Abstract. This paper describes the sources contributing to two seasonal peaks in colunmar SO2 amounts measured with a Brewer spectrophotometer at Thessaloniki, Northern Greece since 1982. The SO2 Brewer measurements combined with those at ground level, meteorological analysis and numerical simulations provide estimates on the contribution of local and remote sources to the SO2 column. It is shown that more than 50% of the observed SO2 column can be attributed to ligniteburning sources in Bulgaria, Romania and former Yugoslavia, this percentage rising to 70% at periods with NE flow at 850 hPa. Winds from the NW-N-NE contribute around 60% to the observed mean SO2 column during winter and 75% during the summer. When including all wind directions at 850 hPa, the Greek sources, including the lignite-burning power plant complexes to the WSW of the city, contribute around 40% to the SO2 colurn. These results are in qualitative agreement with independent observations from inversion of GOME measurements.

A numerical algorithm is developed to examine the transient processes in an electrolytic cell with blocking electrodes when an external electric field is switched on and switched off stepwise in time. The algorithm is applicable for arbitrary mobilities and valence numbers of the ions and for large electric potentials, but is limited at present to small cell thickness, up to about 20 D lengths. The ion charge fluxes and the displacement current contributing to the total current in the cell have been evaluated and analysed. The ion charge behaviour in the linear approximation regime (low excitation voltage) and nonlinear regime (high voltage) have been compared. The dependences of the polarization and diffusion relaxation times on the ion parameters (valence and mobility), on the cell length and on the external electric potential are studied.

The nonlinear differential equations, describing the migration and diffusion of ions in electrolytic cell with blocking electrodes, driven by external electric field, have been solved with the help of a numerical algorithm. Usually, the dynamical equations are simplified by applying the Einstein-Nernst relation between diffusion and mobility, although this relation is valid for stationary, timeindependent variables. In the present work, we have introduced correction terms, to take into account transient ion currents when external stepwise voltage is switched on. The correction terms are defined and numerically evaluated. The transient behavior of the system described without corrections is compared to the transients when corrections are applied. The results are examined for different regimes and parameters of the system.

Transient correction terms to the nonlinear differential equations, describing the dynamics of migration and diffusion of the ion charges in electrolytes, have been recently defined and numerically evaluated. The system of equations has been modified in accordance with the obtained non-equilibrium corrections and the system variables have been evaluated in the case when the corrections are space-averaged. The purpose of the present work is to obtain a general solution when both the time and space dependencies of the correction terms are preserved i.e. without space-averaging of the corrections. The obtained, under these conditions, set of dynamical equations has been analytically transformed to a simpler form, which is easier to be tackled numerically. The corrected results, in contrast to the results of uncorrected equations, show much faster convergence to equilibrium of the physical system and manifest the presence of characteristic pre-electrode maxima of the transient ion currents.

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