Conversion Of Carbonyl Sulfide Research Articles

Oxygen vacancies-promoted removal of COS via catalytic hydrolysis over CuTiO2-δ nanoflowers

Catalytic hydrolysis was considered as an efficient technology to remove carbonyl sulfide (COS). The introduction of oxygen vacancy (Ov) is a promising strategy to improve the catalytic performance of COS hydrolysis by promoting the adsorption and activation of reactants. Herein, we reported a Cu-doped TiO2 nanoflower with abundant oxygen vacancies for COS hydrolysis. The Cu species successfully entered the crystal lattice of TiO2 and induced more oxygen vacancies than pure TiO2. The Ov sites can effectively reduce the adsorption and activation energy of COS and H2O. Benefiting from the ample Ov sites, the resulting CuTiO2-δ-F achieved nearly 100 % COS conversion at 70 °C and 93.5 % H2S yield at 130 °C, which is better than that of pure TiO2. Furthermore, in situ FT-IR measurements and density functional theory (DFT) calculations were performed to reveal the reaction pathway in COS hydrolysis.

CuY@NiAl-Layered double oxides bi-functional composites for “Catalytic–Adsorptive” removal of carbonyl sulfide

Carbonyl sulfide (COS) removal processes, typically containing catalytic hydrolysis and subsequent capture of hydrogen sulfide (H2S) generated, are energy-intensive and of high complexity. Bi-functional materials with the capability for synergistic catalytic conversion of COS and subsequent adsorption capture of H2S can enable effective removal of COS at low energy cost. Here, we report the development of a bi-functional CuY@NiAl-LDO composite material with integrated catalytic conversion of COS and adsorptive separation of H2S with high efficiency and stability. Fundamental material characterizations elucidate that the CuY@NiAl-LDO composite materials feature a core–shell structure. Coupled dynamic breakthrough experiments and DFT simulations highlight the unique synergistic “catalytic–adsorptive” effect between CuY and NiAl-LDO in the COS removal processes. Specifically, compared with the pure zeolite CuY and NiAl-LDO, the CuY@NiAl-LDO composites present significantly higher COS removal performance. The highest degree of desulfurization was observed on CuY@NiAl-LDO(46 %) at 80 °C with a breakthrough sulfur capacity of 20.57 mg-S/g and no detectable H2S in the outlet gas. In sum, this study highlights the synergistic “catalytic–adsorptive” mechanism of the bi-functional CuY@NiAl-LDO core–shell composite in desulfurization. The insights obtained may benefit chemical engineers by providing a new strategy for the development of multi-functional desulfurization materials.

Rational design of poisoning-resistant catalyst based on γ-Al2O3 for hydrolysis of carbonyl sulfide

Catalytic hydrolysis of carbonyl sulfide (COS) is an efficient way of desulfurization for industrial flue gas, while the frequently used Al2O3-based catalysts always suffer from sulfur poisoning leading to poor stability, especially in the presence of O2. With this regard, an analysis of the reaction pathway has been conducted, and the tri-coordinated Al in γ-Al2O3 is identified as the responsible site for poisoning, on which produced H2S is over-oxidized resulting in active sites covering. Density functional theory (DFT) has been adopted for dopant screening to eliminate tri-coordinated Al on γ-Al2O3, inhibit H2S over-oxidation, and prevent sulfur poisoning. Results show La, Fe, Ni, and Cu all can eliminate tri-coordinated Al, while only La-doped γ-Al2O3 exhibits inhibition to H2S oxidation, thus, improving activity and stability for γ-Al2O3. Experiments show COS conversion of 99 % over 30 h can be attained in the presence of O2, which agrees well with the DFT calculation.

Performance boosting by induction heating for hydrolysis of carbonyl sulfide using magnetic ordered mesoporous alumina as catalyst

Hydrolysis of carbonyl sulfide (COS) has been recognized as an efficient way for COS removal, however, searching for an effective way of minimizing the catalyst deactivation is always the most important task to realize its industrial application. In this study, a high-performance alkali-free catalyst of magnetic ordered mesoporous Al2O3 has been synthesized for the COS hydrolysis. At the same time, induction heating has been introduced as a novel heating method to boost the performance of the catalyst. Results show that the inducting heating can boost the performance of COS hydrolysis, as well as minimize the catalyst deactivation compared to the joule heating under the same reaction conditions. The reaction performance under induction heating mode, in terms of COS conversion, is at least 7.0% on average higher than the joule heating mode. This is attributed to precise control of the localized temperature on active sites since the heat is generated directly from the catalyst under induction heating mode. While in the joule heating mode, the heat should be transferred to the catalyst from environments, which induces a huge temperature gradient. This study implies that induction heating can be used as a unique way of minimizing the side reactions over the gas-solid heterogeneous catalytic processes and boosting the catalyst performance.

