Equivalent Effective Stratospheric Chlorine Research Articles

Damage factors of stratospheric ozone depletion on human health impact with the addition of nitrous oxide as the largest contributor in the 2000s

PurposeStratospheric ozone (O3) depletion caused by O3-depleting substances (ODSs) remains an unsolved issue. The leakage of older ODSs in the atmosphere continue to affect stratospheric O3, and nitrous oxide (N2O) remains the largest contributor to stratospheric O3 depletion. The purpose of this study was to update the damage factors of stratospheric O3 depletion on human health impacts, particularly skin cancers and eye cataracts, for the years 2010 and 2015 by adding N2O.MethodsThe framework to derive damage factors followed that of our previous study; the marginal increase in total incidence per unit ODS emission was estimated using the following terms: ground surface emission, tropospheric chlorine loading, equivalent effective stratospheric chlorine (EESC), total O3 in the air column, ultraviolet-B (UV-B) at the ground surface, incidence due to erythemal UV-B exposure, standardized age structure, population, and ODS atmospheric lifetime. By multiplying the disability-adjusted life years (DALYs) per incidence by the marginal increase in total incidence per unit emission, the damage factor was obtained as the DALY per unit emission. The following update was made in this study: the addition of N2O and revisions of the relationship between EESC and total O3, ODS lifetime, population, and DALY per incidence.Results and discussionDamage factors of all ODSs regulated by the Montreal Protocol and of N2O were calculated for melanoma, non-melanoma skin cancers, and eye cataracts. The total damage factors of N2O were 2.1 × 10–5 and 2.2 × 10–5 DALY per kg nitrogen (N) in 2010 and 2015, respectively. These values were smaller than those of chlorofluorocarbons and halons; however, the global effect of N2O on stratospheric O3 depletion was approximately 170,000 DALYs or 3.9 billion USD in 2010, accounting for 48% of the total damage. The damage factor of N2O on climate change was estimated, based on existing literature, to be 27 times higher than that for stratospheric O3 depletion estimated in this study.ConclusionsN2O is currently the largest contributor to stratospheric O3 depletion, which accounted for approximately 50% of the total health impact induced by all ODSs in 2010. Although another important impact of N2O, i.e., climate change, was demonstrated to be 27 times more damaging than stratospheric O3 depletion, this means that N2O emissions contribute to two global environmental issues simultaneously. Thus, efforts to reduce N2O emissions should be increased.

Impact of the Indian Ocean SST on Wintertime Total Column Ozone Over the Tibetan Plateau

AbstractUsing various observations and a chemistry‐climate model, this study investigates the impact of the Indian Ocean sea surface temperature (SST) on the wintertime total column ozone (TCO) over the Tibetan Plateau (TP). Our analysis reveals that increases in TCO over the TP are accompanied by anomalously high Indian Ocean SSTs, and vice versa. Further analysis reveals that high Indian Ocean SSTs are accompanied by a narrowing of the northern branch of the Hadley cell, and consequently, the subtropical westerly jet in the Northern Hemisphere (NH) tends to shift southward. The southward shift of subtropical westerly jet can lead to a descent of tropopause and higher lower‐stratospheric temperatures over the TP. The latter causes a descent of isentropic surfaces in the upper troposphere and lower stratosphere. Rossby waves triggered by diabatic heating in the upper troposphere over the Indian Ocean can also lead to a descent of isentropic surfaces over the TP. On the other hand, local descending motions over the TP caused by the Gill Pattern circulation and enhanced Brewer‐Dobson circulation bring ozone‐rich air in the lower stratosphere downward. The descent of tropopause and isentropic surfaces and the downward transport of ozone lead to a downward shift of ozone profiles, and consequently, TCO increases over the TP. Based on multiple linear regression analysis, during 1996–2020 the Indian Ocean SST increase and the equivalent effective stratospheric chlorine decline dominate the wintertime TCO increase over the TP, which is partly offset by the TCO decline due to the TP warming.

