Abstract
Abstract. Hydrogen chloride (HCl) and hydrogen fluoride (HF) are respectively the main chlorine and fluorine reservoirs in the Earth's stratosphere. Their buildup resulted from the intensive use of man-made halogenated source gases, in particular CFC-11 (CCl3F) and CFC-12 (CCl2F2), during the second half of the 20th century. It is important to continue monitoring the evolution of these source gases and reservoirs, in support of the Montreal Protocol and also indirectly of the Kyoto Protocol. The Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) is a space-based instrument that has been performing regular solar occultation measurements of over 30 atmospheric gases since early 2004. In this validation paper, the HCl, HF, CFC-11 and CFC-12 version 2.2 profile data products retrieved from ACE-FTS measurements are evaluated. Volume mixing ratio profiles have been compared to observations made from space by MLS and HALOE, and from stratospheric balloons by SPIRALE, FIRS-2 and Mark-IV. Partial columns derived from the ACE-FTS data were also compared to column measurements from ground-based Fourier transform instruments operated at 12 sites. ACE-FTS data recorded from March 2004 to August 2007 have been used for the comparisons. These data are representative of a variety of atmospheric and chemical situations, with sounded air masses extending from the winter vortex to summer sub-tropical conditions. Typically, the ACE-FTS products are available in the 10–50 km altitude range for HCl and HF, and in the 7–20 and 7–25 km ranges for CFC-11 and -12, respectively. For both reservoirs, comparison results indicate an agreement generally better than 5–10% above 20 km altitude, when accounting for the known offset affecting HALOE measurements of HCl and HF. Larger positive differences are however found for comparisons with single profiles from FIRS-2 and SPIRALE. For CFCs, the few coincident measurements available suggest that the differences probably remain within ±20%.
Highlights
Under unperturbed atmospheric conditions, hydrogen chloride (HCl) and hydrogen fluoride (HF) are the two most abundant halogenated species of the inorganic chlorine and fluorine families (respectively denoted Cly and Fy; see e.g. Prinn et al (1999)) in the stratosphere
Comparisons between version (v19) HALOE and v1.0 Atmospheric Chemistry Experiment (ACE)-FTS data were described by McHugh et al (2005), who found that ACE-FTS HCl was within ±10% of HALOE below km, and 10–20% higher than HALOE from 20 to 48 km
We have compared ACE-FTS v2.2 products with a series of available coincident or comparable profile or column measurements performed from space, balloons and from the ground, for HCl, HF, CFC-11 and CFC-12
Summary
Hydrogen chloride (HCl) and hydrogen fluoride (HF) are the two most abundant halogenated species of the inorganic chlorine and fluorine families (respectively denoted Cly and Fy; see e.g. Prinn et al (1999)) in the stratosphere. Even before the unambiguous confirmation of the significant role of anthropogenic chlorine in the destruction of the Earth’s protective ozone layer, monitoring networks such as the AGAGE (Advanced Global Atmospheric Gases Experiment) and NOAA/ESRL (National Oceanic and Atmospheric Administration – Earth System Research Laboratory) have been measuring increases in a large number of source gases, including all major long-lived CFCs and HCFCs, by in situ surface sampling (e.g., O’Doherty et al, 2004; Montzka et al, 1999, and references therein) These increasing tropospheric loadings were at the heart of the alarming threat to ozone suggested by Molina and Rowland (1974). While profiles of more than 30 species are retrieved from the ACE-FTS measurements, a group of primary (“baseline”) data products have been selected by the ACE Science Team as the focus of this validation exercise These include O3, CH4, H2O, NO, NO2, ClONO2, HNO3, N2O, N2O5, HCl, CCl3F, CCl2F2, HF, CO, aerosols, temperature and pressure. It should be noted that, to date, no formal complete error budget has been produced for the ACE-FTS v2.2 retrievals
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