Reactive Trace Gases 3

High accuracy global total ozone measurements from space are key data for evaluating the amplitude of ozone depletion, understanding its contribution and consequences on climate change and predicting the future evolution of both ozone and climate. Long series of total ozone data are available, but showing still systematic significant differences between them. Here most recent data available from GOME-ERS2, SCIAMACHY-Envisat, GOME2-Metop reprocessed with the GODFIT V3 (GOME-Type Direct FITing) algorithm developed in the frame of the ESA Ozone Climate Initiative (Lerot et al., 2014), from NASA SBUV v8.6 (McPeters et al., 2013), NASA-AURA-OMI (Levelt et al, 2007) and IASI Metop A and B (Clerbaux et al., 2009), are compared with SAOZ total ozone ground-based NDACC measurements available at several stations distributed in latitude in the world since the early 90’s (Pommereau and Goutail, 1988).

The comparison does show some small mean biases between satellites and SAOZ, as well as between satellites, depending on the satellites observing periods. But most significant is the amplitude of the seasonal variations of the satellite-SAOZ difference varying with the satellite data used varying for example at northern mid-latitude from 2% amplitude with the GODFIT GOME, SCIA and GOME2 data, 3% with SBUV and 4% with IASI. Although the seasonal cycle of stratospheric temperature contributes partly in the case of SBUV measurements in the UV, the largest identified cause of seasonal variation of the satellite-SAOZ difference appears related to a Sun Zenith Angle dependence affecting possibly both satellites and/or ground-based measurements. The origin of this SZA dependence is further explored by looking at the satellite-SAOZ differences at various NDACC stations distributed in latitude.

The evolution of water vapor in the lower stratosphere is still a case for controversy, while its exact trend in response to climate change is important. The GOMOS instrument on board ENVISAT (launched 2002 in a helio-synchronous polar orbit) contains a high resolution spectrometer for the measurement of O₂ at 760 nm (A band) and H₂O at 936 nm, with a spectral resolution of  0.073 nm for O₂ and 0.093 nm for H₂O. GOMOS is the first instrument measuring H₂O by the technique of stellar occultation, which in principle is insensitive to long term drift, an essential feature for long term monitoring of a possible trend. Seven stars are sufficiently bright to provide useful measurements for stratospheric and tropospheric H₂O in the altitude range from 5-10 to 45 km, the lower limit being dictated by the presence of clouds. Over the lifetime of GOMOS, more than 800,000 vertical profiles have been obtained, about 50,000 of them suitable for H₂O.

The H₂O retrieval is severely hampered by a strong intra-pixel CCD non uniformity. Nonetheless, a comparison with other H₂O instruments is undergoing in 2015 in the frame of WAVAS-II efforts (conducted by Gabriella Stiller and Stefan Lossow), while O₂ measurements may be compared to ECMWF re-analysis of pressure-temperature profiles. Some comparisons will be presented, showing that GOMOS H₂O results are at their best in the upper troposphere and lower stratosphere (UTLS), where spectral H₂O features are larger than artifacts due to CCD non-uniformity and vertical resolution of GOMOS higher than other instruments.

A new type of retrieval algorithm is being developed in the frame of an ESRIN contract, the preliminary results of which will be presented. 

Sulfur dioxide (SO₂) and carbonyl sulfide (OCS) are the most important trace gases of the atmospheric sulfur budget. These gases are the main players responsible for the existence and modulation of the stratospheric aerosol layer. For example, an increase of the stratospheric aerosol burden is discussed as contribution to the so-called global warming hiatus, a decrease in the rise of global temperatures since about 1998. Further, it is essential to correctly understand and model the stratospheric sulfur cycle with regard to the proposals of possible artificial climate cooling (so-called geo-engineering) by injection of sulfur into the stratosphere. This can only be achieved by validation of model results with observations.  

Another important aspect about OCS is related to the global carbon cycle. OCS can be used as proxy for the photosynthetic uptake of CO₂ due to its similar diffusion pathways into leaves. But – in contrast to CO2 - this uptake is irreversible. Thus, understanding of global OCS fluxes may contribute to assess gross primary productivity in the biosphere. 

