Cyanogen Halides (X-CN, X = Cl, Br, I) could be significant contributors to reactive halogen chemistry in the troposphere.

Cyanogen halides are produced in the environment through the reaction of reactive halogen compounds, e.g. HOX, with reduced nitrogen species characteristic of biological substrates. Known sources include water chlorination, including waste water and swimming pools, and algal photosynthetic processes. XCN compounds are volatile and relatively un-reactive in traditional gas phases processes, e.g. reaction with OH radicals, and photolysis, with the exception of ICN, which has a photolysis lifetime of about a day. There are observations of XCN compounds in the aqueous phase: ClCN and BrCN have been observed in chlorinated waters, and BrCN has been observed in algal cultures. There are only a few preliminary observations of XCN compounds in the gas phase. We have quantified the XCN detection sensitivities of a high-resolution time of flight chemical ionization mass spectrometer using iodide reagent ions. Sensitivities range from 1.5 ion counts/pptv for BrCN to 40 for ICN, and 3-σ detection limits on the order of 5 pptv BrCN and 0.5 pptv ICN for 1 second measurements can be achieved. This instrument has sampled the troposphere over the Pacific and Atlantic oceans in 2017 and 2018 during the NASA ATom mission and can be used to investigate the atmospheric abundance of XCN compounds in remote atmospheres. We have measured the aqueous solubilities of ClCN and BrCN at temperatures pertinent to the lower troposphere, and found them to be fairly low, ranging from 1.4 to 32 M/atm. Using that data, and upper limits on hydrolysis rates that come from our measurements and literature values, we conclude that heterogeneous chemistry will be the major sink for ClCN and BrCN and could also be important for ICN. The mechanisms of XCN hydrolysis are all base catalyzed and XCN lifetimes due to those reactions range from a few hours to weeks at environmental pHs. Hydrolysis proceeds through oxidation of the cyanide group to form cyanate, OCN^-, the conjugate anion of isocyanic acid, HNCO. The condensed phase chemistry of XCN compounds suggests they have potential to contribute to reactive halogen and HNCO atmospheric budgets.