Abstract
Abstract. Halogenated very short-lived substances (VSLSs), such as bromoform (CHBr3), can be transported to the stratosphere and contribute to the halogen loading and ozone depletion. Given their highly variable emission rates and their short atmospheric lifetimes, the exact amount as well as the spatio-temporal variability of their contribution to the stratospheric halogen loading are still uncertain. We combine observational data sets with Lagrangian atmospheric modelling in order to analyse the spatial and temporal variability of the CHBr3 injection into the stratosphere for the time period 1979–2013. Regional maxima with mixing ratios of up to 0.4–0.5 ppt at 17 km altitude are diagnosed to be over Central America (1) and over the Maritime Continent–west Pacific (2), both of which are confirmed by high-altitude aircraft campaigns. The CHBr3 maximum over Central America is caused by the co-occurrence of convectively driven short transport timescales and strong regional sources, which in conjunction drive the seasonality of CHBr3 injection. Model results at a daily resolution reveal isolated, exceptionally high CHBr3 values in this region which are confirmed by aircraft measurements during the ACCENT campaign and do not occur in spatially or temporally averaged model fields. CHBr3 injection over the west Pacific is centred south of the Equator due to strong oceanic sources underneath prescribed by the here-applied bottom-up emission inventory. The globally largest CHBr3 mixing ratios at the cold point level of up to 0.6 ppt are diagnosed to occur over the region of India, Bay of Bengal, and Arabian Sea (3); however, no data from aircraft campaigns are available to confirm this finding. Inter-annual variability of stratospheric CHBr3 injection of 10 %–20 % is to a large part driven by the variability of coupled ocean–atmosphere circulation systems. Long-term changes, on the other hand, correlate with the regional sea surface temperature trends resulting in positive trends of stratospheric CHBr3 injection over the west Pacific and Asian monsoon region and negative trends over the east Pacific. For the tropical mean, these opposite regional trends balance each other out, resulting in a relatively weak positive trend of 0.017±0.012 ppt Br per decade for 1979–2013, corresponding to 3 % Br per decade. The overall contribution of CHBr3 together with CH2Br2 to the stratospheric halogen loading accounts for 4.7 ppt Br, in good agreement with existing studies, with 50 % and 50 % being injected in the form of source and product gases, respectively.
Highlights
It has long been recognized that the depletion of stratospheric ozone over the last 30 years is mainly caused by humanmade chlorine- and bromine-containing substances, often referred to as ozone-depleting substances (ODSs) (Carpenter et al, 2014)
Once brominated very short-lived substances (VSLSs) have reached the stratosphere in the form of source gases (SGs) or product gases (PGs), they participate in ozone depletion at middle and high latitudes (Braesicke et al, 2013; Yang et al, 2014; Sinnhuber and Meul, 2015)
In order to derive the amount of VSLS source and product gases entrained into the stratosphere from the model simulations in Sect. 3.6, we explicitly calculate the cold point along each trajectory based on the ERA-Interim meteorological fields as stratospheric entrainment point
Summary
It has long been recognized that the depletion of stratospheric ozone over the last 30 years is mainly caused by humanmade chlorine- and bromine-containing substances, often referred to as ozone-depleting substances (ODSs) (Carpenter et al, 2014). Randel et al, 2010; Tissier and Legras, 2016), entraining mostly South East Asian planetary boundary layer air with the potential to include emissions from the Indian Ocean and Bay of Bengal (Fiehn et al, 2017, 2018b) In both regions, the west Pacific and the Indian Ocean, these effective transport pathways may coincide with strong oceanic emissions (e.g. Ziska et al, 2013), potentially leading to anomalously large injection of brominated VSLSs. While aircraft measurements in the west Pacific have confirmed high concentrations of brominated VSLSs such as CHBr3 (Wales et al, 2018), the role of the Asian monsoon as an entrainment mechanism for VSLS is not clear due to the lack of observations in this region.
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