The Montreal Protocol has banned the production of long-lived chlorine-containing gases such as chlorofluorocarbons (CFCs) that deplete stratospheric ozone. These halogenated compounds ultimately form HCl in the upper atmosphere; the effectiveness of the Montreal Protocol can therefore be monitored by measuring stratospheric HCl. The Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) measures infrared solar occultation spectra of the Earth’s atmosphere from which altitude profiles of HCl volume mixing ratios (VMRs) are determined. The upper stratospheric HCl VMR time series has a linear trend of −۴٫۸ ± ۰٫۲%/decade for 2004–۲۰۱۷, highlighting the continuing success of the Montreal Protocol. © ۲۰۱۸ Elsevier Ltd. All rights reserved.

Introduction

The Montreal Protocol is an international treaty that controls substances such as chlorofluorocarbons (CFCs) and halons that deplete stratospheric ozone [1]. Stratospheric ozone prevents deleterious near ultraviolet radiation (200–۳۰۰ nm) from reaching the ground. Rowland and Molina [2] discovered that long-lived chlorofluorocarbons (CFCs) emitted by human activities are inert in the troposphere but are photolyzed in the stratosphere and release chlorine atoms. These chlorine atoms destroy ozone in a catalytic cycle involving the ClO free radical. Ultimately CFCs are oxidized to CO2, HF and HCl. The success of the Montreal Protocol can therefore be monitored by measuring the amount of stratospheric HCl. Atmospheric HCl can be measured from the ground by high resolution infrared spectroscopy using lines from the fundamental band in the 3.5 μm region and the Sun as a light source. The total column density of HCl is being measured by a network of infrared Fourier transform spectrometers as part of the NDACC (Network for the Detection of Atmospheric Composition Change; http://www.ndsc.ncep.noaa.gov/) [3]. HCl volume mixing ratio (VMR) measurements as a function of altitude can be obtained from high altitude balloons with in situ measurements [e.g., 4] or with an infrared spectrometer using the Sun as a light source [e.g., 5]. More comprehensive global observations have been made from satellite platforms starting with the HALOE (Halogen Occultation Experiment) instrument on NASA’s Upper Atmosphere Research Satellite (UARS) from 1991 to 2005 [6]. More recent HCl measurements (2004-present) are being made by the Canadian Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) on SCISAT [7] and the Microwave Limb Sounder (MLS) on NASA’s Aura satellite [8]. Many papers have been published on atmospheric HCl trends such as by Froidevaux et al. in 2006 [9] and in 2015 [10]. Froidevaux et al. [10] have combined HALOE, MLS and ACE-FTS data to provide a multi-instrument HCl time series for 1991 to 2012 called GOZCARDS (Global OZone Chemistry And Related trace gas Data records for the Stratosphere). Brown et al. [11] used tropical ACEFTS data for the period 2004–۲۰۱۰٫ HCl trends are also reported every 4 years in the WMO ozone assessment [12]. In the present paper, we update global HCl trends derived from ACE-FTS data for 2004–۲۰۱۷٫ A complicating factor in determining stratospheric HCl trends is dynamical variability. For example, recent HCl volume mixing ratios (VMRs) in the lower stratosphere in the Northern Hemisphere have increased because of dynamics [13], while the overall global stratospheric trend remains negative. The effects of dynamical variability on the HCl VMR time series can be reduced by using the correlation with a long-lived tracer such as N2O [e.g., 14]. The HCl trend values determined by HALOE [6] were in the lower mesosphere at 55 km to avoid problems with dynamical variability in the stratosphere and because almost all the source gases are converted to HCl at high altitude.