An investigation of the origins of reactive gaseous mercury in the Mediterranean marine boundary layer

Abstract

Atmospheric mercury species concentrations were measured during two oceanographic cruise campaigns covering the Adriatic Sea, the first during the autumn in 2004 and the second in the summer of 2005. The inclement weather during the autumn campaign meant that no clear in-situ production of oxidised gas phase mercury was seen. Events where high values of Hg^II_(g) and/or Hg associated with particulates (Hg^P) were observed, could be linked to probable anthropogenic emission source areas. During the summer campaign however, the by now rather familiar diurnal variation of Hg^II_(g) concentration, with maxima around midday, was observed. Again there were events when high Hg^II_(g) and particulates (Hg^P) concentrations were seen which did not fit with the pattern of daily in-situ Hg^II_(g) production. These events were traceable, with the help of back trajectory calculations, to areas of anthropogenic emissions. The back trajectories for all the events during which high Hg species concentrations were encountered showed that the airmass being sampled had passed near port areas in the previous 24 h. Not all these ports are associated with major industrial installations, it is possible therefore (bearing in mind the uncertainty associated with the back trajectory calculations) that either shipping or port activities are a Hg source. Box modelling studies of the summer 2005 campaign show that although the in-situ production of Hg^II_(g) occurs in the MBL, the exact chemical mechanism responsible is difficult to determine. However given the high O₃ concentrations encountered during this campaign it seems clear that if Hg⁰ does react with O₃, it does not produce gas phase Hg^II. Equally, the reaction between Hg⁰ and OH if it occurs, does not contribute appreciably to Hg^II_(g) production.