Indicators of the ozone recovery for selected sites in the Northern Hemisphere mid-latitudes derived from various total column ozone datasets (1980–2020)

Abstract. We propose a method to examine the current status of the ozone recovery attributed to changes of ozone-depleting substances (ODS) in the stratosphere. The total column ozone (TCO3) datasets used are based on the ground-based (by the Dobson and/or Brewer spectrophotometer) measurements, satellite observations (from the Solar Backscatter Ultraviolet (SBUV) and Ozone Mapping and Profiler Suite (OMPS) instruments), and output of reanalyses (Multi-Sensor Reanalysis version 2 (MSR2) and Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA2)). The TCO3 time series are calculated for selected sites in the mid-latitudes of the Northern Hemisphere (NH, 35–60∘ N), which are station locations with long-term TCO3 observations archived at the World Ozone and Ultraviolet Radiation Data Centre (WOUDC). The TCO3 monthly means (1980–2020) are averaged over the April–September period to obtain TCO3 time series for the warm sub-period of the year. Two types of the averaged TCO3 time series are considered: the original one and non-proxy time series with removed natural variability by a standard multiple regression model. The TCO3 time series were smoothed by the locally weighted scatterplot smoother (LOWESS) and the super smoother (SS). The smoothed TCO3 values in 1980, 1988, 1997, and 2020 were used to build ozone recovery indices (ORIs) in 2020. These are key years in the equivalent effective stratospheric chlorine (EESC) time series for the period 1980–2020, i.e., the stratosphere was only slightly contaminated by ODS in 1980, 1988 is the year in which the EESC value is equal to its value at the end (2020), and in 1997, the EESC maximum was in mid-latitude stratosphere. The first proposed ORI, ORI1, is the normalized difference between the TCO3 values in 2020 and 1988. The second one, ORI2, is the percentage of the recovered TCO3 in 2020 since the ODS maximum. Following these definitions, the corresponding reference ranges (from −0.5 % to 1 % for ORI1 and from 40 % to 60 % for ORI2) are obtained by analyzing a set of possible EESC time series simulated via the Goddard automailer. The ozone recovery phases are classified comparing the current ORI values and their uncertainty ranges (by the bootstrapping) with these reference ranges. In the analyzed TCO3 time series, for specific combinations of datasets, data types, and the smoother used, we find faster (for ORI1 or ORI2 above the reference range) and slower (for ORI1 or ORI2 below the reference range) recovery in 2020 than that inferred from the EESC change, and a continuation of the TCO3 decline after the EESC peak (ORI2<0 %). Strong signal of the slower TCO3 recovery is found in Toronto, Hohenpeissenberg, Hradec Kralove, and Belsk. A continuation of ozone decline after the turnaround in ODS concentration is found in both the original and non-proxy time series from WOUDC (Toronto), SBUV and OMPS (Toronto, Arosa, Hohenpeissenberg, Uccle, Hradec Kralove, and Belsk), and MERRA2 data (Arosa, Hohenpeissenberg, Hradec Kralove, and Belsk).

Evolution of Ozone above Togo during the 1979–2020 Period

The objective of this paper is to estimate the trend of the Total Ozone Column (TOC) over Togo. A Multi-Sensor Reanalysis-2 (MSR-2) of the TOC over the entire territory of Togo was used. A Multiple Linear Regression (MLR) method has been applied to retrieve the interannual contributions of different forcings and the long-term variability. It was found that the Annual Oscillation (AnO), the Quasi Biennial Oscillation at 30 mb (QBO30), the Solar Flux (SF), and the El Niño–Southern Oscillation (ENSO) has a statistically significant influence on the interannual variability of the TOC. The strongest contribution (22 ± 1.4 DU) is allocated to the AnO while the weakest (<1 DU) is attributed to the Semi-Annual Oscillations (SAnO). Before the peak year of the Equivalent Effective Stratospheric Chlorine (EESC) in the tropics in 1997, the trend is negative (−0.3% ± 0.9% per decade) and is not statistically significant. After the peak year, a statistically significant positive trend is observed. The trend of the TOC is 0.6% ± 0.2% per decade. The monthly TOC trend over Togo is positive and statistically significant during the rainy season (particularly during the monsoon period) except in April, unlike during the harmattan period (DJF), where the trend is not significant.

Global total ozone recovery trends attributed to ozone-depleting substance (ODS) changes derived from five merged ozone datasets

