The impact of El Chichon and Pinatubo volcanic eruptions on ozonesonde-derived ozone values in 8--16, 16--24 and 24--32 km layers is estimated for north polar and temperature zones, and the north temperate regions of North America, Europe and Asia, by minimizing the influence of solar cycle and trend, and adjusting for the quasi-biennial variation in ozone by use of the superposed epoch method. Based on this procedure, in north temperature 8--16 and 16--24 km layers the impact of El Chichon is a significant 10% ozone decrease 3--4 seasons after eruption, the impact of Pinatubo a significant 15% ozone decrease 7 seasons after eruption. In the 24--32 km layer the respective ozone decreases are 2% and 5%, the latter significant. In the north polar zone the ozone decreases following Pinatubo are a significant 20% and 10% in 8--16 and 16--24 km layers, but there are marginally significant ozone increases of 5% in the 24--32 km layer following both eruptions.