Marine Emissions of Halogenated Trace Gases from the Tropical Ocean
Oceanic bromoform (CHBr3) and dibromomethane (CH2Br2) are the largest contributors to organic bromine in the atmosphere, while atmospheric organoiodine is significantly influenced by marine methyl iodide (CH3I) and diiodomethane (CH2I2). Halogenated hydrocarbons (halocarbons) and their degradation products are involved in ozone chemistry in both the troposphere and stratosphere. With decreasing anthropogenic atmospheric halocarbons, the impact of naturally produced halocarbons on atmospheric processes will likely increase. Many uncertainties still exist with regard to their production and degradation in the ocean, as well as to their emissions. While macroalgae have been identified as important sources of them, microalgae were shown to be halocarbon producers as well. Hence, oceanic upwelling systems might play a crucial role for open ocean emissions. The tropical ocean has not only been hypothesized to contribute largely to global halocarbon emissions, but it also may contribute to their transport into upper atmospheric layers. They may be transported in significant amounts into the tropical stratosphere by tropical deep convection. This thesis aims at reducing some of the uncertainties regarding halocarbon emissions from the tropical ocean to understand their role in a future climate. Two campaigns are covered here: MSM18/3 onboard RV Maria S. Merian investigating the Eastern tropical equatorial Atlantic during the cooling season in June and July 2011, and DRIVE (Diurnal and RegIonal Variability of halogen Emissions) onboard RV Poseidon, which focused on the Mauritanian upwelling region in June 2010. Oceanic and atmospheric halocarbon data, biological, meteorological and oceanographic parameters were collected to investigate impact factors on halocarbon emissions. The oceanic and atmospheric data were also included in the most complete halocarbon database so far, HalOcAt (Halocarbons in the Ocean and Atmosphere). Manuscripts, prepared and published on the basis of this data set, include the first manuscript (Hepach et al., in prep) that focuses on the first measurements of CHBr3, CH2Br2, CH3I and CH2I2 in the surface and the water column of the equatorial Atlantic during the Atlantic Cold Tongue (ACT) season. The second (Hepach et al., 2014) and third manuscript (Fuhlbrügge et al., 2013) cover oceanic and atmospheric abundances of CHBr3, CH2Br2 and CH3I in the Mauritanian upwelling region on a diel and regional scale. While the second manuscript investigates impact factors on emissions of these compounds, the third manuscript analyzes meteorological constraints on atmospheric halocarbons. The fourth manuscript (Ziska et al., 2013) uses the HalOcAt database to determine global emissions of CHBr3, CH2Br2 and CH3I, and estimates global contributions from different regions. In the fifth manuscript (Stemmler et al., 2013), depth profiles of CH3I measured during the DRIVE campaign are used to validate modeled profiles from the tropical open ocean using the General Ocean Turbulence Model (GOTM). The transport of emissions of CH3I into the stratosphere is calculated in the sixth manuscript (Tegtmeier et al., 2013), indicating that CH3I from the DRIVE campaign is entrained in small amounts into the stratosphere. Both upwelling systems, the Mauritanian upwelling and the equatorial Atlantic, were shown here to be source regions for CHBr3 and CH2Br2, contributing to the large emissions of these compounds from the tropical ocean. While CH3I has been found to be ubiquitously distributed in the Mauritanian upwelling region hinting towards photochemical formation there, strong implications for biological formation were found in the ACT. This agrees well to the modeled depth profiles of CH3I indicating they may be influenced both by photochemical and biological formation of this compound. Although it has been hypothesized that the tropical ocean may not contribute to CH2I2 emissions to the atmosphere due to its very rapid photolysis, CH2I2 could be detected in low concentrations in the surface water of the ACT. The first determination of diapycnal fluxes of CHBr3, CH2Br2, CH3I and CH2I2 in the ACT indicate that their production takes place within the mixed layer regardless of deeper biomass maxima, which may be very important for their emissions. In the Mauritanian upwelling, oceanic halocarbon production was identified as the main driver of halocarbon emissions with wind speed having impact on a diel scale. For the first time, the height of the Marine Atmospheric Boundary Layer (MABL) has been found to indirectly impact halocarbon emissions due to its decreasing and increasing effect on atmospheric halocarbons. Together with enhanced emissions of halocarbons, the largely elevated atmospheric halocarbons above the Mauritanian upwelling could be explained solely by local emissions in contrast to previous hypotheses. This process could be of importance in other coastal upwelling systems as well.
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