Abstract. We report on updated trends using different merged zonal mean total ozone datasets from satellite and ground-based observations for the period from 1979 to 2020. This work is an update of the trends reported in Weber et al. (2018) using the same datasets up to 2016. Merged datasets used in this study include NASA MOD v8.7 and NOAA Cohesive Data (COH) v8.6, both based on data from the series of Solar Backscatter Ultraviolet (SBUV), SBUV-2, and Ozone Mapping and Profiler Suite (OMPS) satellite instruments (1978–present), as well as the Global Ozone Monitoring Experiment (GOME)-type Total Ozone – Essential Climate Variable (GTO-ECV) and GOME-SCIAMACHY-GOME-2 (GSG) merged datasets (both 1995–present), mainly comprising satellite data from GOME, SCIAMACHY, OMI, GOME-2A, GOME-2B, and TROPOMI. The fifth dataset consists of the annual mean zonal mean data from ground-based measurements collected at the World Ozone and Ultraviolet Radiation Data Centre (WOUDC). Trends were determined by applying a multiple linear regression (MLR) to annual mean zonal mean data. The addition of 4 more years consolidated the fact that total ozone is indeed slowly recovering in both hemispheres as a result of phasing out ozone-depleting substances (ODSs) as mandated by the Montreal Protocol. The near-global (60∘ S–60∘ N) ODS-related ozone trend of the median of all datasets after 1995 was 0.4 ± 0.2 (2σ) %/decade, which is roughly a third of the decreasing rate of 1.5 ± 0.6 %/decade from 1978 until 1995. The ratio of decline and increase is nearly identical to that of the EESC (equivalent effective stratospheric chlorine or stratospheric halogen) change rates before and after 1995, confirming the success of the Montreal Protocol. The observed total ozone time series are also in very good agreement with the median of 17 chemistry climate models from CCMI-1 (Chemistry-Climate Model Initiative Phase 1) with current ODS and GHG (greenhouse gas) scenarios (REF-C2 scenario). The positive ODS-related trends in the Northern Hemisphere (NH) after 1995 are only obtained with a sufficient number of terms in the MLR accounting properly for dynamical ozone changes (Brewer–Dobson circulation, Arctic Oscillation (AO), and Antarctic Oscillation (AAO)). A standard MLR (limited to solar, Quasi-Biennial Oscillation (QBO), volcanic, and El Niño–Southern Oscillation (ENSO)) leads to zero trends, showing that the small positive ODS-related trends have been balanced by negative trend contributions from atmospheric dynamics, resulting in nearly constant total ozone levels since 2000.

Stratospheric Impacts of Continuing CFC‐11 Emissions Simulated in a Chemistry‐Climate Model

AbstractTrichlorofluoromethane (CFC‐11, CFCl3) is a major anthropogenic ozone‐depleting substance and greenhouse gas, and its production and consumption are controlled under the Montreal Protocol. However, recent studies show that CFC‐11 emissions increased during 2014–2017 relative to 2008–2012. In this study, we use a chemistry‐climate model to investigate the stratospheric impacts of potential CFC‐11 emissions continuing into the future. As a sensitivity test, we use a high CFC‐11 scenario in which the inferred 2013–2016 average emissions of 72.5 Gg/yr is sustained to year 2100. This increases equivalent effective stratospheric chlorine by 15% in 2100, relative to the WMO (2018) baseline scenario in which future emissions decay with a bank release rate of 6.4%/year. Consistent with recent studies, the resulting ozone response has a linear dependence on the accumulated CFC‐11 emissions, yielding global and Antarctic spring total ozone sensitivity per 1,000 Gg of −0.37 and −3.9 DU, respectively, averaged over 2017–2100. The deepened ozone hole reduces UV heating, causing a colder Antarctic lower stratosphere in spring/early summer. Through thermal wind balance, this accelerates the circumpolar jet which in turn alters planetary and gravity wave propagation through the Southern Hemisphere stratosphere, and modifies the Brewer‐Dobson circulation. Age of air in the high scenario is slightly younger than the baseline in the lower stratosphere globally during 2090–2099, with a maximum change of −0.1 years. Coupled atmosphere‐ocean model simulations show that the resulting greenhouse gas impact of CFC‐11 is small and not statistically significant throughout the troposphere and stratosphere.

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

AbstractRecent 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.

Future changes in stratospheric quasi-stationary wave-1 in the extratropical southern hemisphere spring and summer as simulated by ACCESS-CCM

Seasonally dependent quasi-stationary planetary wave activity in the southern hemisphere influences the distribution of ozone within and near the equatorward edge of the stratospheric polar vortex. Accurate representation of this zonal asymmetry in ozone is important in the characterisation of stratospheric circulation and climate and their associated effects at the surface. In this study, we used the Australian Community and Climate Earth System Simulator-Chemistry Climate Model to investigate the influence of greenhouse gases (GHGs) and ozone depleting substances (ODSs) on the zonal asymmetry of total column ozone (TCO) and 10 hPa zonal wind between 50 and 70°S. Sensitivity simulations were used from 1960 to 2100 with fixed ODSs and GHGs at 1960 levels and a regression model that uses equivalent effective stratospheric chlorine and carbon dioxide equivalent radiative forcing as the regressors. The model simulates the spring and summer zonal wave-1 reasonably well, albeit with a slight bias in the phase and amplitude compared to observations. An eastward shift in the TCO and 10 hPa zonal wave-1 is associated with both decreasing ozone and increasing GHGs. Amplitude increases are associated with ozone decline and amplitude decreases with GHG increases. The influence of ODSs typically outweigh those by GHGs, partly due to the GHG influence on TCO phase at 50°S likely being hampered by the Andes. Therefore, over the 21st century, influence from ozone recovery causes a westward shift and a decrease in amplitude. An exception is at 70°S during spring, where the GHG influence is larger than that of ozone recovery, causing a continued eastward trend throughout the 21st century. Also, GHGs have the largest influence on the 10 hPa zonal wave-1 phase, but still only induce a small change in the wave-1 amplitude. Different local longitudes also experience different rates of ozone recovery due to the changes in phase of the zonal wave-1. The results from this study have important implications for understanding future ozone layer distribution in the Southern Hemisphere under changing GHG and ODS concentrations. Important future work would involve conducting a similar study using a large ensemble of models to gain more statistically significant results.

