Filling in the Gaps in the Global Budget: Halocarbon Emissions from Salt-Affected Wetlands

Abstract

Saltwater intrusion resulting from sea level rise and storm surges transports large quantities of halides into coastal freshwater wetlands, creating salt-affected wetland environments. Reactions between halides and the carbon in the soil produce halogenated organic compounds, known as organohalogens, which can then be broken down into smaller, volatile halocarbons, specifically methyl bromide (MeBr) and methyl chloride (MeCl), that are released into the atmosphere and break down ozone through photolysis. The initial emissions of halocarbons from freshwater wetlands that are exposed to seawater for the first time are unable to be measured in the field and it is for this reason that salt-affected wetlands have been found to be a negligible source of MeBr and a small sink of MeCl. To more thoroughly capture these first exposure reactions, this study performed laboratory-based simulations of natural halogenation reactions by adding seawater to freshwater wetland soil. This study showed that exposure to halides increased the amount of halide incorporation in the soil with bromide (Br-) being incorporated preferentially over chloride (Cl-). Furthermore, this study showed that soil exposed to seawater for the first time emitted peak concentrations of halocarbons after several hours at rates greater than background emissions from known halocarbon sources. These first exposure reactions indicate that halocarbon emissions from salt-affected wetlands are significantly greater than what have been measured in the field and may fill in the current imbalances in the global budgets for these gases. These results suggest that further sea level rise from global warming could lead to greater concentrations of halocarbons in the atmosphere which would promote further ozone depletion.