New threats to the ozone layer: exploring ozone-depleting substances in the troposphere and stratosphere.

The release into the atmosphere of trace gases that contain chlorine and bromine can significantly impact stratospheric ozone depletion. The production and consumption of many ozone-depleting substances has been phased out under an international agreement called the Montreal Protocol on Substances that Deplete the Ozone Layer. Further monitoring is required to ensure the overall abundance of ozone-depleting substances continues to decrease in the atmosphere. This thesis focuses on East Asia which is a region where there can be rapid transport of ozone-depleting substances into the stratosphere and where continuing emissions of many ozone-depleting substances have been reported in recent years. In this thesis, a variety of halogenated trace gases were measured in air samples collected at ground level measurement sites and during aircraft campaigns via a gas chromatograph mass spectrometer system (GC-MS) to investigate new threats to the ozone layer. As part of the StratoClim project, a number of halogenated trace gases were analysed in air samples collected on board the Geophysica high altitude research aircraft over the Mediterranean, Nepal and northern India during the summers of 2016 and 2017 to investigate the composition of the Asian summer monsoon in the upper troposphere and lower stratosphere. Chlorinated very short-lived ozone-depleting substances (VSLSs) were found to be enhanced above mixing ratios measured in the tropical tropopause layer in 2013-2014. This indicates that large emissions of chlorinated VSLSs in East and South Asia can be rapidly transported by the Asian monsoon into the lower stratosphere, before they have broken down to insignificant levels, and therefore they can contribute to ozone depletion. Chlorinated VSLSs contribute only a small fraction of the total amount of chlorine in the tropopause region and the lower stratosphere. However, their contribution could delay the long-term recovery of the ozone layer. This thesis also presents updated long-term trends and global annual emissions of CFC-113a (CCl3CF3). The mixing ratios of CFC-113a are still increasing substantially and global atmospheric emissions of CFC-113a remained at about 1.7 Gg yr-1 between 2012 and 2016 after an increase in emissions in 2010−2012. Complementary ground-based observations in Taiwan suggest the presence of persistent emissions of CFC-113a in East Asia. The emissions are relatively small and it is possible they are due to the few remaining allowed uses of CFC-113a. Furthermore, atmospheric observations of multiple halogenated trace gases in Taiwan used with backward trajectory modelling found that CFC-11 (CCl3F) emissions coming from eastern China had increased from 12 (10-14) Gg yr-1 in 2008−2011 to 19 (14−23) Gg yr-1 in 2014−2018. This is about one-quarter of global emissions and the increase contributed to the recently discovered global increase in CFC-11 emissions. These results independently support the findings of other recent studies. The increase in emissions is possibly due to illegal production of CFC-11

Trends and emissions of six perfluorocarbons in the Northern Hemisphere and Southern Hemisphere

Perfluorocarbons (PFCs) are potent greenhouse gases with global warming potentials up to several thousand times greater than CO2 on a 100-year time horizon. The lack of any significant sinks for PFCs means that they have long atmospheric lifetimes of the order of thousands of years. Anthropogenic production is thought to be the only source for most PFCs. Here we report an update on the global atmospheric abundances of the following PFCs, most of which have for the first time been analytically separated according to their isomers: c-octafluorobutane (c-C4F8), n-decafluorobutane (n-C4F10), n-dodecafluoropentane (n-C5F12), n-tetradecafluorohexane (n-C6F14), and n-hexadecafluoroheptane (n-C7F16). Additionally, we report the first data set on the atmospheric mixing ratios of perfluoro-2-methylpentane (i-C6F14). The existence and significance of PFC isomers have not been reported before, due to the analytical challenges of separating them. The time series spans a period from 1978 to the present. Several data sets are used to investigate temporal and spatial trends of these PFCs: time series of air samples collected at Cape Grim, Australia, from 1978 to the start of 2018; a time series of air samples collected between July 2015 and April 2017 at Tacolneston, UK; and intensive campaign-based sampling collections from Taiwan. Although the remote “background” Southern Hemispheric Cape Grim time series indicates that recent growth rates of most of these PFCs are lower than in the 1990s, we continue to see significantly increasing mixing ratios that are between 6 % and 27 % higher by the end of 2017 compared to abundances measured in 2010. Air samples from Tacolneston show a positive offset in PFC mixing ratios compared to the Southern Hemisphere baseline. The highest mixing ratios and variability are seen in air samples from Taiwan, which is therefore likely situated much closer to PFC sources, confirming predominantly Northern Hemispheric emissions for most PFCs. Even though these PFCs occur in the atmosphere at levels of parts per trillion molar or less, their total cumulative global emissions translate into 833 million metric tonnes of CO2 equivalent by the end of 2017, 23 % of which has been emitted since 2010. Almost two-thirds of the CO2 equivalent emissions within the last decade are attributable to c-C4F8, which currently also has the highest emission rates that continue to grow. Sources of all PFCs covered in this work remain poorly constrained and reported emissions in global databases do not account for the abundances found in the atmosphere

