Seasonal impact of biogenic very short-lived bromocarbons on lowermost stratospheric ozone between 60◦ N and 60◦ S during the 21st century

Abstract. Biogenic very short-lived bromocarbons (VSL Br ) currently represent ∼25  % of the total stratospheric bromine loading. Owing to their much shorter lifetime compared to anthropogenic long-lived bromine (e.g. halons) and chlorine (e.g. chlorofluorocarbons), the impact of VSL Br on ozone peaks in the lowermost stratosphere, which is a key climatic and radiative atmospheric region. Here we present a modelling study of the evolution of stratospheric ozone and its chemical loss within the tropics and at mid-latitudes during the 21st century. Two different experiments are explored: considering and neglecting the additional stratospheric injection of 5 ppt biogenic bromine naturally released from the ocean. Our analysis shows that the inclusion of VSL Br results in a realistic stratospheric bromine loading and improves the agreement between the model and satellite observations of the total ozone column (TOC) for the 1980–2015 period at mid-latitudes. We show that the overall ozone response to VSL Br at mid-latitudes follows the stratospheric evolution of long-lived inorganic chlorine and bromine throughout the 21st century. Additional ozone loss due to VSL Br is maximized during the present-day period (1990–2010), with TOC differences of −8  DU ( −3  %) and −5.5  DU ( −2  %) for the Southern Hemisphere and Northern Hemisphere mid-latitudes (SH-MLs and NH-MLs), respectively. Moreover, the projected TOC differences at the end of the 21st century are ∼50  % lower than the values found for the present-day period. We find that seasonal VSL Br impact on lowermost stratospheric ozone at mid-latitude is influenced by the seasonality of the heterogeneous inorganic-chlorine reactivation processes on ice crystals. Indeed, due to the more efficient reactivation of chlorine reservoirs (mainly ClONO2 and HCl) within the colder SH-ML lowermost stratosphere, the seasonal VSL Br impact shows a small but persistent hemispheric asymmetry through the whole modelled period. Our results indicate that, although the overall VSL Br -driven ozone destruction is greatest during spring, the halogen-mediated (Halog x-Loss ) ozone loss cycle in the mid-latitude lowermost stratosphere during winter is comparatively more efficient than the HOx cycle with respect to other seasons. Indeed, when VSL Br are considered, Halog x-Loss dominates wintertime lowermost stratospheric ozone loss at SH-MLs between 1985 and 2020, with a contribution of inter-halogen ClOx – BrOx cycles to Halog x-Loss of ∼50  %. Within the tropics, a small ( - 2.5  DU) and relatively constant ( ∼ - 1  %) ozone depletion mediated by VSL Br is closely related to their fixed emissions throughout the modelled period. By including the VSL Br sources, the seasonal Halog x-Loss contribution to lowermost stratospheric ozone loss is practically dominated by the BrOx cycle, reflecting the low sensitivity of very short-lived (VSL) bromine to background halogen abundances to drive tropical stratospheric ozone depletion. We conclude that the link between biogenic bromine sources and seasonal changes in heterogeneous chlorine reactivation is a key feature for future projections of mid-latitude lowermost stratospheric ozone during the 21st century.