We have developed a detailed chemical scheme for the degradation of the short-lived source gases bromoform (CHBr3) and dibromomethane (CH2Br2) 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 2O 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 CH2Br 2, and to a lesser extent CHBr3, although the data is sparse and comparisons are not conclusive. Better agreement in the TTL is obtained in the sensitivity run using Î̧ levels (SLIMCAT) and vertical motion from iabatic heating rates. Trajectory estimates of residence times in the two model versions show slower vertical transport in the SLIMCAT Î̧-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 stratospheric source gas injection (SGI) is only reduced by ∼16% for CHBr3 and ∼2% for CH2Br 2 without convection. Overall, the relative importance of the SG pathway and the PG pathway for transport of bromine to the stratospheric overworld (Î̧>380 K) has been assessed. Assuming a 10-day washout lifetime of Bry in TOMCAT, we find the delivery of total Br from Br3 to be 0.72 pptv with ∼53% of this coming from SGI. Similary, for CH2Br2 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.

Bayesian inference has been used to determine rigorous estimates of hydroxyl radical concentrations ([OH]) and air mass dilution rates (K) averaged following air masses between linked observations of nonmethane hydrocarbons (NMHCs) spanning the North Atlantic during the Intercontinental Transport and Chemical Transformation (ITCT)-Lagrangian-2K4 experiment. The Bayesian technique obtains a refined (posterior) distribution of a parameter given data related to the parameter through a model and prior beliefs about the parameter distribution. Here, the model describes hydrocarbon loss through OH reaction and mixing with a background concentration at rate K. The Lagrangian experiment provides direct observations of hydrocarbons at two time points, removing assumptions regarding composition or sources upstream of a single observation. The estimates are sharpened by using many hydrocarbons with different reactivities and accounting for their variability and measurement uncertainty. A novel technique is used to construct prior background distributions of many species, described by variation of a single parameter α. This exploits the high correlation of species, related by the first principal component of many NMHC samples. The Bayesian method obtains posterior estimates of [OH], K and α following each air mass. Median [OH] values are typically between 0.5 and 2.0 × 10⁶ molecules cm^-3, but are elevated to between 2.5 and 3.5 × 10⁶ molecules cm^-3, in low-level pollution. A comparison of [OH] estimates from absolute NMHC concentrations and NMHC ratios assuming zero background (the "photochemical clock" method) shows similar distributions but reveals systematic high bias in the estimates from ratios. Estimates of K are ∼0.1 day^-1 but show more sensitivity to the prior distribution assumed. Copyright 2007 by the American Geophysical Union.