The impact of volcanic emission of halogenated compounds on the Southern Hemisphere and Antarctic environment

The study aims to estimate and compare the global emission for 20 halocarbons from volcanic and hydrothermal sources into the Earth’s atmosphere. It follows from the results that the contribution of volcanic emission for these species in the depletion of stratospheric ozone in the catalytic halogen cycles does not exceed 0.1%. Still, they significantly impair the level of tropospheric ozone near the volcanoes. The scheme of gas-phase free radical chain halogenation of the hydrocarbons is proposed and confirmed by thermodynamic and kinetic calculations. This explains the experimental ratios between concentrations of CH3I : CH3Br : CH3Cl and CCl4 : CHCl3 : CH2Cl2 : CH3Cl in the volcanic gases. The possible volcanic emission of halocarbons from Erebus and explosive eruptions in the Southern Hemisphere during the Holocene do not have a notable impact on their content in the Antarctic ice. However, volcanic emission of hydrogen halides (HX, X = Cl, Br or I) from powerful eruptions in the Southern Hemisphere during Holocene could deplete the stratospheric ozone substantially, causing a drastic impact of the harmful UV-B radiation on the biota of continents and ocean. We calculated the injected Equivalent Effective Stratospheric Chlorine values and estimated the column ozone percentage change, Δ%O3, for 20 known volcano eruptions in the tropical belt and Southern latitudes. The estimates lead to more than 50% depletion of stratospheric ozone after past powerful volcanic eruptions. The range is estimated for possible ozone depletion after the eruption of Deception Island’s volcano occurred near 4000 BP (from 44 to 56%), which is comparable with those from Krakatoa, Samalas, and Tambora eruptions. A similar analysis was carried out for 192 yrs series of Mt Takahe (West Antarctica) halogen-rich volcanic eruptions at 17,7 kyr, showing extensive stratospheric ozone depletion over Antarctica. Crude estimations of stratospheric ozone depletion (Δ%O3) after Ferrar Large Igneous Province eruptions (183 Ma) in Antarctica were performed, considering the whole LIP volume of basaltic lavas, and they range from 49 to 83%. Given the very low emission rate of HCl due to non-eruptive degassing of the Mt. Erebus volcano, the volcanic emission of Erebus could not be a fundamental reason for modern springtime ozone hole formation over Antarctica.

Reformulating the bromine alpha factor and equivalent effective stratospheric chlorine (EESC): evolution of ozone destruction rates of bromine and chlorine in future climate scenarios

Abstract. Future trajectories of the stratospheric trace gas background will alter the rates of bromine- and chlorine-mediated catalytic ozone destruction via changes in the partitioning of inorganic halogen reservoirs and the underlying temperature structure of the stratosphere. The current formulation of the bromine alpha factor, the ozone-destroying power of stratospheric bromine atoms relative to stratospheric chlorine atoms, is invariant with the climate state. Here, we refactor the bromine alpha factor, introducing normalization to a benchmark chemistry–climate state, and formulate Equivalent Effective Stratospheric Benchmark-normalized Chlorine (EESBnC) to reflect changes in the rates of both bromine- and chlorine-mediated ozone loss catalysis with time. We show that the ozone-processing power of the extrapolar stratosphere is significantly perturbed by future climate assumptions. Furthermore, we show that our EESBnC-based estimate of the extrapolar ozone recovery date is in closer agreement with extrapolar ozone recovery dates predicted using more sophisticated 3-D chemistry–climate models than predictions made using equivalent effective stratospheric chlorine (EESC).

