We present a new approach to calculate a trace gas budget for the lowermost stratosphere based on airborne CO measurements. The measurements were obtained during the SPURT-project (SPURT in German: Trace gas transport in the tropopause region) and covered the tropopause region over Europe from 35¿--75¿N during different seasons. Using CO as a tracer for tropospheric air we quantified the contribution of three source regions to the composition of the lowermost stratosphere. Influence from the extratropical troposphere mainly contributes to the mixing layer above the local tropopause, where it decreases rapidly to less than 25% within a potential temperature range of 25 K above the local tropopause. With increasing distance from the tropopause mixing layer, the tropospheric fraction is dominated by influence from the tropics. During winter and spring the latter contributes about 35% to the chemical composition of the lowermost stratosphere over Europe, increasing to about 55% in summer and autumn.