Investigation of East Asian emissions of CFC-11 using atmospheric observations in Taiwan

Recent findings of an unexpected slowdown in the decline of CFC-11 mixing ratios in the atmosphere have led to the conclusion that global CFC-11 emissions have increased over the past decade and have been attributed in part to eastern China. This study independently assesses these findings by evaluating enhancements of CFC-11 mixing ratios in air samples collected in Taiwan between 2014 and 2018. Using the NAME (Numerical Atmospheric Modeling Environment) particle dispersion model, we find the likely source of the enhanced CFC-11 observed in Taiwan to be East China. Other halogenated trace gases were also measured, and there were positive interspecies correlations between CFC-11 and CHCl3, CCl4, HCFC-141b, HCFC-142b, CH2Cl2, and HCFC-22, indicating co-location of the emissions of these compounds. These correlations in combination with published emission estimates of CH2Cl2 and HCFC-22 from China, and of CHCl3 and CCl4 from eastern China, are used to estimate CFC-11 emissions. Within the uncertainties, these estimates do not differ for eastern China and the whole of China, so we combine them to derive a mean estimate that we term as being from "(eastern) China". For 2014-2018, we estimate an emission of 19 ± 5 Gg year-1 (gigagrams per year) of CFC-11 from (eastern) China, approximately one-quarter of global emissions. Comparing this to previously reported CFC-11 emissions estimated for earlier years, we estimate CFC-11 emissions from (eastern) China to have increased by 7 ± 5 Gg year-1 from the 2008-2011 average to the 2014-2018 average, which is 50 ± 40% of the estimated increase in global CFC-11 emissions and is consistent with the emission increases attributed to this region in an earlier study

Investigating stratospheric changes between 2009 and 2018 with halogenated trace gas data from aircraft, AirCores, and a global model focusing on CFC-11

We present new observations of trace gases in the stratosphere based on a cost-effective sampling technique that can access much higher altitudes than aircraft. The further development of this method now provides detection of species with abundances in the parts per trillion (ppt) range and below. We obtain mixing ratios for six gases (CFC-11, CFC-12, HCFC-22, H-1211, H-1301, and SF6), all of which are important for understanding stratospheric ozone depletion and circulation. After demonstrating the quality of the data through comparisons with ground-based records and aircraft-based observations, we combine them with the latter to demonstrate its potential. We first compare the data with results from a global model driven by three widely used meteorological reanalyses. Secondly, we focus on CFC-11 as recent evidence has indicated renewed atmospheric emissions of that species relevant on a global scale. Because the stratosphere represents the main sink region for CFC-11, potential changes in stratospheric circulation and troposphere–stratosphere exchange fluxes have been identified as the largest source of uncertainty for the accurate quantification of such emissions. Our observations span over a decade (up until 2018) and therefore cover the period of the slowdown of CFC-11 global mixing ratio decreases measured at the Earth's surface. The spatial and temporal coverage of the observations is insufficient for a global quantitative analysis, but we do find some trends that are in contrast with expectations, indicating that the stratosphere may have contributed to the slower concentration decline in recent years. Further investigating the reanalysis-driven model data, we find that the dynamical changes in the stratosphere required to explain the apparent change in tropospheric CFC-11 emissions after 2013 are possible but with a very high uncertainty range. This is partly caused by the high variability of mass flux from the stratosphere to the troposphere, especially at timescales of a few years, and partly by large differences between runs driven by different reanalysis products, none of which agree with our observations well enough for such a quantitative analysis

Aircraft‐Based Observations of Ozone‐Depleting Substances in the Upper Troposphere and Lower Stratosphere in and Above the Asian Summer Monsoon

Recent studies show that the Asian summer monsoon anticyclone (ASMA) transports emissions from the rapidly industrializing nations in Asia into the tropical upper troposphere. Here, we present a unique set of measurements on over 100 air samples collected on multiple flights of the M55 Geophysica high altitude research aircraft over the Mediterranean, Nepal, and Northern India during the summers of 2016 and 2017 as part of the European Union project StratoClim. These air samples were measured for 27 ozone‐depleting substances (ODSs), many of which were enhanced above expected levels, including the chlorinated very short‐lived substances, dichloromethane (CH2Cl2), 1,2‐dichloroethane (CH2ClCH2Cl), and chloroform (CHCl3). CH2Cl2 mixing ratios in the tropopause region were 65–136 parts per trillion (ppt) in comparison to previous estimates of mixing ratios in the tropical tropopause layer of 30–44 ppt in 2013–2014. Backward trajectories, calculated with the trajectory module of the chemistry‐transport model CLaMS and driven by the ERA5 reanalysis, indicate possible source regions of CH2Cl2 in South Asia. We derived total equivalent chlorine (ECl), and equivalent effective stratospheric chlorine (EESC) and found that these quantities were substantially higher than previous estimates in the literature. EESC at mean age‐of‐air of 3 years based on the 2016 measurements was 1,861–1,872 ppt in comparison to a previously estimated EESC of 1,646 ppt. Our findings show that the ASMA transports larger than expected mixing ratios of long‐lived and very short‐lived ODSs into the upper troposphere and lower stratosphere, likely leading to an impact on the stratospheric ozone layer

