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. Ballast water treatment is required for vessels toprevent the introduction of potentially invasive neobiota. Some treatmentmethods use chemical disinfectants which produce a variety of halogenatedcompounds as disinfection by-products (DBPs). One of the most abundant DBPs from oxidative ballast water treatment is bromoform (CHBr3), for which wefind an average concentration of 894±560 nmol L−1 (226±142 µg L−1) in the undiluted ballast water frommeasurements and the literature. Bromoform is a relevant gas for atmosphericchemistry and ozone depletion, especially in the tropics where entrainmentinto the stratosphere is possible. The spread of DBPs in the tropics overmonths to years is assessed here for the first time. With Lagrangiantrajectories based on the NEMO-ORCA12 model velocity field, we simulate DBPspread in the sea surface and quantify the oceanic bromoformconcentration and emissions to the atmosphere from ballast water discharge atmajor harbours in the tropical region of Southeast Asia. The exemplarysimulations of two important regions, Singapore and the Pearl River Delta,reveal major transport pathways of DBPs and anthropogenic bromoformconcentrations in the sea surface. Based on our simulations, we expect DBPsto spread into the open ocean, along the coast and through advection withmonsoon-driven currents into the North Pacific and Indian Ocean.Furthermore, anthropogenic bromoform concentrations and emissions arepredicted to increase locally around large harbours. In the sea surfacearound Singapore, we estimate an increase in bromoform concentration by 9 % compared to recent measurements. In a moderate scenario in which 70 % ofthe ballast water is chemically treated, bromoform emissions to theatmosphere can locally exceed 1000 pmol m−2 h−1 and doubleclimatological emissions. In the Pearl River Delta all bromoform is directlyoutgassed, which leads to an additional bromine (Br) input into theatmosphere of 495 kmol Br a−1 (∼42 t CHBr3). ForSingapore ports the additional atmospheric Br input is calculated as 312 kmol Br a−1 (∼ 26 t CHBr3). We estimate a globalanthropogenic Br input from ballast water into the atmosphere of up to 13 Mmol a−1. This is 0.1 % of global Br input from background bromoformemissions and thus not relevant for stratospheric ozone depletion.

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