On the Cause of Recent Variations in Lower Stratospheric Ozone

Abstract

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. (2018Ball et al. ( , https://doi.org/10.5194/acp-18-1379Ball et al. ( -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.

Recent Trends in Stratospheric Chlorine From Very Short‐Lived Substances

“…All simulations were performed for the period 2000 to 2017. The version of the stratospheric model employed here was recently used by Chipperfield et al (2018) to investigate lower stratospheric ozone trends and is used by Harrison et al (2018) to interpret long-term COCl 2 observations from ACE. It has also been extensively evaluated in terms of both chemistry and transport (e.g., Harrison et al, 2016;Wales et al, 2018).…”

“…They estimated a mean increase of 3.7 ppt Cl/year over this period, with the positive trend reflecting increasing CH 2 Cl 2 . Although the impact of this additional chlorine on stratospheric ozone is expected to have been modest over the last decade (Chipperfield et al, ), possible future increases in Cl‐VSLS emissions could influence the time scale for stratospheric ozone recovery, particularly in polar regions (Hossaini et al, ). Such projections carry large uncertainties; however, they are important to consider to provide bounds on potential resulting ozone changes, especially given that large CH 2 Cl 2 emission increases from major economies are projected under business as usual scenarios until 2030 (Feng et al, ).…”

On the Regional and Seasonal Ozone Depletion Potential of Chlorinated Very Short‐Lived Substances

“…Chlorinated Very Short-Lived Substances (Cl-VSLS), including chloroform (CHCl 3 ) and dichloromethane (CH 2 Cl 2 ), are a significant source of stratospheric chlorine (e.g., Hossaini et al, 2015;Laube et al, 2008) and therefore contribute to ozone depletion (e.g., Chipperfield et al, 2018;Hossaini et al, 2017). These compounds have surface atmospheric lifetimes of~6 months or less (e.g., Ko et al, 2003) and are used in a variety of commercial and industrial applications.…”

“…Owing to increasing emissions, tropospheric CH 2 Cl 2 mixing ratios have approximately doubled since the early 2000s, evidenced by long-term surface monitoring data (e.g., Hossaini et al, 2015Hossaini et al, , 2017 and measurements in the upper troposphere (Leedham Elvidge et al, 2015). Although the influence of CH 2 Cl 2 on ozone has been modest in the recent past (Chipperfield et al, 2018), if sustained CH 2 Cl 2 growth continues in ©2019. The Authors.…”

Modeling the Effect of Potential Nitric Acid Removal During Convective Injection of Water Vapor Over the Central United States on the Chemical Composition of the Lower Stratosphere

“…Finally, despite potential recovery of upper stratosphere ozone, extrapolar lower stratosphere ozone has been declining since 1998 without an attributed cause (Ball et al, ). However, this trend has been disputed by analysis suggesting that the trend is not as significant when including the year 2017 and that ozone trends in the midlatitudes are largely due to dynamic variability (Chipperfield et al, ).…”