Convalescent plasma in patients admitted to hospital with COVID-19 (RECOVERY): a randomised controlled, open-label, platform trial

SummaryBackground Azithromycin has been proposed as a treatment for COVID-19 on the basis of its immunomodulatoryactions. We aimed to evaluate the safety and efficacy of azithromycin in patients admitted to hospital with COVID-19.Methods In this randomised, controlled, open-label, adaptive platform trial (Randomised Evaluation of COVID-19Therapy [RECOVERY]), several possible treatments were compared with usual care in patients admitted to hospitalwith COVID-19 in the UK. The trial is underway at 176 hospitals in the UK. Eligible and consenting patients wererandomly allocated to either usual standard of care alone or usual standard of care plus azithromycin 500 mg once perday by mouth or intravenously for 10 days or until discharge (or allocation to one of the other RECOVERY treatmentgroups). Patients were assigned via web-based simple (unstratified) randomisation with allocation concealment andwere twice as likely to be randomly assigned to usual care than to any of the active treatment groups. Participants andlocal study staff were not masked to the allocated treatment, but all others involved in the trial were masked to theoutcome data during the trial. The primary outcome was 28-day all-cause mortality, assessed in the intention-to-treatpopulation. The trial is registered with ISRCTN, 50189673, and ClinicalTrials.gov, NCT04381936.Findings Between April 7 and Nov 27, 2020, of 16 442 patients enrolled in the RECOVERY trial, 9433 (57%) wereeligible and 7763 were included in the assessment of azithromycin. The mean age of these study participants was65·3 years (SD 15·7) and approximately a third were women (2944 [38%] of 7763). 2582 patients were randomlyallocated to receive azithromycin and 5181 patients were randomly allocated to usual care alone. Overall,561 (22%) patients allocated to azithromycin and 1162 (22%) patients allocated to usual care died within 28 days(rate ratio 0·97, 95% CI 0·87–1·07; p=0·50). No significant difference was seen in duration of hospital stay (median10 days [IQR 5 to >28] vs 11 days [5 to >28]) or the proportion of patients discharged from hospital alive within 28 days(rate ratio 1·04, 95% CI 0·98–1·10; p=0·19). Among those not on invasive mechanical ventilation at baseline, nosignificant difference was seen in the proportion meeting the composite endpoint of invasive mechanical ventilationor death (risk ratio 0·95, 95% CI 0·87–1·03; p=0·24).Interpretation In patients admitted to hospital with COVID-19, azithromycin did not improve survival or otherprespecified clinical outcomes. Azithromycin use in patients admitted to hospital with COVID-19 should be restrictedto patients in whom there is a clear antimicrobial indication

12 years of continuous atmospheric O2, CO2 and APO data from Weybourne Atmospheric Observatory in the United Kingdom

We present analyses of a 12-year time series of continuous atmospheric measurements of O2 and CO2 at the Weybourne Atmospheric Observatory in the United Kingdom. These measurements are combined into the term Atmospheric Potential Oxygen (APO), a tracer that is conservative with respect to terrestrial biosphere processes. The CO2, O2 and APO datasets discussed are hourly averages between May 2010 and December 2021. We include details of our measurement system and calibration procedures, and describe the main long-term and seasonal features of the time series. The 2-minute repeatability of the measurement system is approximately ±3 per meg for O2 and approximately ±0.005 ppm for CO2. The time series shows average long-term trends of 2.40 ppm yr-1 (2.38 to 2.42) for CO2, -24.0 per meg yr-1 for O2 (-24.3 to -23.8) and -11.4 per meg yr-1 (-11.7 to -11.3) for APO, over the 12-year period. The average seasonal cycle peak-to-peak amplitudes are 16 ppm for CO2, 134 per meg for O2, and 68 per meg for APO. The diurnal cycles of CO2 and O2 vary considerably between seasons. The datasets are publicly available at https://doi.org/10.18160/Z0GF-MCWH (Adcock et al., 2023) and have many current and potential scientific applications in constraining carbon cycle processes, such as investigating air-sea exchange of CO2 and O2, and top-down quantification of fossil fuel CO2