A comparison between Claus and THIOPAQ sulfur recovery techniques in natural gas plants

A sulfur recovery process is one of the most important processes in the oil and gas industry to get rid of hydrogen sulfide (H2S) which is produced from the acid gas removal process of sour natural gas to convert it into sweet natural gas. Actual data from a gas field is used to obtain a realistic comparison between two sulfur recovery techniques, through which researchers and/or manufacturers can obtain information that will help them choose the most appropriate and cheapest method. A total feed acid gas flow rate of 5.1844 MMSCFD with an H2S concentration of 24.62% by mole percent was produced from amine acid gas removal units. Claus sulfur recovery technique is a traditional chemical process that uses thermal and catalytic reactors. Therefore, an acid gas enrichment unit is applied to increase the H2S concentration to approximately 50% mole to provide reliable and flexible operation in the thermal and catalytic reactors. Moreover, a tail gas treatment unit is applied to increase the overall conversion efficiency to 99.90% with the Claus technique instead of 95.08% without it to achieve high sulfur recovery and reliable operation through the conversion of carbonyl sulfide (COS) and mercaptans. Studies on the safety and simplicity of the Claus technique revealed many important hazards and a large number of transmitters (379) and control loops (128) in one Claus train. THIOPAQ sulfur recovery as a new technology is a biological desulfurization process that uses a natural mixture of sulfide-oxidizing bacteria. It is also a unique H2S removal process with an efficiency of 99.999%. In addition, studies on the safety and simplicity of the THIOPAQ technique have shown that the hazards, the number of transmitters (74), and the number of control loops (29) of a one THIOPAQ train are lower. The THIOPAQ technique showed higher efficiency, was safer, simpler, and had lower CAPEX and OPEX. This study was conducted using Aspen HYSYS V11 and actual data.

Enhancing Kinetics of Carbonyl Sulfide Hydrolysis Using Pt-Supported Al2O3 Catalysts: First-Principles-Informed Energetic Span Analysis

Catalytic hydrolysis is considered an effective strategy for treating carbonyl sulfide (COS)─a toxic sulfur-containing gas that causes problems to the environment and petrochemical industries. Al2O3-based materials are commonly used as catalysts for COS hydrolysis owing to their stability and cost effectiveness. However, they still suffer from sulfur poisoning leading to partial COS conversion after long time use. To improve their catalytic performances, herein, we computationally designed and studied the catalytic activity of the Pt-supported Al2O3 catalysts by means of density functional calculations. We mechanistically explored the COS hydrolysis on both bare and Pt-decorated surfaces to reveal the role of Pt catalysts in the reaction kinetics. We find that bare Al2O3 suffers from difficult C–S bond breaking as its barrier is relatively high. Pt facilitates C–S bond breaking where its barrier is reduced by more than half (1.34 to 0.60 eV). However, the Pt–Al2O3 catalyst could encounter sulfur poisoning as the sulfur-containing intermediates are rather stable. Such issues could be remedied by increasing the operating temperature to destabilize the intermediates and promote product desorption. The energetic span models reveal that the important states of bare and Pt–Al2O3 are indeed C–S bond breaking and product desorption corresponding to an energy span of 2.72 and 1.67 eV at 773 K, respectively─suggesting that Pt dramatically enhances the catalytic activity of Al2O3-based catalysts toward COS hydrolysis. The suggested operating temperatures are above 673 K to avoid sulfur poisoning. Our findings will be useful for the development of more efficient Al2O3-based catalysts for treating COS.

