Oceanic Bromoform Emissions Weighted by Their Ozone Depletion Potential

Atmos. Chem. Phys., 15, 13647–13663, 2015 www.atmos-chem-phys.net/15/13647/2015/ doi:10.5194/acp-15-13647-2015 © Author(s) 2015. CC Attribution 3.0 License. Oceanic bromoform emissions weighted by their ozone depletion potential S. Tegtmeier1, F. Ziska1, I. Pisso2, B. Quack1, G. J. M. Velders3, X. Yang4, and K. Krüger5 1GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany 2Norwegian Institute for Air Research (NILU), Kjeller, Norway 3National Institute for Public Health and the Environment, Bilthoven, the Netherlands 4British Antarctic Survey, Cambridge, UK 5University of Oslo, Oslo, Norway Correspondence to: S. Tegtmeier ([email protected]) Received: 12 March 2015 – Published in Atmos. Chem. Phys. Discuss.: 26 May 2015 Revised: 17 November 2015 – Accepted: 27 November 2015 – Published: 10 December 2015 Abstract. At present, anthropogenic halogens and oceanic in a future climate. However, at the same time, it is reduced emissions of very short-lived substances (VSLSs) both con- by less effective bromine-related ozone depletion due to de- tribute to the observed stratospheric ozone depletion. Emis- clining stratospheric chlorine concentrations. The compari- sions of the long-lived anthropogenic halogens have been son of the ODP-weighted emissions of short- and long-lived reduced and are currently declining, whereas emissions of halocarbons provides a new concept for assessing the over- the biogenic VSLSs are expected to increase in future cli- all impact of oceanic halocarbon emissions on stratospheric mate due to anthropogenic activities affecting oceanic pro- ozone depletion for current conditions and future projections. duction and emissions. Here, we introduce a new approach for assessing the impact of oceanic halocarbons on stratospheric ozone by calculating their ozone depletion potential (ODP)-weighted emissions. Seasonally and spatially de- 1 Introduction pendent, global distributions are derived within a case-study framework for CHBr3 for the period 1999–2006. At present, The overall abundance of ozone-depleting substances in the ODP-weighted emissions of CHBr3 amount up to 50 % of atmosphere has been decreasing since the beginning of the ODP-weighted anthropogenic emissions of CFC-11 and to 21st century as a result of the successful implementation of 9 % of all long-lived ozone depleting halogens. The ODP- the 1987 Montreal Protocol and its later adjustments and weighted emissions are large where strong oceanic emissions amendments (Carpenter and Reimann, 2014). In contrast coincide with high-reaching convective activity and show to the long-lived halocarbons, the halogenated very short- pronounced peaks at the Equator and the coasts with largest lived substances (VSLSs) with chemical lifetimes of less contributions from the Maritime Continent and western Pa- than 6 months are not controlled by the Montreal Protocol cific Ocean. Variations of tropical convective activity lead to and are even suggested to increase in the future (Hepach seasonal shifts in the spatial distribution of the trajectory- et al., 2014; Hossaini et al., 2015). Brominated VSLSs are derived ODP with the updraught mass flux, used as a proxy known to have large natural sources; however, evidence has for trajectory-derived ODP, explaining 71 % of the variance emerged that their oceanic production and emissions are en- of the ODP distribution. Future climate projections based on hanced through anthropogenic activities which are expected the RCP 8.5 scenario suggest a 31 % increase of the ODP- to increase in the future (Leedham et al., 2013; Ziska et weighted CHBr3 emissions by 2100 compared to present val- al., 2015). At present, oceanic VSLSs provide a significant ues. This increase is related to a larger convective updraught contribution to the stratospheric bromine budget (Carpenter mass flux in the upper troposphere and increasing emissions and Reimann, 2014). In the future, the decline of anthropogenic chlorine and bromine will further increase the rel- Published by Copernicus Publications on behalf of the European Geosciences Union. 