We will give an overview of the highlights of OCS and SO₂ observations from MIPAS/Envisat between 2002 and 2012 in combination with results from global chemistry-transport models. In case of SO2, for the first time a global picture of the vertically resolved distribution of SO₂ between 15 and 45 km altitude has been obtained. It indicates evaporation of H₂SO₄ aerosols above the Junge-layer, conversion to SO2 by photolysis, the poleward transport due to the Brewer-Dobson circulation and the ‘CCN explosion’, i.e. the rapid reformation of aerosol particles over the poles during sunrise in springtime. Further, the distribution of SO₂ in the region of the upper troposphere and lower stratosphere has been investigated gaining an altitude-resolved time-series of SO₂-injections from small and medium-size volcanic eruptions into the stratosphere. The global picture of SO2 in the upper troposphere indicates e.g. elevated levels at the top of the Asian monsoon circulation which might be connected to an aerosol layer recently discovered at similar altitudes.  In the MIPAS dataset of OCS several features related to the stratospheric aerosol layer as well as its tropospheric sources (oceanic release, biomass burning) and sinks (vegetative and soil uptake) are analysed.

This contribution presents analyses of stratospheric methane (CH₄) and nitrous oxide (N₂O) based on assimilation of EOS Aura MLS (v3.3) and MIPAS (IPF v6.0) by the Belgian Assimilation System of Chemical ObsErvations (BASCOE) during the period 2005-2012. Several assimilation aspects will be discussed that intent to improve the analyses and then add value to the observations, namely (1) the use of the Averaging Kernels of the observations, (2) the use, for MIPAS, of the observational error vertical correlation matrix in addition to the standard deviation error given at the tangent point, (3) the removal of the systematic differences between the MLS and MIPAS observations using ACEFTS as anchor and (4) the implementation of CH₄-N₂O correlations in the background error covariance matrix of BASCOE. Analyses will be validated against independent data from ACEFTS and NDACC FTIR observations. In particular, the impact of the relatively poor availability of MIPAS data during the years 2005-2007 on the time consistency of the analyses will be evaluated.

The majority of fluorine in the atmosphere has resulted from the anthropogenic emission of chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), and hydrofluorocarbons (HFCs).  Most tropospheric fluorine is present in its emitted ‘organic’ form due to the molecules having long lifetimes (up to a decade or longer).  Thus they are able to reach the stratosphere where they are broken down, liberating fluorine.  The principal ‘inorganic’ degradation products found in the stratosphere are carbonyl fluoride (COF₂), carbonyl chloride fluoride (COClF), and hydrogen fluoride (HF); of these HF is the most abundant.  In fact at the top of the stratosphere most of the fluorine is present as HF, which, due to its extreme stability, is an almost permanent reservoir of stratospheric fluorine.  The second most abundant stratospheric ‘inorganic’ fluorine reservoir is carbonyl fluoride (COF₂).  Whereas all fluorine-containing species are effectively sources of HF, COF₂ is formed largely from the atmospheric degradation of CFC-12 (CCl₂F₂), which is now banned under the Montreal Protocol, and HCFC-22 (CHF₂Cl), the most abundant HCFC and classed as a transitional substitute under the Montreal Protocol.  All COF₂ ultimately degrades to HF.

The use of satellite remote-sensing techniques allows the measurement of COF₂ and HF atmospheric abundances with impressive global coverage, and the investigation of trends, and seasonal and latitudinal variability.  This work presents global distributions and trends of COF₂ using data from two satellite limb instruments: the Atmospheric Chemistry Experiment Fourier transform spectrometer (ACE-FTS), onboard the SCISAT-1 satellite, which has been recording atmospheric spectra since 2004, and the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) instrument onboard the ENVIronmental SATellite (Envisat), which has recorded thermal emission atmospheric spectra between 2003 and 2012.  Global distributions and trends of HF are derived using data from the ACE-FTS, and the HALogen Occultation Experiment (HALOE) instrument, onboard the Upper Atmosphere Research Satellite (UARS), which recorded atmospheric spectra between 1991 and 2005.  All observations are compared with the output of SLIMCAT, a state-of-the-art three-dimensional chemical transport model (CTM).  The model aids in the interpretation of the satellite observations, and the comparison provides a validation of emission inventories and the atmospheric degradation reaction schemes used in the model.