Contribution of Na/K Doping to the Activity and Mechanism of Low-Temperature COS Hydrolysis over TiO2-Al2O3 Based Catalyst in Blast Furnace Gas

As an organic sulfur pollutant generated in blast furnace gas, carbonyl sulfide (COS) has attracted more attention due to its negative effects on the environment and economy. The TiO2-Al2O3 composite metal oxide (Ti0.5Al) with uniformly dispersed particles was prepared by the co-precipitation method. And on this basis, a series of Na/K-doped catalysts were prepared separately. The activity evaluation results showed that the introduction of Na/K significantly improved the low-temperature COS hydrolysis activity, which exhibited a COS conversion of 98% and H2S yield of 95% at 75 °C with 24,000 h–1. And K showed a better promoting effect than Na. Brunauer–Emmett–Teller (BET) results revealed the increased mesopore proportion of Na/K-modified catalysts. X-ray diffraction (XRD) and scanning electron microscopy (SEM) showed that Na and K formed prismatic and nanorod-like structures, respectively. More weakly basic sites with enhanced intensity and decreased Oads/Olat content contributed to the excellent catalytic activity, as certified by the results of CO2 temperature-programmed desorption (CO2-TPD) and X-ray photoelectron spectroscopy (XPS). It was also proposed that the decrease of weakly basic sites ultimately deactivated catalyst activity. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) showed that the introduction of Na/K enhanced the dissociation of H2O, and the generated abundant hydroxyl groups promoted the adsorption of COS and formed surface transition species, such as HSCO2– and HCO3–.

Selection Mechanism of Synergistic Effect between Dielectric Barrier Discharge Plasma and Cu/Γ-Al2o3 Catalyst in Carbonyl-Sulfide Conversion: Electric Field Changes Energy Barrier and Reaction Path

Selection Mechanism of Synergistic Effect between Dielectric Barrier Discharge Plasma and Cu/Γ-Al2o3 Catalyst in Carbonyl-Sulfide Conversion: Electric Field Changes Energy Barrier and Reaction Path

Selection Mechanism of Synergistic Effect between Dielectric Barrier Discharge Plasma and Cu/Γ-Al2o3 Catalyst in Carbonyl-Sulfide Conversion: Electric Field Changes Energy Barrier and Reaction Path

Selection Mechanism of Synergistic Effect between Dielectric Barrier Discharge Plasma and Cu/Γ-Al2o3 Catalyst in Carbonyl-Sulfide Conversion: Electric Field Changes Energy Barrier and Reaction Path

Catalysts of Ordered Mesoporous Alumina with a Large Pore Size for Low-Temperature Hydrolysis of Carbonyl Sulfide

Carbonyl sulfide (COS) generated from various chemical and metallurgical industry sectors has been receiving considerable attention due to its environmental impact and threat to production chains. The catalytic hydrolysis of COS has been considered an economic way for converting the unremovable COS into H₂S that subsequently can be easily removed through the mature wet or dry desulfurization process. Searching for an effective catalyst with stable activity is always crucial for COS hydrolysis. In this study, alkali-free ordered mesoporous Al₂O₃ with a larger pore size, incorporated with Ni, Cu, or Fe as an additive, and synthesized via a one-pot approach has been investigated for the catalytic hydrolysis of COS at low temperatures. It has been demonstrated that alumina with ordered mesoporous structures exhibits significant catalytic activity over COS hydrolysis whose activity improved further when incorporated with transition metals. More importantly, the enlargement of mesopore size for all the catalysts is critical for maintaining their catalytic performance. By optimizing the pore size and the type of additive, a highly active and stable catalyst of Ni-containing mesoporous Al₂O₃ with a large pore size has been established, which exhibits a long-term COS conversion above 95% and strong sulfur resistance. With the help of various characterization techniques, the details of the hydrolysis reaction, as well as the sulfur poisoning mechanism, have been revealed. It is envisaged that the synthesized catalyst of alkali-free ordered mesoporous Al₂O₃ holds great application potential for the catalytic hydrolysis of COS in practical industrial applications.

Use of Dithiasuccinoyl-Caged Amines Enables COS/H2 S Release Lacking Electrophilic Byproducts.