Analysis and attribution of total column ozone changes over the Tibetan Plateau during 1979–2017

Abstract. Various observation-based datasets have confirmed positive zonal mean column ozone trends at midlatitudes as a result of the successful implementation of the Montreal Protocol. However, there is still uncertainty about the longitudinal variation of these trends and the direction and magnitude of ozone changes at low latitudes. Here, we use the extended Copernicus Climate Change Service (C3S) dataset (1979–2017) to investigate the long-term variations in total column ozone (TCO) over the Tibetan Plateau (TP) for different seasons. We use piecewise linear trend (PWLT) and equivalent effective stratospheric chlorine loading (EESC)-based multivariate regression models with various proxies to attribute the influence of dynamical and chemical processes on the TCO variability. We also compare the seasonal behaviour of the relative total ozone low (TOL) over the TP with the zonal mean at the same latitude. Both regression models show that the TP column ozone trends change from negative trends from 1979 to 1996 to small positive trends from 1997 to 2017, although the later positive trend based on PWLT is not statistically significant. The wintertime positive trend starting from 1997 is larger than that in summer, but both seasonal TP recovery rates are smaller than the zonal means over the same latitude band. For TP column ozone, both regression models suggest that the geopotential height at 150 hPa (GH150) is a more suitable and realistic dynamical proxy compared to a surface temperature proxy used in some previous studies. Our analysis also shows that the wintertime GH150 plays an important role in determining summertime TCO over the TP through persistence of the ozone signal. For the zonal mean column ozone at this latitude, the quasi-biennial oscillation (QBO) is nonetheless the dominant dynamical proxy. We also use a 3-D chemical transport model to diagnose the contributions of different proxies for the TP region. The role of GH150 variability is illustrated by using two sensitivity experiments with repeating dynamics of 2004 and 2008. The simulated ozone profiles clearly show that wintertime TP ozone concentrations are largely controlled by tropics to midlatitude pathways, whereas in summer variations associated with tropical processes play an important role. These model results confirm that the long-term trends of TCO over the TP are dominated by different processes in winter and summer. The different TP recovery rates relative to the zonal means at the same latitude band are largely determined by wintertime dynamical processes.

The Impact of Continuing CFC‐11 Emissions on Stratospheric Ozone

AbstractTrichlorofluoromethane (CFC‐11, CFCl3) is a major anthropogenic ozone‐depleting substance and greenhouse gas, and its production and consumption are controlled under the Montreal Protocol. However, recent studies show that CFC‐11 emissions have been near constant or increasing since 2002. In this study, we use a two‐dimensional chemistry‐climate model to investigate the stratospheric ozone response to a range of future CFC‐11 emissions scenarios. A scenario with future emissions sustained at 10 gigagrams per year (Gg/year) above the baseline WMO (2018) A1 scenario results in minor additional global (90°S–90°N) ozone depletion of 0.13% by 2100, and a 1.5‐year delay in the global ozone recovery to 1980 levels, relative to the baseline. A scenario with 72.5 Gg/year (the 2013–2016 average) sustained to 2100 results in a substantial 15% increase in effective equivalent stratospheric chlorine and nearly 1% additional global ozone depletion by 2100, with a 7.5‐year delay in the recovery to 1980 global ozone levels, relative to the baseline. The ozone response averaged over time has a strong linear dependence on the cumulative amount of future CFC‐11 emissions under a wide range of scenarios. The resulting ozone response sensitivity gives a simple metric relating the time‐averaged ozone change to the cumulative CFC‐11 emissions. This sensitivity has an inverse dependence on future greenhouse gas concentrations (CO2, CH4, and N2O). For the medium Intergovernmental Panel on Climate Change Representative Concentration Pathway‐6.0 scenario, the sensitivity per 1,000 Gg of cumulative CFC‐11 emissions is −0.1% and −1% for global and Antarctic spring ozone, respectively.

Effect of lower stratospheric temperature on total ozone column (TOC) during the ozone depletion and recovery phases

Abstract Using the Solar Back-scatter Ultraviolet merged ozone data during 1980–2016, the recovery of total ozone column (TOC) is examined in 5° wide latitude bands from 65°S to 65°N across the globe. Since the variability of TOC is influenced by various natural and anthropogenic proxies, a multiple linear regression (MLR) is employed to remove the dynamic variability. Among the proxies, lower stratospheric temperature at ~100hPa plays a significant role in the dynamics of the TOC variability. The MLR model has significantly improved after including the lower stratospheric temperature along with other proxies and the standard deviation of the estimated TOC trend has significantly reduced about 35 to 43% in 50°N-65°N and 16 to 20% in 45°S-65°S after using the lower stratospheric temperature during both the depletion (1980–1996) and recovery (1997–2016) phases. Cooling of lower-middle stratospheric temperature are observed during the depletion phase with −0.09 °C to −0.02 °C year−1. However, during the recovery phase, the temperature at the lower stratosphere is warming. Due to weak signals, the estimated warming trends are not statistically significant. The TOC trends are examined by the increasing or decreasing emission of chlorofluorocarbon, indicated by Equivalent Effective Stratospheric Chlorine (EESC) index. During the recovery phase, the EESC varies from 0.82±0.01 to 0.29±0.005 DU year−1 at 45–65°S using the lower stratospheric temperature. In the northern hemisphere, the recovery trends are weak and varied from 0.21±0.004 to 0.25±0.005 DU year−1 and these trends are statistically significant. This study suggests the importance of the lower stratospheric temperature to derive proper trends using the MLR model.