New Fractional Release Factors, Ozone Depletion Potentials, and Lifetimes for Four Long-Lived CFCs: CFC-13, CFC-114, CFC-114a, and CFC-115

Knowing the stratospheric lifetime of an Ozone Depleting Substance (ODS), and its potential depletion of ozone during that time, is vital to reliably monitor and control the use of ODSs. Here, we present improved policy-relevant parameters: Fractional Release Factors (FRFs), Ozone Depletion Potentials (ODPs), and stratospheric lifetimes, for four understudied long-lived CFCs: CFC-13 (CClF3), CFC-114 (CClF2CCCLF2), CFC-114a (CCl2FCF3), and CFC-115 (C2ClF5). Previously derived lifetime estimates for CFC-114 and CFC-115 have substantial uncertainties, while lifetime uncertainties for CFC-13 and CFC-114a are absent from the peer-reviewed literature (Carpenter & Danie et al, 2018).This study used both observational and model data to investigate these compounds and this work derives, for the first time, observation-based lifetimes utilising measurements of air samples collected in the stratosphere. We also used a version of the NASA Goddard Space Flight Center (GSFC) 2-D atmospheric model driven by temperature and transport fields derived from MERRA/MERRA-2 reanalysis.FRFs for these compounds, which had been lacking until now, were derived using stratospheric air samples collected from several research flights with the high-altitude aircraft M55-Geophysica, and the background trend from archived surface air samples from Cape Grim, Tasmania.&#160;By using a previously-published correlation between lifetime and FRF for seven well-characterised compounds (CF4, C2F6, C3F8, CHF3, HFC-125, HFC-227ea and SF6), we were able to derive lifetimes for these four new species. Lifetime estimates for CFC-114a agreed within the uncertainties (agreement to one sigma) with the lifetime estimates compiled in Burkholder et al. (2018), while the one for CFC-114 agreed within 2 sigma (measurement-related uncertainties) with those cited in Burkholder et al. (2018). However, observation-based lifetimes for CFC-13 and CFC-115 only agreed with those in Burkholder et al. (2018) within 3 sigma. The lifetime uncertainties in this study were reduced compared to those in Carpenter & Danie et al (2018).As our lifetime estimates for these latter two compounds are notably lower than previous estimates, this suggests that these two compounds may have had greater emissions than previously thought, in order to account for their abundance. It also implies that they will be removed from the atmosphere more quickly than previously thought.New ODPs were derived for these compounds from their new lifetimes and FRFs. Since for two of these compounds (CFC-13 and CFC-114a), there is an absence of observation-derived ODPs in the peer-reviewed literature, this is new and relevant information. The ODPs for CFC-114 and CFC-115 are comparable with estimates from the most recent Scientific Assessment of Ozone Depletion (Burkholder et al., 2018). Providing new and updated lifetimes, FRFs and ODPs for these compounds will help improve future estimates of their tropospheric emissions and their resulting damage to the stratospheric ozone layer.&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;ReferencesBurkholder et al. (2018). Appendix A, Table A-1 in Scientific Assessment of Ozone Depletion: 2018, Global Ozone Research and Monitoring Project, Report No. 58, World Meteorological Organization, Geneva, Switzerland,&#160; http://ozone.unep.org/science/assessment/sap.Carpenter, L.J., Danie, J.S. et al (2018). Scenarios and Information for Policymakers Chapter 6, Table 6-1 in Scientific Assessment of Ozone Depletion: 2018, Global Ozone Research and Monitoring Project, Report No. 58, World Meteorological Organization, Geneva, Switzerland.</p

Investigating stratospheric circulation and chemistry changes over three decades with trace gas data from aircraft, large balloons, and AirCores

Laube et al. (2020) investigated stratospheric changes between 2009 and 2018 with halogenated trace gas data (CFC-11, CFC-12, H-1211, H-1301, HCFC-22, and SF6) from air samples collected via aircraft and AirCores, and compared the mixing ratios and average stratospheric transit times derived from these observations with those from a global model. We here expand this analysis in three ways: firstly, by adding data from further traces gases such as CFC-115, C2F6, and HCFC-142b to broaden the range of tropospheric trends and stratospheric lifetimes, both of which help to assess the robustness of inferred long-term trends in the stratosphere; secondly, by increasing the temporal span of the observations to nearly three decades using new AirCore observations as well as reanalysed archived air samples collected on board high altitude aircraft and large balloons in the 1990s and 2000s; and thirdly, by investigating the fractional release factors and mean ages of air derived from the aforementioned species as measures of their stratospheric chemistry and the strength of the Brewer-Dobson circulation. In combination with model data from the Chemical Langrangian Model of the Stratosphere (CLaMS) this unique data set allows for an unprecedented evaluation of stratospheric chemistry and dynamics in the mid-latitudes of the Northern Hemisphere