The enzymatic conversion of carbonyl sulfide (COS) to hydrogen sulfide (H2 S) by carbonic anhydrase has been used to develop self-immolating thiocarbamates as COS-based H2 S donors to further elucidate the impact of reactive sulfur species in biology. The high modularity of this approach has provided a library of COS-based H2 S donors that can be activated by specific stimuli. A common limitation, however, is that many such donors result in the formation of an electrophilic quinone methide byproduct during donor activation. As a mild alternative, we demonstrate here that dithiasuccinoyl groups can function as COS/H2 S donor motifs, and that these groups release two equivalents of COS/H2 S and uncage an amine payload under physiologically relevant conditions. Additionally, we demonstrate that COS/H2 S release from this donor motif can be altered by electronic modulation and alkyl substitution. These insights are further supported by DFT investigations, which reveal that aryl and alkyl thiocarbamates release COS with significantly different activation energies.

Hydrolysis of carbonyl sulfide over modified Al2O3 by platinum and barium in a packed-bed reactor

The integrated gasification combined cycle (IGCC) has been applied as an alternative technology for power plant due to the higher thermal efficiency than that of conventional pulverized coal-fired technology. However, syngas produced from coal gasification in IGCC is usually contaminated with carbonyl sulfide (COS). Therefore, it is necessary to be treated before being utilized in gas turbine or chemicals production. In this paper, catalytic treatment of COS by hydrolysis over alumina-supported platinum (Pt/Al2O3) catalysts was investigated. COS was hydrolyzed and converted to hydrogen sulfide (H2S) in a packed-bed reactor under flue-gas in a temperature range of 150–250 °C, and Weight Hourly Space Velocity (WHSV) of 7,000 hr−1. The optimal Pt/Al2O3 catalysts were determined by varying active Pt metal (0.1–1.0 wt%) loadings and barium promoter (1–5 wt%) loadings. The prepared catalysts were characterized by X-ray diffraction (XRD), N2 adsorption and desorption, and Scanning Electron Microscopy with Energy Dispersive X-ray spectroscopy (SEM/EDX). It was found that the activity of Al2O3 drastically dropped from 90 to 10% after 510 min time on stream. The addition of Pt and Ba onto Al2O3 could stabilize the catalytic activity. The 0.5%Pt/5.0%Ba/Al2O3 catalyst provided the best performance in COS hydrolysis. The complete COS conversion was achieved with 0.5%Pt/5.0%Ba/Al2O3 in all initial concentrations, and the lifespan of the catalyst was prolonged. A kinetic model of this reaction was also purposed. Sulfur poisoning and carbon deposition on the catalyst surface caused the catalyst deactivation.

A sustainable design of ZnO-based adsorbent for robust H2S uptake and secondary utilization as hydrogenation catalyst

Low sulfur capacity and low regeneration performance are the long-time problems in the industrial application of ZnO-based adsorbents for ambient H2S removal. However, no efficient approach has been reported up to now to simultaneously solve these two problems. With the increasingly stringent environmental regulations in China, a robust but sustainable ZnO-based adsorbent is strongly desired. Herein, we constructed such an adsorbent for the first time by introducing cobalt species into ZnO/SiO2 to form a heterostructured ZnO-Co3O4/SiO2 nanocomposite. For comparison, Co doped ZnO/SiO2 and Zn doped Co3O4/SiO2 were also fabricated. According to the breakthrough tests with initial H2S partial pressure of 57 Pa, the heterostructured ZnO-Co3O4/SiO2 adsorbent exhibited highest sulfur capacity of 188.4 mg S/g sorbent at breakthrough level of 1%, the corresponded partition coefficient (at 0.57 Pa, 303 K) was 10.3 mol. kg−1. Pa−1. The introduced Co species not only improved the dispersion of ZnO nanograins, but also served as active sites for reacting and oxidizing H2S molecular. It also found that the formed ZnO-Co3O4 heterostructure could boost the hydroxylation of H2O, which provided an alkali environment for H2S dissociation. More meaningfully, this exhausted sorbent could be secondarily utilized as catalyst for carbonyl sulfide (COS) hydrogenation and exhibited 100% COS conversion even at a temperature lower than 200 °C, which was almost 3.3 times the conversion achieved by a commercial Co-Mo/Al2O3 catalyst (30%). This study will provide a new idea for rational design of metal oxide-based adsorbents to avoid causing secondary pollution by the spend adsorbents.

