Evidence for the loss of snow-deposited MSA to the interstitial gaseous phase in central Antarctic firn

We have examined several MSA (methanesulfonic acid) records from the upper 200 m of the Antarcticice sheet and in particular the new Dome F profile. At all the four sites studied, concentration profilesexhibit similar patterns as a function of depth. They suggest that snow metamorphism and solid phasemigration are responsible for a marked release of gaseous MSA to interstitial firn air as well as probablyto the free atmosphere, in particular at extremely low accumulation sites. Snow acidity can also modifyMSA concentration. It is proposed that, below the upper few metres where the communication withthe free atmosphere is possible, gaseous MSA may remain in the firn layers and be entrapped later inair bubbles at pore close-off, i.e. when firn is transformed into ice. Chemical measurements on the firncore do not take into account the MSA released to the gaseous phase, but this fraction is measurable inice samples. In spite of these alterations occurring in the firn layers, relative changes of the atmosphericMSA concentration in the past are probably still there deep within the Antarctic ice sheet. However, for glacial periods, different processes have to be considered in relation to modified aerosol properties.