CH
is the most abundant reactive greenhouse gas and a complete understanding of its atmospheric fate is needed to formulate mitigation policies. Current chemistry-climate models tend to underestimate ...the lifetime of CH
, suggesting uncertainties in its sources and sinks. Reactive halogens substantially perturb the budget of tropospheric OH, the main CH
loss. However, such an effect of atmospheric halogens is not considered in existing climate projections of CH
burden and radiative forcing. Here, we demonstrate that reactive halogen chemistry increases the global CH
lifetime by 6-9% during the 21st century. This effect arises from significant halogen-mediated decrease, mainly by iodine and bromine, in OH-driven CH
loss that surpasses the direct Cl-induced CH
sink. This increase in CH
lifetime helps to reduce the gap between models and observations and results in a greater burden and radiative forcing during this century. The increase in CH
burden due to halogens (up to 700 Tg or 8% by 2100) is equivalent to the observed atmospheric CH
growth during the last three to four decades. Notably, the halogen-driven enhancement in CH
radiative forcing is 0.05 W/m
at present and is projected to increase in the future (0.06 W/m
by 2100); such enhancement equals ~10% of present-day CH
radiative forcing and one-third of N
O radiative forcing, the third-largest well-mixed greenhouse gas. Both direct (Cl-driven) and indirect (via OH) impacts of halogens should be included in future CH
projections.
As a result of the 1987 Montreal Protocol and its amendments, the atmospheric loading of anthropogenic ozone-depleting substances is decreasing. Accordingly, the stratospheric ozone layer is expected ...to recover. However, short data records and atmospheric variability confound the search for early signs of recovery, and climate change is masking ozone recovery from ozone-depleting substances in some regions and will increasingly affect the extent of recovery. Here we discuss the nature and timescales of ozone recovery, and explore the extent to which it can be currently detected in different atmospheric regions.
With the successful implementation of the Montreal Protocol on Substances that Deplete the Ozone Layer, the atmospheric abundance of ozone-depleting substances continues to decrease slowly and the ...Antarctic ozone hole is showing signs of recovery. However, growing emissions of unregulated short-lived anthropogenic chlorocarbons are offsetting some of these gains. Here, we report an increase in emissions from China of the industrially produced chlorocarbon, dichloromethane (CH
Cl
). The emissions grew from 231 (213-245) Gg yr
in 2011 to 628 (599-658) Gg yr
in 2019, with an average annual increase of 13 (12-15) %, primarily from eastern China. The overall increase in CH
Cl
emissions from China has the same magnitude as the global emission rise of 354 (281-427) Gg yr
over the same period. If global CH
Cl
emissions remain at 2019 levels, they could lead to a delay in Antarctic ozone recovery of around 5 years compared to a scenario with no CH
Cl
emissions.
It is well established that anthropogenic chlorine-containing chemicals contribute to ozone layer depletion. The successful implementation of the Montreal Protocol has led to reductions in the ...atmospheric concentration of many ozone-depleting gases, such as chlorofluorocarbons. As a consequence, stratospheric chlorine levels are declining and ozone is projected to return to levels observed pre-1980 later this century. However, recent observations show the atmospheric concentration of dichloromethane-an ozone-depleting gas not controlled by the Montreal Protocol-is increasing rapidly. Using atmospheric model simulations, we show that although currently modest, the impact of dichloromethane on ozone has increased markedly in recent years and if these increases continue into the future, the return of Antarctic ozone to pre-1980 levels could be substantially delayed. Sustained growth in dichloromethane would therefore offset some of the gains achieved by the Montreal Protocol, further delaying recovery of Earth's ozone layer.
We use height‐resolved and total column satellite observations and 3‐D chemical transport model simulations to study stratospheric ozone variations during 1998–2017 as ozone‐depleting substances ...decline. In 2017 extrapolar lower stratospheric ozone displayed a strong positive anomaly following much lower values in 2016. This points to large interannual variability rather than an ongoing downward trend, as reported recently by Ball et al. (2018, https://doi.org/10.5194/acp‐18‐1379‐2018). The observed ozone variations are well captured by the chemical transport model throughout the stratosphere and are largely driven by meteorology. Model sensitivity experiments show that the contribution of past trends in short‐lived chlorine species to the ozone changes is small. Similarly, the potential impact of modest trends in natural brominated short‐lived species is small. These results confirm the important role that atmospheric dynamics plays in controlling ozone in the extrapolar lower stratosphere on multiannual time scales and the continued importance of monitoring ozone profiles as the stratosphere changes.
