In August 2019, the Canadian-led Atmospheric Chemistry Experiment (ACE) mission will complete its sixteenth year in orbit on board the SCISAT satellite. The long lifetime of ACE has provided a valuable time series of composition measurements that contribute to our understanding of ozone recovery, climate change and pollutant emissions. These profiles of atmospheric trace gases and aerosols provide altitude-resolved data that are necessary for understanding processes that occur at specific altitudes or over limited vertical length scales.

The SCISAT/ACE mission uses infrared and UV-visible spectroscopy to make its solar occultation measurements. There are two instruments on board SCISAT. The ACE Fourier Transform Spectrometer (ACE-FTS) is an infrared FTS operating between 750 and 4400 cm-1 and the ACE-MAESTRO (Measurements of Aerosol Extinction in the Stratosphere and Troposphere Retrieved by Occultation) is a dual UV-visible-NIR spectrophotometer which was designed to extend the ACE wavelength coverage to the 280-1030 nm spectral region. From these measurements, altitude profiles of atmospheric trace gas species, temperature and pressure are retrieved. From SCISAT’s 74º inclination and 650 km altitude orbit, ACE makes measurements over a latitude range of ~85 ºN to ~85 ºS with a focus on the high latitudes. In addition to the mission and instrument status, a review of current validation and science results from the ACE mission will be presented in this paper.

Accurate evaluations of long-term ozone trends and their uncertainty and attribution constitute the basis for the WMO / UNEP Scientific Assessments of Ozone Depletion which are published every four years. The success of the Montreal Protocol in terms of the recovery of the ozone layer since the 1990s until the present is evaluated through comprehensive studies of space- and ground-based observations and model data. These also allow us to project atmospheric ozone in the next decades and derive realistic dates of return to 1980 ozone levels. The 2014 Ozone Assessment concluded that observations showed a statistically significant recovery in the upper stratosphere. In 2015, the SI2N initiative reported that upper stratospheric recovery was not necessarily significant, based on different data records and different analysis methods and assumptions. This inspired a new coordinated activity endorsed by SPARC, WMO (GAW) and IO3C named LOTUS (Long-term Ozone Trends and Uncertainties in the Stratosphere). This initiative aimed primarily at bringing the profile ozone community together in order to reach an as wide consensus as possible on the significance of stratospheric ozone trends, all of which should be concluded prior to the 2018 Ozone Assessment. LOTUS succeeded in this goal, taking advantage of state of the art profile ozone CDRs and models and analysis tools. In addition to estimating ozone trends, best practices for data handling, data merging and regression analyses were put forward. We will present the methods developed and used by this effort and we’ll summarize the main conclusions regarding stratospheric ozone recovery between 2000 and 2016. We also look forward and discuss lessons learned by LOTUS and in particular how these relate to the challenges faced by ozone CDR developers and providers such as ESA’s Climate Change Initiative and EC’s Copernicus Climate Change Service.

The recent World Meteorological Organization (WMO) Scientific Assessment of Ozone Depletion: 2018 has stated that satellite measurements show an increase in upper stratospheric ozone since the late part of the last century. Vertically resolved ozone profiles retrieved from optical measurements made by SAGE II, OSIRIS and OMPS-LP were used to construct multiple, long term, data records that were used to derive this conclusion.
In the period leading up to the 2018 assessment analysis was done within the SPARC LOTUS Initiative on multiple satellite based data records. The primary technique used to determine long term change, and its statistical significance, involved Multivariable Linear Regression (MLR). Recently, new techniques that involve Dynamic Linear Models (DLM) have been developed. These techniques may add insight to beyond that available using MLR as DLM does not constrain long term change to a given functional form, such as a straight line trend, but instead allows the change to curve and bend in response to the data. Also available within DLM is the instantaneous change associated with this curve that is prescribed by the data.
Presented within this paper are the latest results of a DLM algorithm, implemented by our team at the University of Saskatchewan, applied to our new merged SAGE II/OSIRIS/OMPS-LP USask 2D/ozonesonde data record. These results are compared to results achieved using MLR techniques and advantages of DLM over MLR are highlighted.

The creation of homogenized long-term ozone datasets based on limb-viewing measurements from ENVISAT sensors (GOMOS, MIPAS, SCIAMACHY) as well as from ESA Third Party Missions (OSIRIS, SMR and ACE-FTS) is one of the objectives of the ESA ozone-CCI project.
In the framework of the ozone-CCI project, different datasets are created. The datasets from individual instruments are collected in the user-friendly HARMonized dataset of OZone profiles (HARMOZ). For trend analyses, the ozone profile measurements from seven ESA and NASA satellite instruments (SAGE II, GOMOS, MIPAS, SCIAMACHY, ACE-FTS, OSIRIS, OMPS) are merged in a climate data record—the merged SAGE-CCI-OMPS dataset, which covers more than 30 years, from 1984 to 2017. This dataset has been used for evaluating ozone trends in the stratosphere.
In the presentation, we will show the obtained results, including updated analyses of stratospheric ozone trends, and discuss future developments of ozone climate data records.

