Description: Environmental context: Halocarbons are trace gases important in atmospheric ozone chemistry whose biogenic production – among other factors – depends on light-induced stress of marine algae. Several studies have confirmed this effect in laboratory experiments but knowledge in natural systems remains sparse. In mesocosm experiments, which are a link between field and laboratory studies, we observed that the influence of natural levels of ultraviolet radiation on halocarbon dynamics in the marine surface waters was either insignificant or concealed by the complex interactions in the natural systems. Abstract: The aim of the present study was to evaluate the influence of different light quality, especially ultraviolet radiation (UVR), on the dynamics of volatile halogenated organic compounds (VHOCs) at the sea surface. Short term experiments were conducted with floating gas-tight mesocosms of different optical qualities. Six halocarbons (CH3I, CHCl3, CH2Br2, CH2ClI, CHBr3 and CH2I2), known to be produced by phytoplankton, together with a variety of biological and environmental variables were measured in the coastal southern Baltic Sea and in the Raunefjord (North Sea). These experiments showed that ambient levels of UVR have no significant influence on VHOC dynamics in the natural systems. We attribute it to the low radiation doses that phytoplankton cells receive in a normal turbulent surface mixed layer. The VHOC concentrations were influenced by their production and removal processes, but they were not correlated with biological or environmental parameters investigated. Diatoms were most likely the dominant biogenic source of VHOCs in the Baltic Sea experiment, whereas in the Raunefjord experiment macroalgae probably contributed strongly to the production of VHOCs. The variable stable carbon isotope signatures (δ13C values) of bromoform (CHBr3) also indicate that different autotrophic organisms were responsible for CHBr3 production in the two coastal environments. In the Raunefjord, despite strong daily variations in CHBr3 concentration, the carbon isotopic ratio was fairly stable with a mean value of –26 ‰. During the declining spring phytoplankton bloom in the Baltic Sea, the δ13C values of CHBr3 were enriched in 13C and showed noticeable diurnal changes (–12 ‰ ± 4). These results show that isotope signature analysis is a useful tool to study both the origin and dynamics of VHOCs in natural systems.

Description: Environmental context: Once released to the atmosphere, halocarbons are involved in key chemical reactions. Stable carbon isotope measurements of halocarbons can provide valuable information on their sources and fate in the atmosphere. Here, we report δ13C values of 13 polyhalomethanes released from brown algae, which may provide a basis for inferring their sources and fate in future studies. Abstract: Halocarbons are important vectors of reactive halogens to the atmosphere, where the latter participate in several key chemical processes. An improved understanding of the biogeochemical controls of the production–destruction equilibrium on halocarbons is of vital importance to address potential future changes in their fluxes to the atmosphere. Carbon stable isotope ratios of halocarbons could provide valuable additional information on their sources and fate that cannot be derived from mixing ratios alone. We determined the δ13C values of 13 polyhalomethanes from three brown algae species (Laminaria digitata, Fucus vesiculosus, Fucus serratus) and one seagrass species (Zostera noltii). The δ13C values were determined in laboratory incubations under variable environmental conditions of light, water levels (to simulate tidal events) and addition of hydrogen peroxide (H2O2). The δ13C values of the polyhalomethanes ranged from –42.2 ‰ (±3.5 s.d.) for CHCl3 to 6.9 ‰ (±4.5) for CHI2Br and showed a systematic effect of the halogen substituents that could empirically be described in terms of linear free energy relationships. We further observed an enrichment in the δ13C of the polyhalomethanes with decreasing polyhalomethane yield that is attributed to the competing formation of halogenated ketones. Though variable, the isotopic composition of polyhalomethanes may provide useful additional information to discriminate between marine polyhalomethane sources.