The relevance of volatile arsenic emissions from volcanic areas

DFG PL 302/13-1

Arsenic is ubiquitous and one of the best investigated elements in rock, soil, or water. Little, however, is known about arsenic in the atmosphere. The quantitatively most important fluxes originate from point sources with the most dominant natural ones being volcanic areas. The fate in the atmosphere has previously been assumed to be determined by particulate arsenic while volatile arsenic species were neglected despite their high toxicity even at low concentrations. They were regarded as exotic, of low abundance, and with a life time too short to be of environmental relevance. Recent investigations, however, indicate that volatile arsenic stability was underestimated so far. A lack of sampling, stabilization, and analytical techniques has prevented calculating a species-selective mass balance of atmospheric arsenic release and distribution as well as differentiating abiotic and biotic formation mechanisms.

The hypothesis for the present project is that volatile arsenic species contribute significantly more to the global biogeochemical cycle and transportation distances are much larger than currently assumed. It is further postulated that besides primary abiotic release, microbial communities around volcanic sources cause secondary arsenic volatilization and determine its speciation.

The first goal is to develop a new, field-suitable method for volatile arsenic sampling using extraction traps with polymeric sorbents inside a stainless steel needle (Needle Trap Devices, NTDs). NTDs can be loaded by active pumping allowing flow quantification and calculation of absolute concentrations. NTDs are routinely applied in organic chemistry. Their potential for quantitative and species-conserving trapping of volatile arsenic is not known, yet. Sorbent material, pump rates, and storage conditions must be optimized and competitive effects by other volatile metal(loid)s or major volcanic gas components (H2O, SO2, H2S) must be eliminated. For analysis, a sophisticated coupling technique is applied (GC-MS split ICP-MS): After chromatographic separation the sample flow is split; mass spectrometry enables molecular identification of unknown species, inductively coupled plasma-MS enables element quantification.

The second goal is to reassess the relevance of volatile arsenic release and distribution from various sources in three volcanic areas, selected for their different levels of volcanic activity (Mt.Etna > Vulcano > Yellowstone National Park). Transects will be monitored to determine the fate of volatile arsenic in amount and speciation during transport. On-site incubation tests using extremophilic bacteria will show whether microbial volatilization of methylated arsenates and methylation of arsine in the gas phase occurs.

The overall goal is to provide a field method and show the role of volatile arsenic, exemplary in volcanic areas, thereby triggering a new assessment of the importance of volatile metal(loid)s for global biogeochemical cycles.