of SLIMCAT, a state-of-the-art three-dimensional chemical transport model.

In general the agreement between observation and model is good, although the ACE-FTS measurements are biased high by ~ 10% relative to HALOE. The observed global HF trends reveal a substantial slowing down in the rate of increase of HF since the 1990s: 4.97 ± 0.12 % year–1 (1991–1997; HALOE), 1.12 ± 0.08 % year–1 (1998–2005; HALOE), and 0.52 ± 0.03 % year–1 (2004–2012; ACE-FTS). In comparison, SLIMCAT calculates trends of 4.01, 1.10, and 0.48 % year–1, respectively, for the same periods.

Furthermore, the observations reveal variations in the HF trends with latitude and altitude; for example, between 2004 and 2012 HF actually decreased in the Southern Hemisphere below ~ 35 km. An additional SLIMCAT simulation with repeating meteorology for the year 2000 produces much cleaner trends in HF with minimal variations with latitude and altitude. Therefore, the variations with latitude and altitude in the observed HF trends are due to variability in stratospheric dynamics on the timescale of a few years.