Effects of methane outgassing on the Black Sea atmosphere

[摘要] Methane in air and seawater was measured in the Eastern Black Sea during the10–18 December 1999 BIGBLACK project cruise. The measurements allowed for thecalculation of supersaturation ratios and methane fluxes across the air-seainterface. CH₄ mixing ratios in air were generally in the 1.8–2.0 ppmvrange, while surface (4 m depth) seawater concentrations varied from 4 to 93 ppmv. Above active seep areas, the water was supersaturated to around500% with respect to the overlying atmosphere. Accordingly, fluxdensities varied greatly and were up to 3300 umol m⁻² day⁻¹. Inthe Sevastopol harbour, supersaturations up to around 3000%, similar tothose at the Danube Delta, were observed, while in the Istanbul harboursupersaturations could not be determined because the very high values ofwater concentrations led to detector saturation. Simple modelling shows thatthe observed fluxes do not have any substantial impact on the methanecontent of the Black Sea atmosphere, as they would only raise itsconcentrations by less than 50 ppt. On the other hand, calculations performedas part of the CRIMEA project show that mud volcano eruptions couldepisodically raise the methane concentrations well above their regionalbackground for several tens of kilometres downwind. These calculations,which also apply to mud volcano eruptions elsewhere on the globe, indicatethat the spatial extend and the magnitude of the atmospheric perturbation issuch that its observation might lie within the capabilities of existingsatellite instrumentation such as SCIAMACHY on ENVISAT.

[学科分类] 大气科学