Plain Language Summary
Emission of long‐lived chlorine and bromine‐containing ozone‐depleting substances has led to the depletion of the ozone layer, most notably the Antarctic ozone hole. Policy action through the Montreal Protocol has phased out the production of the major long‐lived ozone‐depleting substances. Consequently, stratospheric chlorine and bromine amounts are declining, and we expect the ozone layer to slowly recover. However, although the tropical lower stratosphere is not a region where large ozone loss has so‐far been observed, a recent study by Ball et al. (2018) suggested that ozone there is decreasing, in disagreement with models and expectations of ozone recovery. We use updated observations and an atmospheric model to investigate these issues. First, we use an additional year of observations which show that ozone values in the lower stratosphere increased in 2017, which is a consequence of variations in atmospheric dynamics. Second, our 3‐D model performs well in reproducing the observed ozone variations. Although the model is not perfect, the comparisons suggest that we do have a good understanding of the lower stratospheric ozone. Third, we quantify the role of short‐lived chlorine and bromine compounds, which are not controlled by the Montreal Protocol, on the recent ozone changes. The effect is small.
Key Points
Observations show that lower stratospheric ozone at extrapolar latitudes increased strongly in 2017 relative to a negative anomaly in 2016
Model simulations reproduce the observed ozone variations well, and the main driver in the lower stratosphere is atmospheric dynamics
The contribution of an observation‐based trend in short‐lived chlorine species to recent lower stratospheric ozone variations is small
Chlorine atoms (Cl) are highly reactive toward hydrocarbons in the Earth's troposphere, including the greenhouse gas methane (CH4). However, the regional and global CH4 sink from Cl is poorly ...quantified as tropospheric Cl concentrations (Cl) are uncertain by ~2 orders of magnitude. Here we describe the addition of a detailed tropospheric chlorine scheme to the TOMCAT chemical transport model. The model includes several sources of tropospheric inorganic chlorine (Cly), including (i) the oxidation of chlorocarbons of natural (CH3Cl, CHBr2Cl, CH2BrCl, and CHBrCl2) and anthropogenic (CH2Cl2, CHCl3, C2Cl4, C2HCl3, and CH2ClCH2Cl) origin and (ii) sea‐salt aerosol dechlorination. Simulations were performed to quantify tropospheric Cl, with a focus on the marine boundary layer, and quantify the global significance of Cl atom CH4 oxidation. In agreement with observations, simulated surface levels of hydrogen chloride (HCl), the most abundant Cly reservoir, reach several parts per billion (ppb) over polluted coastal/continental regions, with sub‐ppb levels typical in more remote regions. Modeled annual mean surface Cl exhibits large spatial variability with the largest levels, typically in the range of 1–5 × 104 atoms cm−3, in the polluted northern hemisphere. Chlorocarbon oxidation provides a tropospheric Cly source of up to ~4320 Gg Cl/yr, sustaining a background surface Cl of <0.1 to 0.5 × 103 atoms cm−3 over large areas. Globally, we estimate a tropospheric methane sink of ~12–13 Tg CH4/yr due the CH4 + Cl reaction (~2.5% of total CH4 oxidation). Larger regional effects are predicted, with Cl accounting for ~10 to >20% of total boundary layer CH4 oxidation in some locations.
Key Points
Monthly mean surface Cl >104 atoms cm−3 in polluted northern hemisphere
Cl atoms account for up to 25% of regional boundary layer CH4 oxidation
Global tropospheric sink of ~12–13 Tg CH4/yr from CH4 + Cl reaction (~2.5% of total CH4 oxidation)
Chloroform (CHCl3), dichloromethane (CH2Cl2), perchloroethylene (C2Cl4), and 1,2‐dichloroethane (C2H4Cl2) are chlorinated Very Short‐Lived Substances (Cl‐VSLS) with a range of commercial/industrial ...applications. Recent studies highlight the increasing influence of Cl‐VSLS on the stratospheric chlorine budget and therefore their possible role in ozone depletion. Here we evaluate the ozone depletion potential (ODP) of these Cl‐VSLS using a three‐dimensional chemical transport model and investigate sensitivity to emission location/season. The seasonal dependence of the ODPs is small, but ODPs vary by a factor of 2–3 depending on the continent of emission: 0.0143–0.0264 (CHCl3), 0.0097–0.0208 (CH2Cl2), 0.0057–0.0198 (C2Cl4), and 0.0029–0.0119 (C2H4Cl2). Asian emissions produce the largest ODPs owing to proximity to the tropics and efficient troposphere‐to‐stratosphere transport of air originating from industrialized East Asia. The Cl‐VSLS ODPs are generally small, but the upper ends of the CHCl3 and CH2Cl2 ranges are comparable to the mean ODP of methyl chloride (0.02), a longer‐lived ozone‐depleting substance.