Representativeness of single lidar stations for zonally averaged ozone profiles, their trends and attribution to proxies

Abstract. This paper is focusing on the representativeness of single lidar stations for zonally averaged ozone profile variations over the middle and upper stratosphere. From the lower to the upper stratosphere, ozone profiles from single or grouped lidar stations correlate well with zonal means calculated from the Solar Backscatter Ultraviolet Radiometer (SBUV) satellite overpasses. The best representativeness with significant correlation coefficients is found within ±15∘ of latitude circles north or south of any lidar station. This paper also includes a multivariate linear regression (MLR) analysis on the relative importance of proxy time series for explaining variations in the vertical ozone profiles. Studied proxies represent variability due to influences outside of the earth system (solar cycle) and within the earth system, i.e. dynamic processes (the Quasi Biennial Oscillation, QBO; the Arctic Oscillation, AO; the Antarctic Oscillation, AAO; the El Niño Southern Oscillation, ENSO), those due to volcanic aerosol (aerosol optical depth, AOD), tropopause height changes (including global warming) and those influences due to anthropogenic contributions to atmospheric chemistry (equivalent effective stratospheric chlorine, EESC). Ozone trends are estimated, with and without removal of proxies, from the total available 1980 to 2015 SBUV record. Except for the chemistry related proxy (EESC) and its orthogonal function, the removal of the other proxies does not alter the significance of the estimated long-term trends. At heights above 15 hPa an “inflection point” between 1997 and 1999 marks the end of significant negative ozone trends, followed by a recent period between 1998 and 2015 with positive ozone trends. At heights between 15 and 40 hPa the pre-1998 negative ozone trends tend to become less significant as we move towards 2015, below which the lower stratosphere ozone decline continues in agreement with findings of recent literature.

A refined method for calculating equivalent effective stratospheric chlorine

Abstract. Chlorine and bromine atoms lead to catalytic depletion of ozone in the stratosphere. Therefore the use and production of ozone-depleting substances (ODSs) containing chlorine and bromine is regulated by the Montreal Protocol to protect the ozone layer. Equivalent effective stratospheric chlorine (EESC) has been adopted as an appropriate metric to describe the combined effects of chlorine and bromine released from halocarbons on stratospheric ozone. Here we revisit the concept of calculating EESC. We derive a refined formulation of EESC based on an advanced concept of ODS propagation into the stratosphere and reactive halogen release. A new transit time distribution is introduced in which the age spectrum for an inert tracer is weighted with the release function for inorganic halogen from the source gases. This distribution is termed the release time distribution. We show that a much better agreement with inorganic halogen loading from the chemistry transport model TOMCAT is achieved compared with using the current formulation. The refined formulation shows EESC levels in the year 1980 for the mid-latitude lower stratosphere, which are significantly lower than previously calculated. The year 1980 is commonly used as a benchmark to which EESC must return in order to reach significant progress towards halogen and ozone recovery. Assuming that – under otherwise unchanged conditions – the EESC value must return to the same level in order for ozone to fully recover, we show that it will take more than 10 years longer than estimated in this region of the stratosphere with the current method for calculation of EESC. We also present a range of sensitivity studies to investigate the effect of changes and uncertainties in the fractional release factors and in the assumptions on the shape of the release time distributions. We further discuss the value of EESC as a proxy for future evolution of inorganic halogen loading under changing atmospheric dynamics using simulations from the EMAC model. We show that while the expected changes in stratospheric transport lead to significant differences between EESC and modelled inorganic halogen loading at constant mean age, EESC is a reasonable proxy for modelled inorganic halogen on a constant pressure level.

