Bromoform and dibromomethane in the tropics: a 3-D model study of chemistry and transport

Abstract

Abstract.We have developed a detailed chemical scheme for the degradation of the short-lived source gases bromoform (CHBr 3 ) and dibromomethane (CH 2 Br 2 ) and implemented it in the TOMCAT/SLIMCAT three-dimensional (3-D) chemical transport model (CTM). The CTM has been used to predict the distribution of the two source gases (SGs) and 11 of their organic product gases (PGs). These first global calculations of the organic PGs show that their abundance is small. The longest lived organic PGs are CBr 2 O and CHBrO, but their peak tropospheric abundance relative to the surface volume mixing ratio (vmr) of the SGs is less than 5%. We calculate their mean local tropospheric lifetimes in the tropics to be ∼7 and ∼2 days (due to photolysis), respectively. Therefore, the assumption in previous modelling studies that SG degradation leads immediately to inorganic bromine seems reasonable.We have compared observed tropical SG profiles from a number of aircraft campaigns with various model experiments. In the tropical tropopause layer (TTL) we find that the CTM run using p levels (TOMCAT) and vertical winds from analysed divergence overestimates the abundance of CH 2 Br 2 , and to a lesser extent CHBr 3 , although the data is sparse and comparisons are not conclusive. Better agreement in the TTL is obtained in the sensitivity run using θ levels (SLIMCAT) and vertical motion from diabatic heating rates. Trajectory estimates of residence times in the two model versions show slower vertical transport in the SLIM-CAT θ-level version. In the p-level model even when we switch off convection we still find significant amounts of the SGs considered may reach the cold point tropopause; the Correspondence to: R. Hossaini (chm3rh@leeds.ac.uk) stratospheric source gas injection (SGI) is only reduced by ∼16% for CHBr 3 and ∼2% for CH 2 Br 2 without convection.Overall, the relative importance of the SG pathway and the PG pathway for transport of bromine to the stratospheric overworld (θ >380 K) has been assessed. Assuming a 10-day washout lifetime of Br y in TOMCAT, we find the delivery of total Br from CHBr 3 to be 0.72 pptv with ∼53% of this coming from SGI. Similary, for CH 2 Br 2 we find a total Br value of 1.69 pptv with ∼94% coming from SGI. We infer that these species contribute ∼2.4 pptv of inorganic bromine to the lower stratosphere with SGI being the dominant pathway. Slower transport to and through the TTL would decrease this estimate.

Estimates of tropical bromoform emissions using an inversion method

“…In contrast, recent global emission estimates of the longer-lived CH 2 Br 2 (which has a lifetime of months, e.g. Warwick et al, 2006;Hossaini et al, 2010) have converged on a value of ∼ 60 Gg yr −1 (e.g. Liang et al, 2010;Ordóñez et al, 2012;Ziska et al, 2013).…”

“…We will refer to the full three-dimensional (longitudelatitude-time) grid of dilution values as the "dilution matrix". We approximate the effect of photochemical loss of CHBr 3 by allowing the mass associated with each trajectory to decay with an e-folding time of 15 days (similar to the expected lifetime of CHBr 3 in the tropical lower troposphere, Warwick et al, 2006;Hossaini et al, 2010;Liang et al, 2010). This has the effect of decreasing the dilution value in grid cells further from the measurement locations.…”

“…Warwick et al, 2006;Hossaini et al, 2010), which ensures that measured mixing ratios are often highly variable, and are likely to be influenced by only a small fraction of global sources. This difficulty is reflected in the following range of recent global emission estimates, all given in Gg CHBr 3 yr −1 :…”

Short-lived brominated hydrocarbons – observations in the source regions and the tropical tropopause layer

“…(2) Is also justified, given that only the ratios of the lifetimes must be similar. Hossaini et al (2010) found altitude dependent ratios of the local tropical lifetimes for Ko et al (2003) for average tropospheric conditions at about 500 hPa altitude. This shows that the lifetime ratios vary mainly with altitude and less with latitude.…”

“…These values have been updated in the new WMO report to 24, 59 and 123 days for CHBr 3 , CHBr 2 Cl and CH 2 Br 2 (Montzka et al, 2011) the degradation rate especially of CH 2 Br 2 relative to CHBr 3 , as shown by Hossaini et al (2010), who calculated local lifetimes of 52 days for CH 2 Br 2 and 16 days for CHBr 3 in these regions. Using these values and an estimate of 29 days for the corresponding lifetime of CHBr 2 Cl, the slope of the chemical decay line is even lower (2.84).…”