Finding the missing stratospheric Br<sub>y</sub>: a global modeling study of CHBr<sub>3</sub> and CH<sub>2</sub>Br<sub>2</sub>

Abstract

Abstract. Recent in situ and satellite measurements suggest a contribution of ∼5 pptv to stratospheric inorganic bromine from short-lived bromocarbons. We conduct a modeling study of the two most important short-lived bromocarbons, bromoform (CHBr 3 ) and dibromomethane (CH 2 Br 2 ), with the Goddard Earth Observing System Chemistry Climate Model (GEOS CCM) to account for this missing stratospheric bromine. We derive a "top-down" emission estimate of CHBr 3 and CH 2 Br 2 using airborne measurements in the Pacific and North American troposphere and lower stratosphere obtained during previous NASA aircraft campaigns. Our emission estimate suggests that to reproduce the observed concentrations in the free troposphere, a global oceanic emission of 425 Gg Br yr −1 for CHBr 3 and 57 Gg Br yr −1 for CH 2 Br 2 is needed, with 60% of emissions from open ocean and 40% from coastal regions. Although our simple emission scheme assumes no seasonal variations, the model reproduces the observed seasonal variations of the short-lived bromocarbons with high concentrations in winter and low concentrations in summer. This indicates that the seasonality of short-lived bromocarbons is largely due to seasonality in their chemical loss and transport. The inclusion Correspondence to: Q. Liang (qing.liang@nasa.gov) of CHBr 3 and CH 2 Br 2 contributes ∼5 pptv bromine throughout the stratosphere. Both the source gases and inorganic bromine produced from source gas degradation (Br VSLS y ) in the troposphere are transported into the stratosphere, and are equally important. Inorganic bromine accounts for half (2.5 pptv) of the bromine from the inclusion of CHBr 3 and CH 2 Br 2 near the tropical tropopause and its contribution rapidly increases to ∼100% as altitude increases. More than 85% of the wet scavenging of Br VSLS y occurs in large-scale precipitation below 500 hPa. Our sensitivity study with wet scavenging in convective updrafts switched off suggests that Br VSLS y in the stratosphere is not sensitive to convection. Convective scavenging only accounts for ∼0.2 pptv (4%) difference in inorganic bromine delivered to the stratosphere.

Isotopic composition of polyhalomethanes from marine macrophytes – systematic effects of the halogen substituents on isotopic composition

“…The d 13 C values of CHBr 3 from the seagrass incubations and the concurrent seawater controls do not fit this trend. They are more enriched in 13 C and as mentioned before, they show a tendency towards more enriched values during daytime when production was higher. However, this tendency has to be taken with care because it may also result from unaccounted contributions of CHBr 3 enriched in 13 C from the sea water over the course of the day.…”

“…CHBr 3 was depleted in 13 C during the night when production was low and showed an enrichment in 13 C over the day when production was elevated. In comparison with this, available field data from seagrassdominated coastal sites suggest somewhat depleted d 13 C values for CHBr 3 from Z. noltii communities ranging from À8 to À18%.…”

“…[32] This background may confound the isotopic composition of PHMs of macrophyte incubations in the present study. Indeed, the d 13 C of CHBr 3 in the seawater controls performed along with the seagrass incubations indicates an enrichment in 13 C for the non-macrophyte-derived CHBr 3 . However, currently we cannot unambiguously attribute this enrichment to extracellular CHBr 3 production.…”

“…However, the current emissions estimates for these climaterelevant compounds remain fairly uncertain owing to the large spatial and temporal variability in observed halocarbon mixing ratios and fluxes. This is, in particular, true for short-lived halocarbons such as iodomethane (CH 3 I) and bromoform (tribromomethane, CHBr 3 ), [8][9][10][11][12][13][14] with the largest uncertainties in the known sources being observed for coastal and near-shore emissions. [15] Despite the uncertainties in global halocarbon emission budgets, it is well accepted that in the marine realm, diverse autotrophic organisms contribute to halocarbon production.…”

The Chemistry Climate Model ECHAM6.3-HAM2.3-MOZ1.0

“…Table 2. Halogenated compounds in JAM2 with relevance for stratospheric ozone chemistry and their approximate lifetimes in years (Miller et al, 1998;Liang et al, 2010;WMO, 2014;Harrison et al, 2016 3 Chemical parametrizations…”