Lifetime of Alumina Catalysts in the Claus Reactor during Sulfur Removal from Coke-Oven Gas

To assess the deactivation of Claus catalysts in production conditions, monitoring data for two alumina catalysts in the Claus reactor during sulfur removal from coke-oven gas at PAO Magnitogorskii Metallurgicheskii Kombinat (MMK) are analyzed, when the mean temperature (input/output) is tcat = 251°C and 254°C and the bulk flow rate is W = 800 h–1 and 1100 h–1. Statistical analysis of the rate constants $$K_{{{\text{COS}}}}^{*}$$ of carbonyl-sulfide conversion over three years shows that, in both catalysts, aging over an operational period between a month and two years corresponds to irreversible exponential loss of activity: $$K_{{{\text{COS}}}}^{*}$$ ~ exp(–λt). The parameter λ is constant over time t; that is typical of hydrothermal aging. On that basis, the activity of alumina catalysts in the Claus reactor may be predicted during their basic operating period, which is of most practical interest. The rapid deactivation of the alumina catalysts in the hydrolysis of carbonyl sulfide, which is dominant at first, is also characterized by an exponential time dependence. However, the period of rapid catalyst deactivation is limited to the first two or three weeks of operation on account of the large value of λ.

Tuning the growth of Cu-MOFs for efficient catalytic hydrolysis of carbonyl sulfide

Development of the high activity, promoter-free catalysts for carbonyl sulfide (COS) hydrolysis is important for the efficient utilization of various feedstocks. In this study, the Cu-based metal-organic framework HKUST-1 is synthesized by a simple and mild anodic-dissolution electrochemical method. The physical and chemical properties of the samples are characterized by several techniques, including scanning electron microscopy, X-ray diffraction, Brunauer-Emmett-Teller analysis and X-ray photoelectron spectroscopy. The results reveal that the synthesis voltage plays a crucial role in controlling the morphology of the resulting HKUST-1. The obtained samples function as novel catalysts for the hydrolysis of COS. A high efficiency, approaching 100%, can be achieved for the conversion of COS at 150 °C over the optimal HKUST-1 synthesized at 25 V. This is significantly higher than that of the sample prepared by the traditional hydrothermal method. Additionally, the effects of the water temperature and the flow velocity on the hydrolysis of COS are also investigated in detail. Finally, a possible reaction pathway of COS hydrolysis over HKUST-1 is also proposed. This work represents the first example of MOFs applied to the catalytic hydrolysis of COS. The results presented in this study can be anticipated to give a feasible impetus to design novel catalysts for removing the sulfur-containing compounds.

Synthesis of metallic Ni-Co/graphene catalysts with enhanced hydrodesulfurization activity via a low-temperature plasma approach

Graphene sheets (GS) supported monometal (Ni, Co) or bimetal (Ni-Co) nanoparticle composites have been conveniently prepared with the assistance of dielectric barrier discharge (DBD) plasma at low temperature. Both graphene oxide and the metal ions (Ni2+, Co2+) can be simultaneously reduced during the DBD plasma treatment in H2 atmosphere, which gives rise to the uniformly distributed metal nanoparticles on the surface of GS. The graphene-based catalysts are used in the catalytic hydrodesulfurization (HDS) of carbonyl sulfide (COS). It is revealed that the bimetallic Ni-Co/GS catalyst exhibits outstanding performance for higher COS conversion compared with the monometal catalysts, suggesting the synergetic effect between Ni and Co active species in the HDS reaction. The presented strategy demonstrates a new pathway for the preparation of graphene-based catalysts with high HDS catalytic performance.

CS2 and COS conversion under different combustion conditions

Carbon disulfide (CS2) and carbonyl sulfide (COS) can be generated from sulfur-containing species in a combustion chamber, and thus may be present at the exhaust gas and even emitted to the atmosphere. Therefore, it is of great interest to study and understand the mechanism through which the conversion of CS2 and COS takes place under combustion conditions, both to develop strategies to control these emissions and, consequently, to preserve the environment.In this context, a combined experimental and modeling study addressing the conversion of CS2 and COS under different combustion conditions has been undertaken. The developed kinetic mechanism has been validated against experimental data obtained in the present work of the moist conversion of CS2 and COS in a laboratory quartz flow reactor. Experiments were performed at atmospheric pressure, in the 300–1400K temperature range, and under different combustion environments, ranging from fuel rich to fuel lean conditions, specifically air excess ratios of λ=0.2, 0.7, 1, 2 and 20. The experimental results have been analyzed and interpreted in terms of this detailed gas phase kinetic mechanism, and the elementary steps involving CS2 and COS conversion as a function of the different operating conditions have been identified.Results indicate that the oxidation of both, CS2 and COS, is produced by interaction with the radical pool, mainly with O radicals, and to a lesser extent with S radicals, being this last consumption step more relevant as the reaction environment becomes fuel richer. The CS2 conversion involves a more complex process compared to COS, since COS is an intermediate product during the oxidation of CS2 and thus its conversion process also involves the COS chemistry.