13648 S. Tegtmeier et al.: Oceanic bromoform emissions weighted ative impact of oceanic VSLSs on stratospheric chemistry. tensively used measure and plays an important role in the The amount of ozone loss for given bromine emission, how- Montreal Protocol for control metrics and reporting of emis- ever, is expected to decrease due to decreasing stratospheric sions. Some recent studies have applied the ODP concept to chlorine concentrations and thus a less efficient BrO = ClO VSLSs (e.g., Brioude et al., 2010; Pisso et al., 2010), which ozone loss cycle (Yang et al., 2014). Furthermore, the im- have also natural sources. Depending on the meteorological pacts of climate change on surface emissions, troposphere- conditions, only a fraction of the originally released VSLSs to-stratosphere transport, stratospheric chemistry and resi- reaches the stratosphere. As a consequence, the ODP of a dence time will change the role of VSLSs (Pyle et al., 2007; VSLS is not one number as for the long-lived halocarbons Hossaini et al., 2012). While stratospheric ozone depletion but needs to be quantified as a function of time and location due to long-lived halocarbons is expected to level off and re- of emission. ODPs of VSLSs have been estimated based on verse (Austin and Butchart, 2003), assessing oceanic VSLSs Eulerian (Wuebbles et al., 2001) and Lagrangian (Brioude and their impact on stratospheric ozone in a future changing et al., 2010; Pisso et al., 2010) studies, showing strong ge- climate remains a challenge. ographical and seasonal variations, in particular within the Over the last years, there has been increasing evidence tropics. The studies demonstrated that the ODPs of VSLSs from observational (e.g., Dorf et al., 2006; Sioris et al., are to a large degree determined by the efficiency of vertical 2006) and modeling (e.g., Warwick et al., 2006; Liang et transport from the surface to the stratosphere and that uncer- al., 2010; Tegtmeier et al., 2012) studies that VSLSs pro- tainties in the ODPs arise mainly from uncertainties associ- vide a significant contribution to stratospheric total bromine ated with the representation of convection. (Bry/. The current best-estimate range of 2–8 ppt (Carpen- Combining the emission strength and the ozone- ter and Reimann, 2014) includes observation-derived esti- destroying capabilities of a substance in a meaningful way mates of 2.9 ppt (Sala et al., 2014) and model-derived esti- can be achieved by calculating the ODP-weighted emissions. mates of 4 ppt (Hossaini et al., 2013), 4.5–6 ppt (Aschmann For the long-lived halocarbons, global ODP-weighted emis- and Sinnhuber, 2013) and 7.7 ppt (Liang et al., 2014). Bromi- sions can be calculated as the product of two numbers, their nated VSLSs reduce ozone in the lower stratosphere with mean global emissions and their ODPs (e.g., Velders et al., current estimates of a 3–11 % contribution to ozone depletion 2007; Ravishankara et al., 2009). For the VSLSs, however, (Hossaini et al., 2015) or a 2–10 % contribution (Braesicke et the concept of ODP-weighted emissions has not yet been ap- al., 2013; Yang et al., 2014). Through the relatively large im- plied. To do so requires combining estimates of the emis- pact of VSLSs on ozone in the lower stratosphere, VSLSs sions with the ODPs, both of which are highly variable in contribute −0.02 W m−2 to global radiative forcing (Hos- space and time. Among the brominated VSLSs, the calcu- −2 saini et al., 2015) (∼ 6 % of the 0.33 W m from all halo- lation of CHBr3 ODP-weighted emissions is now possible, carbons of ozone-depleting substances). since global emission inventories (Ziska et al., 2013) and The most abundant bromine-containing VSLSs are dibro- global ODP maps (Pisso et al., 2010) has become available. momethane (CH2Br2/ and bromoform (CHBr3/ with poten- ODP-weighted emissions provide insight in where and when tially important source regions in tropical, subtropical and CHBr3 is emitted that impacts stratospheric ozone. Further- shelf waters (Quack et al., 2007). The contribution of VSLSs more, in a globally averaged framework, the ODP-weighted to stratospheric bromine in form of organic source gases or emissions allow comparisons of the impact of past, present inorganic product gases depends strongly on the efficiency and future long- and short-lived halocarbon emissions. The of troposphere-to-stratosphere transport relative to the pho- ODP-weighted emissions for the anthropogenic component tochemical loss of the source gases and to the wet deposi- of the CHBr3 emission budget cannot be calculated, since no tion of the product gases. Uncertainties in the contribution of reliable estimates of anthropogenic contributions are avail- VSLSs to stratospheric halogen loading mainly result from able at the moment. The concept is introduced here for the uncertainties in the emission inventories (e.g., Hossaini et al., available total emission inventory. 2013) and from uncertainties in the modeled transport and We compile ODP-weighted emissions of CHBr3 in form wet deposition processes (e.g., Schofield et al., 2011). of the seasonal and annual mean distribution in order to The relative contribution of individual halocarbons to assess the overall impact of oceanic CHBr3 emissions on stratospheric ozone depletion is often quantified by the ozone stratospheric ozone. First, we introduce the new approach depletion potential (ODP) defined as the time-integrated of calculating ODP-weighted VSLS emissions,

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