Iodine and bromine speciation in snow and the effect of orographically induced precipitation

[摘要] Iodine is an essential trace element for all mammals and may also influenceclimate through new aerosol formation. Atmospheric bromine cycling is alsoimportant due to its well-known ozone depletion capabilities. Despiteprecipitation being the ultimate source of iodine in the terrestrialenvironment, the processes effecting its distribution, speciation andtransport are relatively unknown. The aim of this study was to determine theeffect of orographically induced precipitation on iodine concentrations in snowand also to quantify the inorganic and organic iodine and bromine species.Snow samples were collected over an altitude profile (~840 m) from thenorthern Black Forest and were analysed by ion-chromatography - inductivelycoupled plasma mass spectrometry (IC-ICP-MS) for iodine and bromine speciesand trace metals (ICP-MS). All elements and species concentrations in snowshowed significant (r²>0.65) exponential decrease relationships withaltitude despite the short (5 km) horizontal distance of the transect. Infact, total iodine more than halved (38 to 13 nmol/l) over the 840 m heightchange. The results suggest that orographic lifting and subsequentprecipitation has a major influence on iodine concentrations in snow. Thisorographically induced removal effect may be more important than lateraldistance from the ocean in determining iodine concentrations in terrestrialprecipitation. The microphysical removal process was common to all elementsindicating that the iodine and bromine are internally mixed within the snowcrystals. We also show that organically bound iodine is the dominant iodinespecies in snow (61–75%), followed by iodide. Iodate was only found intwo samples despite a detection limit of 0.3 nmol/l. Two unknown but mostlikely anionic organo-I species were also identified in IC-ICP-MSchromatograms and comprised 2–10% of the total iodine. The majority ofthe bromine was inorganic bromide with a max. of 32% organo-Br.

[学科分类] 大气科学