Evaluation of the transport and chemistry of climate-relevant species in the lower stratosphere

This thesis investigates aspects of the chemistry and transport of the upper troposphere and lower stratosphere (UTLS), with a particular focus on the Asian Summer Monsoon (ASM). The overall aims have been pursued through simulations of the TOMCAT three-dimensional (3-D) chemical transport model in comparison with aircraft, balloon and satellite observations. Scientific motivation for this work has been provided by the EU StratoClim project which conducted flight campaigns in Greece (2016) and Nepal (2017). Simulations of the transport of chemically active tracers to the UT depend critically on the treatment of convection. In this work I have tested and further developed an improvement to the existing TOMCAT model by using a convection scheme based on mass fluxes from archived meteorological analyses. This leads to more rapid uplift of chemical tracers, which is most apparent for those with short lifetimes (e.g. around 5 days). Both the old and new convection schemes have been evaluated against observations. The model has then been used to quantify the transport associated with the Asian Summer Monsoon (ASM) circulation, focusing on the interannual variability using decadal simulations forced by ERA-Interim reanalysis. The role of large-scale ascent versus convective transport has been investigated, along with the link between the interannual variability of the transport of surface-emitted CO to the UT to the strength of the ASM. Model intercomparisons of tropospheric age-of-air when the old (Tiedtke) convection scheme is applied, shows weak transport, in particular at UTLS levels, when compared with other state-of-the-art 3-D models. In contrast the new (archived mass flux) scheme shows faster and stronger transport reflected in a younger age-of-air in the UT. A multidecadal (1989-2017) simulation with idealized tracers show that the alternative convection schemes vastly impact the related confinement of such tracers in the ASM anticyclonic structure at 100 hPa. However, connecting this confinement with common metrics of the dynamical strength of the ASM circulation is not straightforward and does not lead to conclusive results over the time period modelled. The main chemical observations so far available from the StratoClim campaign are water vapour and CO. Comparison between the in-situ water data from the StratoClim and the ERA-Interim values confirms a negative bias in UTLS in the reanalyses over the Indian Subcontinent region. A full chemistry model simulation is able to capture the observed magnitude and variability of the observed CO well. Analysis of daily model output reveals an interesting tri-modal pattern of elevated CO in the ASM region, which is strongly dependent on convection over the Tibetan Plateau but not entirely due to it. Injection of brominated species into the stratosphere has been investigated using observations from the more extensive American 2013/14 Airborne Tropical Tropopause Experiment (ATTREX) aircraft campaign in the Eastern Pacific. The model simulations with the new convective scheme agree well with UTLS observations of CHBr3, CH3Br, CH2Br2 and H-1211, confirming the injection of around 6 ppt bromine derived from very short-lived substances (VSLS) into the stratosphere. However, comparisons of observed and modeled BrO shows that this cannot account in all cases for the amount of inorganic bromine observed in the lower stratosphere, suggesting direct injection of significant levels (a few ppt) of inorganic bromine into the stratosphere in the Tropics. Finally, I have investigated the impact of artificial injection of particles into the stratosphere – so-called geoengineering through solar radiation management to counteract climate change. I have assessed the possible impact of the underexplored particulate mineral substance, TiO2, on stratospheric ozone through enhanced heterogeneous chemistry. Model simulations, based on loadings causing a similar climate impact to the Mt Pinatubo eruption, show the injection of TiO2 particles in the stratosphere likely has only a small impact on present-day ozone concentrations (decrease of up to 0.06%). With further assumptions about the possible role of TiO2 on chlorine heterogeneous chemistry, a model simulation to 2049 with recurrent large Pinatubo-like volcanic eruptions shows that the impact with declining stratospheric chlorine loading is not more than a -2.5% change in ozone.