Analysis of diurnal, seasonal and annual variations of fair weather atmospheric potential gradient at reduced number concentration of condensation nuclei from long-term measurements at Świder, Poland

Abstract. The ground-level atmospheric potential gradient (PG) has been measured with a radioactive collector method in Stanisław Kalinowski Geophysical Observatory in Świder (52.12° N, 21.23° E), Poland, for several decades. Long-term measurements analysed previously revealed rather typical behaviour in the diurnal and seasonal variations of the PG of a land station controlled by pollution. Observation of the potential gradient at such a station usually show a maximum at local winter months which are mostly affected by anthropogenic pollution. The 1965–2005 series has been newly analysed to describe the Świder PG variations in greater detail, also in connection with an analysis of simultaneous measurements of condensation nuclei measured at 6, 12, 18 UT. An attempt is made to calculate the diurnal and seasonal variations at condensation nuclei number concentrations below 10000 cm^-3. There is a decrease of the PG in the diurnal variation by up to 11 % in the winter, and no significant change in the summer. The reduction in the annual variation is 11–26 % with the biggest difference in February. In the summer months, this difference is negligible. Such differences can be predicted with a simplified model of electrical conductivity including the aerosol composition of water soluble and soot particles, the main components of continental aerosol. With this model we obtained changes in the conductivity and the PG in up to 30 % in the winter, and 6 % in the summer. Despite the efforts to minimise the aerosol effect on the PG, the character of the PG seasonal and annual variation preserves its character with a maximum in the Northern Hemisphere winter and the minimum in the summer.