Episodicity of Sulphate Deposition in Finland

A characteristic of daily acid deposition data, in particular at remote sites, is the occurrence of some extreme peaks above the base level of moderate values. A large proportion of the annual acid deposition is accumulated in a few days: 5 worst days at the Finnish background stations can bring 20–30 % of the annual bulk sulphate load. The aim of this study was to determine whether there is a difference between stations in the number of extreme daily bulk acid depositions. The bulk deposition of sulphate in the 1990's at five Finnish monitoring stations was used as a reference. Days on which the deposition value exceeded the annual median value of the station by a factor of 10 were counted as episode days. The number of episode days in central and northern Finland differed significantly (p<0.05) from the number in the south, when tested with the non-parametric Kruskal-Wallis test. The episodes cannot be completely explained by high precipitation amounts: the mean sulphate concentration in episodic rain was higher than the annual mean concentration. The episodes were mostly imported: the air mass arriving at the station had passed over areas with high emissions outside Finland. A more exact knowledge of the episodicity benefits an examination of the ecological stress caused by acid deposition as well as a study of the critical load and the crossing of this threshold.