Given that the scattering of sunlight by the Earth’s atmosphere above 30-35 km is primarily due to molecular Rayleigh scattering, the intensity of scattered photons can be assumed to be directly proportional to the atmospheric density.
From the measured relative density profile it is possible to retrieve an absolute temperature profile by assuming local hydrostatic equilibrium, the perfect gas law, and an a priori temperature from a climatological model at the top of the atmosphere. This technique is applied to Rayleigh lidar observations for over 35 years. The GOMOS star occultation spectrometer included spectral channels to observe daytime limb scattered sunlight close to the star direction. GOMOS Rayleigh scattering profiles in the spectral range 420-480 nm have been used to retrieve temperature profiles in the altitude range 35-85 km with a 2-km vertical resolution. A database of more than 309,000 temperature profiles has been created.
A global climatology was built and compared to GOMOS external model. In the upper stratosphere, where the external model is based on ECMWF analysis, the agreement is better than 2 K. In the mesosphere, where the external model follows MSIS climatology, 5 to 10 K differences are observed. Comparison with nighttime collocated Rayleigh lidar profiles above south of France shows some differences with a vertical structure that may be at least partially explained by the contribution of thermal diurnal tide.
The temperature evolution obtained at Equator indicates the occurrence of mesospheric inversion layers in the temperature profile with global longitudinal extension, descending in about one month from 80 to 70 km. The climatology shows a semi-annual variation in the upper stratosphere, a stratopause altitude varying between 47 and 54 km and an annual variation in the mesosphere.
The technique to derive temperature profiles from Rayleigh scattering at limb can be applied to any other limb-scatter sounder providing observation in the spectral range 350-500 nm. This is also a good candidate for a future small satellite constellation due to the simplicity of the principle.

The majority of chlorine in the atmosphere has arisen from anthropogenic emissions of 'organic' species such as chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs). Due to their long lifetimes, many of these species reach the stratosphere where they break down, liberating chlorine which catalyses the destruction of ozone. The principal degradation products of Cl-containing organic species are carbonyl chloride (phosgene, COCl₂), carbonyl chloride fluoride (COClF), and hydrogen chloride (HCl). Of these, phosgene is probably the most notorious, having been used as a chemical weapon in World War I.

Atmospheric phosgene, whose mixing ratios peak in the lower stratosphere, is produced via the degradation of the reasonably long-lived species carbon tetrachloride and methyl chloroform, and a number of very short-lived substances (VSLS), including dichloromethane, chloroform and tetrachloroethene. While all these species contribute to stratospheric ozone depletion, only carbon tetrachloride and methyl chloroform are regulated by the Montreal Protocol. It is therefore important that we continue to monitor VSLS and the degradation products to determine how much additional chlorine is reaching the stratosphere. VSLS can either reach the stratosphere directly, via so-called source gas injection (SGI), or degrade in the troposphere into products such as phosgene, which are then delivered into the stratosphere via so-called product gas injection (PGI). Monitoring phosgene in the upper troposphere / lower stratosphere (UTLS) is therefore a marker for PGI due to chlorine-containing VSLS.

In this work we report measurements of phosgene in the UTLS made by the ACE-FTS (Atmospheric Chemistry Experiment Fourier Transform Spectrometer) instrument. These data are compared with the output of the stratospheric configuration of the TOMCAT/SLIMCAT three-dimensional chemical transport model, which contains a detailed treatment of stratospheric chemistry, including the major species in the Ox, NOy, HOx, Fy, Cly, and Bry chemical families; this work represents the most detailed stratospheric modelling study of phosgene to date.

A decrease in stratospheric phosgene, due to the long-lived sources, is confirmed by both a thirteen-year ACE-FTS time series (2004 – 2016) and model calculations. The comparison in the upper troposphere, however, reveals differences, namely the model calculates phosgene VMRs ~ 10–20 ppt lower. Despite this bias, the upper tropospheric positive trend in the ACE-FTS observations, the reverse of that in the stratosphere, is consistent with an increase in phosgene prescribed by the TOMCAT/SLIMCAT outputs when VSLS are included. These observations thus provide the first observational evidence for an increase in chlorine PGI. The model calculations indicate that VSLS accounted for 27% of modelled stratospheric phosgene in 2016, up from 20% in the mid-2000s.