A novel method of monitoring the sulfidation of hydrotreating catalysts: the conversion of carbonyl sulfide

The conversion of COS during the sulfidation of HDS catalysts allows one to determine the temperature range of formation of CoMoS sites.

Fabrication of three-dimensional MoS2-graphene hybrid monoliths and their catalytic performance for hydrodesulfurization

A simple process to synthesize three-dimensional molybdenum disulfide-graphene monolithic catalysts (3D-MoS2-G) was developed. 3D-MoS2-G hybrid monoliths were fabricated by a combined hydrothermal self-assembly and freeze-drying treatment, in which ammonium tetrathiomolybdate and graphite oxide were used as starting materials. The structure and morphology of the samples were characterized by X-ray diffraction, Fourier transform infrared spectrometry, Raman spectrometry, field emission scanning electron microscopy, transmission electron microscopy and nitrogen adsorption. The catalytic performance of the hybrid monoliths was investigated by evaluating the activity for the hydrodesulfurization (HDS) of carbonyl sulfide (COS). In addition, the influence of microwave irradiation on the catalytic property of the 3D-MoS2-G monoliths was also investigated. It is demonstrated that after microwave irradiation the 3D-MoS2-G monoliths show an excellent activity for COS hydrogenation compared with the traditional MoS2/γ-Al2O3 catalyst prepared by impregnation-sulfidation. The 3D-MoS2-G-160 M monolith hydrothermally synthesized at 160 °C exhibits the highest COS conversion of 100% at a relatively low temperature (260 °C) for the HDS reaction. The superior performance of the 3D-MoS2-G-160 M catalyst can be ascribed to the unique hybridized structure of the MoS2 nanoparticles uniformly dispersed on graphene sheets in the monolith. [New Carbon Materials 2014, 29(2): 81–88]

Sulfur dioxide (SO2) as observed by MIPAS/Envisat: temporal development and spatial distribution at 15–45 km altitude

Abstract. We present a climatology of monthly and 10° zonal mean profiles of sulfur dioxide (SO2) volume mixing ratios (vmr) derived from MIPAS/Envisat measurements in the altitude range 15–45 km from July 2002 until April 2012. The vertical resolution varies from 3.5–4 km in the lower stratosphere up to 6–10 km at the upper end of the profiles, with estimated total errors of 5–20 pptv for single profiles of SO2. Comparisons with the few available observations of SO2 up to high altitudes from ATMOS for a volcanically perturbed situation from ACE-FTS and, at the lowest altitudes, with stratospheric in situ observations reveal general consistency of the datasets. The observations are the first empirical confirmation of features of the stratospheric SO2 distribution, which have only been shown by models up to now: (1) the local maximum of SO2 at around 25–30 km altitude, which is explained by the conversion of carbonyl sulfide (COS) as the precursor of the Junge layer; and (2) the downwelling of SO2-rich air to altitudes of 25–30 km at high latitudes during winter and its subsequent depletion on availability of sunlight. This has been proposed as the reason for the sudden appearance of enhanced concentrations of condensation nuclei during Arctic and Antarctic spring. Further, the strong increase of SO2 to values of 80–100 pptv in the upper stratosphere through photolysis of H2SO4 has been confirmed. Lower stratospheric variability of SO2 could mainly be explained by volcanic activity, and no hints of a strong anthropogenic influence have been found. Regression analysis revealed a QBO (quasi-biennial oscillation) signal of the SO2 time series in the tropics at about 30–35 km, an SAO (semi-annual oscillation) signal at tropical and subtropical latitudes above 32 km and annual periodics predominantly at high latitudes. Further, the analysis indicates a correlation with the solar cycle in the tropics and southern subtropics above 30 km. Significant negative linear trends are found in the tropical lower stratosphere, probably due to reduced tropical volcanic activity and at southern mid-latitudes above 35 km. A positive trend is visible in the lower and middle stratosphere at polar to subtropical southern latitudes.