Contribution of very short-lived substances to stratospheric bromine loading: uncertainties and constraints

Abstract

Abstract. Very short-lived substances (VSLS) still represent a major factor of uncertainty in the quantification of stratospheric bromine loading. One of the major obstacles for short-lived source gases in contributing to the stratosphere is generally thought to be loss of inorganic bromine (Bry) in the tropical tropopause layer (TTL) due to dehydration. We use sensitivity calculations with a three-dimensional chemistry transport model comprising a consistent parametrization of convective transport and a comprehensive chemistry scheme to investigate the associated processes. The model considers the two most important bromine VSLS, bromoform (CHBr3) and dibromomethane (CH2Br2). The organic bromine source gases as well as the resulting profile of inorganic bromine in the model are consistent with available observations. In contrast to its organic precursors, Bry is assumed to have a significant sorption capacity regarding sedimenting liquid or frozen particles thus the fraction of intact source gases during their ascent through the TTL is a critical factor. We find that source gas injection is the dominant pathway into the stratosphere, about 50% of CHBr3 and 94% of CH2Br2 is able to overcome the cold point tropopause at approximately 17 km altitude, modulated by the interannual variability of the vertical transport efficiency. In fact, our sensitivity calculations indicate that the extent of source gas injection of CHBr3 is highly sensitive to the strength of convection and large-scale ascent; in contrast, modifying the photolysis or the destruction via OH yields a significantly smaller response. In principle, the same applies as well to CH2Br2, though it is considerably less responsive due to its longer lifetime. The next important aspect we identified is that the partitioning of available Bry from short-lived sources is clearly shifted away from HBr, according to our current state of knowledge the only member of the Bry family which is efficiently adsorbed on ice particles. This effect is caused by very efficient heterogeneous reactions on ice surfaces which reduce the HBr/Bry fraction below 15% at the tropical tropopause. Under these circumstances there is no significant loss of Bry due to dehydration in the model, VSLS contribute fully to stratospheric bromine. In addition, we conduct several sensitivity calculations to test the robustness of this result. If heterogeneous chemistry is ignored, the HBr/Bry fraction exceeds 50% and about 10% of bromine from VSLS is scavenged. Dehydration plays a minor role for Bry removal under the assumption that HOBr is efficiently adsorbed on ice as well since the heterogeneous reactions alter the partitioning equilibrium of Bry in favor of HOBr. In this case, up to 12% of bromine from VSLS is removed. Even in the extreme and unrealistic case that adsorbed species on ice particles are instantaneously removed the maximum loss of bromine does not exceed 25%. Assuming 6 parts per trillion by volume (pptv) of bromine short-lived source gases in convective updrafts, a value that is supported by observational data, we find a most likely contribution of VSLS to stratospheric bromine in the range of 4.5–6 pptv.

Probing the subtropical lowermost stratosphere and the tropical upper troposphere and tropopause layer for inorganic bromine

“…Our findings on LS and TTL Br inorg y are in broad agreement with past experimental and theoretical studies on the processes and the amount of bromine injected by source gas and product gases into the TTL and eventually into the extratropical lowermost stratosphere (Ko et al, 1997;Schauffler et al, 1998;Wamsley et al, 1998;Dvortsov et al, 1999;Pfeilsticker et al, 2000;Montzka et al, 2003;Sinnhuber and Folkins, 2006;Hendrick et al, 2007;Laube et al, 2008;Dorf et al, 2006bDorf et al, , 2008Sinnhuber et al, 2009;Salawitch et al, 2010;Schofield et al, 2011;Aschmann et al, 2011;Hossaini et al, 2012b;Ashfold et al, 2012;Hossaini et al, 2012a;Aschmann and Sinnhuber, 2013;Sala et al, 2014;Wang et al, 2015;Liang et al, 2014;WMO, 2014;Navarro et al, 2015, and many others). Our study, however, sets tighter limits than previous ones existing on the amount of Br inorg y and Br org y , the influx of brominated source and product gases, and the photochemistry of bromine in the TTL and LS.…”

“…Here one may wonder whether (a) this result is significant, (b) some Br inorg y is actually removed by heterogeneous processes in the TTL (e.g., Aschmann et al, 2011;Aschmann and Sinnhuber, 2013), or (c) TTL Br y shows some seasonality analogous to the "tape recorder" for H 2 O (e.g., Levine et al, 2008;Krüger et al, 2008;Fueglistaler et al, 2009;Schofield et al, 2011;Ploeger et al, 2011).…”

A tropical West Pacific OH minimum and implications for stratospheric composition

“…In addition, climate change will probably lead to changes in tropospheric OH abundance and halogen emissions (Hossaini et al, 2012). Short-lived bromine species are thought to contribute roughly 5 ppt (about 25 %) to the stratospheric bromine loading, but with large uncertainties ranging from 1 ppt to 10 ppt in different studies (e.g., Warwick et al, 2006;Liang et al, 2010;Hossaini et al, 2013;Aschmann and Sinnhuber, 2013). Bromine is responsible for a significant part of the heterogeneous ozone depletion in polar spring (WMO, 2011).…”

“…However, tropical and coastal regions are thought to play an important role (e.g., Quack and Wallace, 2003;Warwick et al, 2006;Liang et al, 2010). Numerous modeling and measurement studies have tried to better constrain emissions and input into the stratosphere (e.g., Warwick et al, 2006;Liang et al, 2010;Hossaini et al, 2013;Aschmann and Sinnhuber, 2013).…”

“…The amount of CH 2 Br 2 left at the LCP is a lower limit for the amount of bromine from CH 2 Br 2 reaching the stratosphere: While many of the product species of the degradation are soluble and are subject to washout, unsoluble product species and soluble species not washed out can contribute to the stratospheric bromine budget. There are still large uncertainties in the chemistry, washout and adsorption of the product gases of brominated species (e.g., Liang et al, 2010;WMO, 2011;Aschmann and Sinnhuber, 2013).…”