Supplementary material to "Observations of cyanogen bromide (BrCN) in the global troposphere and their relation to polar surface O ₃ destruction"

Supplement to: BrCN in the global troposphere 1 Formation of BrCN from HOBr chemistryThe formation of XCN compounds from reactions of HCN/CN¯ in the condensed phase is well known, so deserves consideration in the context of the polar boundary layer.The major sources of HCN to the atmosphere are biomass burning and vehicle exhaust, two sources that favor the northern hemisphere.HCN is a very weak acid (pKa=9.3), is not very soluble in water (Yoo et al., 1986), and its gas phase loss processes are quite slow (several years against reaction with OH).Consequently, HCN is widely distributed throughout the troposphere as is apparent in the ATom data shown in the main text.Vertical profiles of HCN imply a surface sink and the effective lifetime of HCN has been estimated to be about 5 months (Li et al., 2003).The data from ATom-3&4, including the vertical profile and inter-hemisphere distribution, are for the large part in agreement with this.