Estimating uncertainties in the SBUV Version 8.6 merged profile ozone data set

Abstract. The combined record of total and profile ozone measurements from the solar backscatter ultraviolet (SBUV) and SBUV/2 series of instruments, known as the SBUV Merged Ozone Data (MOD) product, constitutes the longest satellite-based ozone time series from a single instrument type and as such plays a key role in ozone trend analyses.Following the approach documented in Frith et al. (2014) to analyze the merging uncertainties in the MOD total ozone record, we use Monte Carlo simulations to estimate the potential for uncertainties in the calibration and drift of individual instruments in the profile ozone merged data set. We focus our discussion on the trends and associated merging uncertainty since 2001 in an effort to verify the start of ozone recovery as predicted by chemistry climate models. We find that merging uncertainty dominates the overall estimated uncertainty when considering only the 15 years of data since 2001. We derive trends versus pressure level for the MOD data set that are positive in the upper stratosphere as expected for ozone recovery. These trends appear to be significant when only statistical uncertainties are included but become not significant at the 2σ level when instrument uncertainties are accounted for. However, when we use the entire data set from 1979 through 2015 and fit to the EESC (equivalent effective stratospheric chlorine) we find statistically significant fits throughout the upper stratosphere at all latitudes. This implies that the ozone profile data remain consistent with our expectation that chlorine is the dominant ozone forcing term.

Onset of Stratospheric Ozone Recovery in the Antarctic Ozone Hole in Assimilated Daily Total Ozone Columns

AbstractIn this paper we evaluate the long‐term changes in ozone depletion within the Antarctic ozone hole using a 37 years (1979–2015) of daily ozone mass deficits (OMDs) derived from assimilated total ozone column data. For each year an “average daily OMD” is calculated over a 60 day preferential time period (day of year 220–280). Excluding years with a reduced polar stratospheric cloud (PSC) volume (the so‐called PSC‐limited years), the 1979–2015 time series of spatially integrated average daily OMD correlates very well with long‐term changes in equivalent effective stratospheric chlorine (EESC; R2 = 0.89). We find a corresponding statistically highly significant post‐2000 decrease in OMD of −0.77 ± 0.17 megaton (trend significance of 9.8σ), with an associated post‐2000 change in OMD of approximately −30%, consistent with the post‐2000 change in EESC relative to 1980 EESC levels of approximately −30%. The post‐2000 trend significance is robust to the choice of start year. The spatial distribution of the average daily OMD trends reveals a vortex‐core region (approximately covering the region [90°W–0°–90°E/75°S–85°S]) largely unaffected by dynamics with a post‐2000 trend significance of >8σ, and a vortex‐edge region in which the trend is locally strongly affected by vortex dynamics though not spatially integrated over the whole vortex‐edge region (trend significance >9σ). For the trend significance we do not find consistent evidence for long‐term changes in wave driving, vortex mixing, preozone hole conditions, or the applied assimilation method, playing a role. Our observation/assimilation‐based analysis provides robust evidence of a post‐2000 statistically highly significant decrease in the average daily OMD that is consistent with the long‐term decrease in ozone‐depleting substances since 2000, following international emission regulations.

Roles of ozone depleting substances and solar activity in observed long-term trends in total ozone column over Indian region

ABSTRACTAn additive time-series decomposition analysis was performed on the Multi Sensor Reanalysis-2 (MSR-2) monthly mean total ozone column (TOC) time-series dataset spanning over 34 years (January 1979–December 2012) for Indian region (0.0–40.0 °N; 67.5–97.5° E). Statistically significant (p-value <0.05) long-term trends in TOC were estimated in the deseasonalized TOC time series. The role of multiple natural and anthropogenic factors: quasi biennial oscillations (QBO), El-Nino Southern Oscillations (ENSO), cyclic variation in solar activity (SA), and ozone depleting substances (ODS) was investigated to explain the long-term trends in TOC over Indian region. Over sub-tropical Indian region (25.0° N– 40.0° N), declining long-term linear trends were estimated, which varied from −0.30% to −1.10% per decade. Interestingly a positive long-term linear-trend (0.10–0.30% per decade) was observed over equatorial-tropical part of Indian region. No statistically significant long-term trend was observed for 30mb Equatorial Zonal Winds and Nino 3.4 index – indicators for QBO and ENSO; however, a positive long-term linear trend of magnitude 17.00 ± 1.18% per decade was observed in effective equivalent stratospheric chlorine – a proxy for ODS, and a negative long-term linear trend of magnitude 12.72 ± 2.86% per decade was observed in 10.7 cm Solar Radio Flux – a representative for SF. It is inferred that over the Indian region above tropic of cancer, about 85.00% of the estimated negative long-term linear trend in TOC can be explained by the increase in the stratospheric concentration of ODS; whereas, decrease in the solar activity accounted for 15.00% of the estimated negative long-term linear trend in TOC over sub-tropical Indian region.

