Simulating the spread of disinfection by-products and anthropogenicbromoform emissions from ballast water discharge in Southeast Asia

Abstract. Ballast water treatment is required for vessels to prevent the introduction of potentially invasive neobiota. Some treatment methods use chemical disinfectants which produce a variety of halogenated compounds as disinfection by-products (DBPs). One of the most abundant DBP from oxidative ballast water treatment is bromoform (CHBr 3 ) where we find an average concentration of 894 ± 560 nmol L -1 (226 ± 142 μg L -1 ) in the undiluted ballast water from measurements and literature. Bromoform is a relevant gas for atmospheric chemistry and ozone depletion, especially in the tropics where entrainment into the stratosphere is possible. The spread of DBPs in the tropics over months to years is assessed here for the first time. With Lagrangian trajectories based on the NEMO-ORCA12 model velocity field, we simulate DBP spread in the sea surface and try to quantify the oceanic bromoform concentration and emission to the atmosphere from ballast water discharge at major harbours in the tropical region of Southeast Asia. The exemplary simulations of two important regions, Singapore and the Pearl River Delta, reveal major transport pathways of the DBPs and the anthropogenic bromoform concentrations in the sea surface. Based on our simulations, we expect DBPs to spread into the open ocean, along the coast and also an advection with monsoon-driven currents into the North Pacific and Indian Ocean. Furthermore, anthropogenic bromoform concentrations and emissions are predicted to increase locally around large harbours. In the sea surface around Singapore we estimate an increase in bromoform concentration by 9 % compared to recent measurement. In a moderate scenario where 70 % of the ballast water is chemically treated bromoform emissions to the atmosphere can locally exceed 1000 pmol m -2 h -1 and double climatological emissions. In the Pearl River Delta all bromoform is directly outgassed which leads to an additional bromine (Br) input into the atmosphere of 495 kmol Br (∼42 t CHBr 3 ) a -1 . From Singapore ports the additional atmospheric Br input is calculated as 312 kmol Br (∼26 t CHBr 3 ) a -1 . We estimate the global anthropogenic Br input from ballast water into the atmosphere of up to 13 Mmol a -1 . This is 0.1 % global Br input from background bromoform emissions and thus probably not relevant for stratospheric ozone depletion.