Multi-model estimates of atmospheric lifetimes of long-lived ozone-depleting substances: present and future

We have diagnosed the lifetimes of long-lived source gases emitted at the surface and removed in the stratosphere using six three-dimensional chemistry-climate models and a two-dimensional model. The models all used the same standard photochemical data. We investigate the effect of different definitions of lifetimes, including running the models with both mixing ratio (MBC) and flux (FBC) boundary conditions. Within the same model, the lifetimes diagnosed by different methods agree very well. Using FBCs versus MBCs leads to a different tracer burden as the implied lifetime contained in the MBC value does not necessarily match a model's own calculated lifetime. In general, there are much larger differences in the lifetimes calculated by different models, the main causes of which are variations in the modeled rates of ascent and horizontal mixing in the tropical midlower stratosphere. The model runs have been used to compute instantaneous and steady state lifetimes. For chlorofluorocarbons (CFCs) their atmospheric distribution was far from steady state in their growth phase through to the 1980s, and the diagnosed instantaneous lifetime is accordingly much longer. Following the cessation of emissions, the resulting decay of CFCs is much closer to steady state. For 2100 conditions the model circulation speeds generally increase, but a thicker ozone layer due to recovery and climate change reduces photolysis rates. These effects compensate so the net impact on modeled lifetimes is small. For future assessments of stratospheric ozone, use of FBCs would allow a consistent balance between rate of CFC removal and model circulation rate

Renewed and emerging concerns over the production and emission of ozone-depleting substances

Stratospheric ozone depletion, first observed in the 1980s, has been caused by the increased production and use of substances such as chlorofluorocarbons (CFCs), halons and other chlorine-containing and bromine-containing compounds, collectively termed ozone-depleting substances (ODSs). Following controls on the production of major, long-lived ODSs by the Montreal Protocol, the ozone layer is now showing initial signs of recovery and is anticipated to return to pre-depletion levels in the mid-to-late twenty-first century, likely 2050–2060. These return dates assume widespread compliance with the Montreal Protocol and, thereby, continued reductions in ODS emissions. However, recent observations reveal increasing emissions of some controlled (for example, CFC-11, as in eastern China) and uncontrolled substances (for example, very short-lived substances (VSLSs)). Indeed, the emissions of a number of uncontrolled VSLSs are adding significant amounts of ozone-depleting chlorine to the atmosphere. In this Review, we discuss recent emissions of both long-lived ODSs and halogenated VSLSs, and how these might lead to a delay in ozone recovery. Continued improvements in observational tools and modelling approaches are needed to assess these emerging challenges to a timely recovery of the ozone layer

How Atmospheric Chemistry and Transport Drive Surface Variability of N2O and CFC-11

Nitrous oxide (N2O) is a long-lived greenhouse gas that affects atmospheric chemistry and climate. In this work, we use satellite measurements of N2O, ozone (O3), and temperature from the Aura Microwave Limb Sounder (MLS) instrument to calculate stratospheric loss of N2O, and thus its atmospheric lifetime. Using three chemistry transport models simulating the Aura period 2005–2017, we verify the stratospheric sink using MLS data and follow that loss signal down to the surface and compare with surface observations. Stratospheric loss has a strong seasonal cycle and is further modulated by the Quasi-Biennial Oscillation (QBO); these cycles are seen equally in both observations and the models. When filtered for interannual variability, the modeled surface signal is QBO-caused, and it reproduces the observed pattern, highlighting the potential role of the QBO in tropospheric chemistry and composition, as well as in model evaluation. The observed annual surface signal in the northern hemisphere matches well with the models run without emissions, indicating the annual cycle is driven mostly by stratosphere-troposphere exchange (STE) flux of N2O-depleted air and not surface N2O emissions. In the southern hemisphere (SH), all three models disagree and thus provide no guidance, except for indicating that modeling annual STE in the SH remains a major model uncertainty. Parallel model simulations of CFCl3, which has greater stratospheric loss that N2O and possibly surreptitious emissions, show that its interannual variations parallel those of N2O, and thus the observed N2O variability can identify the stratospheric component of the observed CFCl3 variability

Choosing meteorological input for the global modeling initiative assessment of high-speed aircraft

The global modeling initiative (GMI) science team is developing a three-dimensional chemistry and transport model (CTM) for use in assessment of the atmospheric effects of aviation. This model must be documented, be validated against observations, use a realistic atmospheric circulation, and contain numerical transport and photochemical modules representing atmospheric processes. The model must retain computational efficiency for multiple scenarios and sensitivity studies. To meet these requirements, a facility model concept was developed in which the different components of the CTM are evaluated separately. The assessment of the impact on the stratosphere of the exhaust of supersonic aircraft will depend strongly on the meteorological fields used by the CTM. Three data sets for the stratosphere were considered: the National Center for Atmospheric Research Community Climate Model (CCM2), the Goddard Earth Observing System data assimilation system, and the Goddard Institute for Space Studies general circulation model. Objective criteria were developed to identify the data set that provides the best representation of the stratosphere. Simulations of gases with simple chemical control were chosen to test various aspects of model transport. The data sets were evaluated and graded on their performance on these tests. The CCM2 meteorological data set has the highest score and was selected for GMI. This objective model evaluation establishes a physical basis for interpretation of differences between models and observations. Further, the method provides a quantitative basis for defining model errors, for discriminating between different models, and for ready reevaluation of improved models. This will lead to higher confidence in assessment calculations. Copyright 1999 by the American Geophysical Union

Understanding how adherence goals promote adherence behaviours: a repeated measure observational study with HIV seropositive patients

Background The extent to which patients follow treatments as prescribed is pivotal to treatment success. An exceptionally high level (> 95 %) of HIV medication adherence is required to suppress viral replication and protect the immune system and a similarly high level (> 80 %) of adherence has also been suggested in order to benefit from prescribed exercise programmes. However, in clinical practice, adherence to both often falls below the desirable level. This project aims to investigate a wide range of psychological and personality factors that may lead to adherence/non-adherence to medical treatment and exercise programmes. Methods HIV positive patients who are referred to the physiotherapist-led 10-week exercise programme as part of the standard care are continuously recruited. Data on social cognitive variables (attitude, intention, subjective norms, self-efficacy, and outcome beliefs) about the goal and specific behaviours, selected personality factors, perceived quality of life, physical activity, self-reported adherence and physical assessment are collected at baseline, at the end of the exercise programme and again 3 months later. The project incorporates objective measures of both exercise (attendance log and improvement in physical measures such as improved fitness level, weight loss, improved circumferential anthropometric measures) and medication adherence (verified by non-invasive hair analysis). Discussion The novelty of this project comes from two key aspects, complemented with objective information on exercise and medication adherence. The project assesses beliefs about both the underlying goal such as following prescribed treatment; and about the specific behaviours such as undertaking the exercise or taking the medication, using both implicit and explicit assessments of patients' beliefs and attitudes. We predict that i) the way people think about the underlying goal of their treatments explains medication and exercise behaviours over and above the effects of the behaviour-specific thinking and ii) the relationship between adherence to exercise and to medical treatment is stronger among those with more favourable views about the goal. Results from this study should identify the key contributing factors to inform subsequent adherence research and afford a more streamlined assessment matrix. The project also aims to inform patient care practices. UK Clinical Research Network registration number: UKCRN 7842