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

BARRERA, JAVIER ALEJANDRO; FERNANDEZ, RAFAEL PEDRO; IGLESIAS-SUAREZ, FERNANDO; CUEVAS, CARLOS ALBERTO; LAMARQUE, JEAN-FRANCOIS; SAIZ-LOPEZ, ALFONSO

Atmospheric Chemistry and Physics

Copernicus

Año: 2020 vol. 20 p. 8083 - 8102

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 periodat 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 midlatitudes (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-dayperiod. 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 ClONO 2 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 HO x 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 ClO x ?BrO x cycles to Halog x-Loss of ∼ 50 %. Within the tropics, a small (< −2.5 DU) and relativelyconstant (∼ −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 BrO x 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.