Tracer correlations in the northern high latitude lowermost stratosphere: Influence of cross‐tropopause mass exchange

We present an analysis of trace gas correlations in the lowermost stratosphere. In‐situ aircraft measurements of CO, N 2 O, NO y and O 3 , obtained during the STREAM 1997 winter campaign, have been used to investigate the role of cross‐tropopause mass exchange on tracer‐tracer relations. At altitudes several kilometers above the local tropopause, undisturbed stratospheric air was found with NO y /NO y * ratios close to unity, NO y /O 3 about 0.003–0.006 and CO mixing ratios as low as 20 ppbv (NO y * is a proxy for total reactive nitrogen derived from NO y –N 2 O relations measured in the stratosphere). Mixing of tropospheric air into the lowermost stratosphere has been identified by enhanced ratios of NO y /NO y * and NO y /O 3 , and from scatter plots of CO versus O 3 . The enhanced NO y /O 3 ratio in the lowermost stratospheric mixing zone points to a reduced efficiency of O 3 formation from aircraft NO x emissions.