Emissions and Distribution of Reactive Iodine from Seaweed in Coastal Regions - Investigations using newmobile and in-situ DOAS techniques

Abstract

Reactive iodine species impact atmospheric chemistry in several ways. They play an important role in the process of ozone destruction at mid-latitudes and possibly in polar regions. Reactive iodine compounds (besides bromine) also affect the atmospheric cleaning mechanisms by changing its oxidation capacity. Recent field studies indicate that reactive iodine may impact the local climate in coastal areas by playing a key role in the formation of new particles which could influence cloud micro physical properties. Particularly high concentrations of the reactive iodine are found at mid-latitude coastal sites, which are most likely emitted by seaweed exposed to oxidative stress during low tide. However, there is still very limited knowledge on the involved seaweed species and their contribution to local, regional and global iodine emissions and also the potential iodine mediated particle formation. In the frame of this work a new mobile Open Path CE-DOAS instrument was developed for direct measurements of IO, one of the most important reactive iodine species. It was applied successfully in two field campaigns on the Irish west coast for measurements in the inter-tidal zone, directly above air-exposed seaweed patches. These measurements for the first time identify exposed seaweeds as IO hot-spots with very high IO mixing ratios often exceeding 50 ppt. This shows that local IO concentrations are sufficiently high to initiate the strong particle nucleation events previously observed at the Irish West Coast. Furthermore, during the field campaigns on the Irish west coast, combined LP-DOAS and CE-DOAS measurements were applied, which extended previous observations of reactive iodine at Mace Head and the close by MRI to further locations. These observations now cover ten shores. This study showed that the major sources of reactive iodine at the Irish West coast are wave sheltered shores and not, as previously believed, wave exposed shores, like Mace Head. Since seaweed beds at wave sheltered shores cove larger areas and are longer exposed to air, this shows that Irish, and most likely also global, iodine emissions from coastal areas should be much higher compared to previous estimates based on wave exposed shores. From this finding a yearly emission of 2*10^8 gI/yr to 3*10^9 gI/yr is estimated for global coastal areas. In a third field study on the east coast of the New Zealand south island, for the first time high IO mixing ratios of up to 68 ppt were observed on a southern hemispheric coast. Four, previously uninvestigated, seaweed species were identified as emitters of reactive iodine species and emission rates were estimated. The observations in New Zealand showed also distinct differences in the seaweed distribution to northern hemispheric locations which need to be considered in global estimates of coastal iodine emissions.