Plain Language Summary
Anthropogenic emissions of long‐lived chlorinated substances (e.g., chlorofluorocarbons) have led to global ozone layer depletion since the 1970s/1980s, including the Antarctic Ozone Hole phenomenon. The 1987 Montreal Protocol was enacted to ban production of major ozone‐depleting gases, and in consequence, there are signs that the ozone layer is recovering. However, emissions of so‐called very short‐lived substances, such as dichloromethane, have increased in recent years. Historically, these compounds have not been considered a major threat to stratospheric ozone, due to relatively short lifetimes, and they are not controlled by the Protocol. Given that production of these compounds is projected to increase, it is important to determine their ability to affect stratospheric ozone. We quantify the ozone depletion potential (ODP) of chloroform and perchloroethylene and, for the first time, dichloromethane and 1,2‐dichloroethane, the main chlorinated very short‐lived substances. We show that their ODPs vary depending on where the emission occurs. For example, the ODP from Asian dichloromethane emissions is up to a factor of two greater than that from European emissions. This reflects the relative efficiency of troposphere to stratosphere transport between different geographical areas; the transport of polluted boundary layer air from continental East Asia being one relatively efficient route.
Key Points
Ozone depletion potentials of very short‐lived substances (CHCl3, CH2Cl2, C2Cl4, and C2H4Cl2) were calculated using a chemical transport model
Calculated ozone depletion potentials vary by a factor of 2‐3 depending on emission location, larger ODPs for Asian emissions
Efficient transport of very short‐lived substances from continental East Asia to tropical lower stratosphere lead to larger Asian ODPs
The C-13 isotopic ratio of methane, δC-13 of CH4, provides additional constraints on the CH4 budget to complement the constraints from CH4 observations. The interpretation of δC-13 observations is ...complicated, however, by uncertainties in the methane sink. The reaction of CH4 with Cl is highly fractionating, increasing the relative abundance of 13CH4, but there is currently no consensus on the strength of the tropospheric Cl sink. Global model simulations of halogen chemistry differ strongly from one another in terms of both the magnitude of tropospheric Cl and its geographic distribution. This study explores the impact of the inter-model diversity in Cl fields on the simulated δC-13 of CH4. We use a set of GEOS global model simulations with different predicted Cl fields to test the sensitivity of the δC-13 of CH4 to the diversity of Cl output from chemical transport models. We find that δC-13 is highly sensitive to both the amount and geographic distribution of Cl. Simulations with Cl providing 0.28% or 0.66% of the total CH4 loss bracket the δC-13 observations for a fixed set of emissions. Thus, even when Cl provides only a small fraction of the total CH4 loss and has a small impact on total CH4, it provides a strong lever on δC-13. Consequently, it is possible to achieve a good representation of total CH4 using widely different Cl concentrations, but the partitioning of CH4 loss between the OH and Cl reactions leads to strong differences in isotopic composition depending on which model’s Cl field is used. Comparing multiple simulations, we find that altering the tropospheric Cl field leads to approximately a 0.5‰ increase in δ(13)CH4for each percent increase in how much CH4 is oxidized by Cl. The geographic distribution and seasonal cycle of Cl also impacts the hemispheric gradient and seasonal cycle of δC-13. The large effect of Cl on δC-13 compared to total CH4 broadens the range of CH4 source mixtures that can be reconciled with δC-13 observations. Stronger constraints on tropospheric Cl are necessary to improve estimates of CH4 sources from δC-13 observations.
Clustering – the automated grouping of similar data – can
provide powerful and unique insight into large and complex data sets, in a
fast and computationally efficient manner. While clustering has ...been used in
a variety of fields (from medical image processing to economics), its
application within atmospheric science has been fairly limited to date, and
the potential benefits of the application of advanced clustering techniques
to climate data (both model output and observations) has yet to be fully
realised. In this paper, we explore the specific application of clustering to
a multi-model climate ensemble. We hypothesise that clustering techniques can
provide (a) a flexible, data-driven method of testing model–observation
agreement and (b) a mechanism with which to identify model development
priorities. We focus our analysis on chemistry–climate model (CCM) output of
tropospheric ozone – an important greenhouse gas – from the recent
Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP).
Tropospheric column ozone from the ACCMIP ensemble was clustered using the
Data Density based Clustering (DDC) algorithm. We find that a multi-model
mean (MMM) calculated using members of the most-populous cluster identified
at each location offers a reduction of up to ∼ 20 % in the global
absolute mean bias between the MMM and an observed satellite-based
tropospheric ozone climatology, with respect to a simple, all-model MMM. On a
spatial basis, the bias is reduced at ∼ 62 % of all locations, with
the largest bias reductions occurring in the Northern Hemisphere – where
ozone concentrations are relatively large. However, the bias is unchanged at
9 % of all locations and increases at 29 %, particularly in the
Southern Hemisphere. The latter demonstrates that although cluster-based
subsampling acts to remove outlier model data, such data may in fact be
closer to observed values in some locations. We further demonstrate that
clustering can provide a viable and useful framework in which to assess and
visualise model spread, offering insight into geographical areas of agreement
among models and a measure of diversity across an ensemble. Finally, we
discuss caveats of the clustering techniques and note that while we have
focused on tropospheric ozone, the principles underlying the cluster-based
MMMs are applicable to other prognostic variables from climate models.
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