Anthropogenic bromoform from industrial water treatment
The control and ban of man-made long-lived ozone-depleting substances (ODSs) by the Montreal Protocol is expected to result in the recovery of the stratospheric ozone layer in the course of the 21st century. However, short-lived ODSs, which belong to the very short-lived substances (VSLSs), are not controlled and also participate in stratospheric ozone depletion through catalytic cycles. VSLSs are produced naturally from macroalgae and phytoplankton, and anthropogenically as disinfection by-products (DBPs) from chemical treatment of industrial water. Chemical treatment of seawater, e.g. cooling water of coastal power plants, mainly produces brominated VSLSs, with bromoform as the major DBP. Bromoform is also the largest source of organic bromine from biological marine sources to the atmosphere. However, the contribution of anthropogenic VSLSs to the global bromine budget is still unclear. Industrial water treatment has increased substantially over the last years due to strong economic growth and progressing industrialisation in East Asia and India. The increasing input of anthropogenic VSLSs from emerging industries to the environment has not been quantified yet. Given the growing importance of brominated VSLSs in the face of declining long-lived ODSs, a quantification of anthropogenic VSLS sources is urgently needed. The aim of this thesis is to quantify the environmental input of anthropogenic brominated VSLSs from industrial water treatment, their distribution in the ocean and atmosphere, and their entrainment into the stratosphere. The assessment focusses on the major DBP bromoform. The global distribution of anthropogenic bromoform sources serves as the initial release field for Lagrangian simulations of bromoform pathways in the ocean and atmosphere, and as the basis for air-sea flux calculations. Oceanic trajectory simulations are performed with the ARIANE software to analyse seasonal to annual variations of bromoform distribution. Atmospheric bromoform mixing ratios are simulated with the FLEXPART model in order to analyse the tropospheric and stratospheric distribution. Combining the climatological bottom-up air sea flux estimate with the bromoform flux from coastal power plants, the global bromine budget increases by 8–35 % to 1.9–2.2 Gmol Br a-1. Over 96 % of treated cooling water originate from the regions East-Southeast Asia, Europe, North America, India and Arabia. Bromoform is usually released close to its oceanic source. An effective transport into the stratosphere mainly takes place in the tropics, whereas in extratropical regions the majority of bromoform stays in the lower troposphere and is rapidly removed by deposition, e.g. in Europe. About half of the global anthropogenic bromoform is released in the region around East-Southeast Asia. The majority of this is discharged in the extratropics along the coasts of the Yellow, Japan and East China Seas. Still about 20 % of the bromoform from this region is entrained into the stratosphere during boreal winter due to transport towards the tropical West Pacific by northeasterly winds. The tropical West Pacific, as well as the Indian Ocean are the most efficient regions for stratospheric entrainment of bromoform. Over the Indian Ocean, tropical convection and the monsoon circulation during boreal summer transport 40–50 % of bromoform from the marine boundary layer to the stratosphere. Thereby, anthropogenic emissions contribute 10–43 % to stratospheric bromoform entrainment over the Indian Ocean, and 4–25 % over the global inner tropics. The anthropogenic sources of brominated VSLSs are predicted to increase in the future. In particular, increased bromoform emissions from growing industries in tropical regions will likely lead to more bromine input into the stratosphere. In order to reduce the uncertainties of DBP concentration in treated seawater, as well as the number of missing sources in the bromoform air-sea flux estimate, additional observations along the coasts close to the industrial areas are necessary. A better understanding of all natural and anthropogenic sources of brominated VSLSs and their future trends will thus improve estimates of the global atmospheric bromine input.