Halogen chemistry: from ocean to stratosphere

State University of New York, USA

 

Title: Sources and distribution of non-volatile halogenated organic matter in the ocean

 

Abstract: Marine dissolved organic matter (DOM) is one of Earth’s major carbon reservoirs and is arguably its most dynamic. On the other hand, radiocarbon measurements suggest that the majority of marine DOM is stored for millennia in the deep ocean as refractory dissolved organic carbon (RDOC). Thus, it is often assumed that RDOC, and marine DOM more generally, can persist for thousands of years, but this generalization ignores differences in marine DOM sources, complexity, and reactivity throughout different ocean regions. In fact, despite years of intensive study, pathways and rates of DOM production and removal are still poorly constrained. In this talk, we will discuss why RDOC may not be as stable as we currently assume and explore a largely ignored component of the marine DOM pool – nonvolatile halogenated DOM. While the focus on carbon sequestration is justified, the links between carbon cycling and halogen biogeochemistry have not been established in terms of DOM research, even though it is well-known that abiotic and biological processes can convert halides into reactive halogen species. We will therefore discuss why halogenated DOM may be a large and reactive component of the overall DOM pool, which could provide new insights into DOM reactivity.

 

University of York, UK

 

Title: Developing a new coupled ocean-atmosphere exchange model for iodine emissions

 

Abstract: Iodine in the atmosphere and at the ocean-atmosphere interface is a large sink for tropospheric ozone. Inorganic iodine emissions from the ocean are the largest source of atmospheric iodine. The chemical reaction of ozone and iodide to form HOI at the ocean-atmosphere interface is a large sink for tropospheric ozone, accounting for approximately one-quarter of all ozone loss to deposition. Subsequent aqueous chemistry produces I₂; both of these compounds are then emitted into the marine boundary layer. In the atmosphere, HOI and I₂ are readily photolysed to atomic iodine, then rapidly oxidised by O₃ before cycling back to atomic iodine. To better understand the indirect climate and air quality impacts of iodine emissions, oceanic emissions need to be well-constrained in global models. We have developed and tested a new box model coupling ocean-atmosphere exchange of ozone with inorganic iodine emissions, incorporating recent advancements in the understanding of inorganic SML chemistry. Compared to previous modelling work, the new model has markedly different wind speed dependences, no longer tending to unphysically high emissions at low wind speeds like previous work. We also present a new parameterised equation for inorganic iodine emissions derived from this SML model for use in global models.

 

Institute for Interdisciplinary Science, National Research Council, School of Natural Sciences, National University of Cuyo, Argentina

 

Title: Reactive transport of brominated and iodinated VSLS from the ocean surface to the stratosphere

 

Abstract: The photochemical degradation of ocean-emitted very short-lived halogenated substances (VSLS), mostly brominated and iodinated halocarbons with lifetimes smaller than 6 months, leads to ozone destruction both in the troposphere and stratosphere, altering the oxidative capacity of the whole atmosphere. VSLS are mostly produced in the ocean by biological and photochemical processes, although the anthropogenic production of chlorinated halocarbons has significantly increased in the last decade. Given the widespread and variable distribution of VSLS sources, as well as their different regional and seasonal degradation regimes in the troposphere, addressing the influence of VSLS on lowermost stratospheric ozone requires a detailed representation of the complex photochemical conversion of VSLS source gasses into reactive halogen species within coupled chemistry-climate models, which controls the net Source Gas Injection (SGI) and Product Gas Injection (PGI) of halogens to the stratosphere. These include not only gas-phase and heterogeneous-phase recycling reactions, but also the coupling of halogen reservoir species with anthropogenic air pollutants that have amplified the emission strength of natural halogens in the marine boundary layer. The seminar will provide a brief description of the reactive transport of brominated and iodinated VSLS from the ocean surface to the lower stratosphere, highlighting the main processes controlling the release of reactive halogens to the troposphere, as well as their influence on stratospheric ozone depletion.