Past changes in the vertical distribution of ozone – Part 3: Analysis and interpretation of trends

Abstract. Trends in the vertical distribution of ozone are reported and compared for a number of new and recently revised data sets. The amount of ozone-depleting compounds in the stratosphere (as measured by equivalent effective stratospheric chlorine – EESC) was maximised in the second half of the 1990s. We examine the periods before and after the peak to see if any change in trend is discernible in the ozone record that might be attributable to a change in the EESC trend, though no attribution is attempted. Prior to 1998, trends in the upper stratosphere (~ 45 km, 4 hPa) are found to be −5 to −10 % per decade at mid-latitudes and closer to −5 % per decade in the tropics. No trends are found in the mid-stratosphere (28 km, 30 hPa). Negative trends are seen in the lower stratosphere at mid-latitudes in both hemispheres and in the deep tropics. However, it is hard to be categorical about the trends in the lower stratosphere for three reasons: (i) there are fewer measurements, (ii) the data quality is poorer, and (iii) the measurements in the 1990s are perturbed by aerosols from the Mt Pinatubo eruption in 1991. These findings are similar to those reported previously even though the measurements for the main satellite and ground-based records have been revised. There is no sign of a continued negative trend in the upper stratosphere since 1998: instead there is a hint of an average positive trend of ~ 2 % per decade in mid-latitudes and ~ 3 % per decade in the tropics. The significance of these upward trends is investigated using different assumptions of the independence of the trend estimates found from different data sets. The averaged upward trends are significant if the trends derived from various data sets are assumed to be independent (as in Pawson et al., 2014) but are generally not significant if the trends are not independent. This occurs because many of the underlying measurement records are used in more than one merged data set. At this point it is not possible to say which assumption is best. Including an estimate of the drift of the overall ozone observing system decreases the significance of the trends. The significance will become clearer as (i) more years are added to the observational record, (ii) further improvements are made to the historic ozone record (e.g. through algorithm development), and (iii) the data merging techniques are refined, particularly through a more rigorous treatment of uncertainties.

Bromine and chlorine emissions from Plinian eruptions along the Central American Volcanic Arc: From source to atmosphere

Large explosive volcanic eruptions inject gases, aerosols, and fine ashes into the stratosphere, potentially influencing climate and atmosphere composition on a global scale. Although the potential climate effect of chlorine (Cl) and bromine (Br) injections into the stratosphere is known, the global mass fluxes are poorly constrained. In this study we focus on the magmatic degassing systematics and budgets of Br and Cl, and on constraining the major sources of Br in a subduction setting. We therefore present a regional time series of Br and Cl emissions from 29 highly explosive eruptions throughout the Central American Volcanic Arc (CAVA), covering the last 200 ka, and a range of magmatic compositions and eruption magnitudes. We have measured Br and Cl in matrix glasses and melt inclusions using synchrotron radiation micro X-ray fluorescence spectrometry (SR micro-XRF) and electron microprobe, respectively. Melt inclusions of the CAVA tephras generally have higher Br (0.9 to 17.9 ppm) and Cl (770 to 3800 ppm) contents than the matrix glasses (0.39 to 1.5 ppm Br, 600 to 2800 ppm Cl). Moreover, the difference between maximum and minimum concentrations observed in melt inclusions of a given sample ranges between 9 and 90% of the maximum observed concentration for Br, and between 2 and 40% for Cl. Such intra-sample variations arise from variable pre-eruptive degassing of these halogens into a magmatic fluid phase. The relative loss of Br from the melt is 4 to 68 times higher than that of Cl.The masses of Br (2–1100 kt) and Cl (0.1 to 800 Mt) emitted by the eruptions generate instantaneous additions to the stratosphere potentially amounting to ∼6–5600% of the present-day stratospheric annual global loading of Equivalent Effective Stratospheric Chlorine. As the size of the stratospheric impact is primarily a function of eruption magnitude, we use magnitude-frequency relationships to estimate that eruptions adding ∼10% to resident EESC loading would occur every <40 years while every ∼200 years an eruption would double the EESC loading.Comparing the variations in Br and Cl concentrations and particularly minimum Cl/Br ratios in melt inclusions with geochemical trace-element proxies (e.g. U/La, Ba/Th) and lead-isotope compositions, which change along the arc in response to changing subduction conditions, we suggest that subducted calcareous sediment is a major source of magmatic Br but also infer an important role of fluids expelled from serpentinized subducted mantle. Extrapolation of CAVA volcanic Br emissions to the global subduction system thus needs to consider variations in the nature of subducted lithologies.