The distribution of beryllium‐7 in the troposphere: Implications on stratospheric/tropospheric air exchange

Abstract

The distribution of cosmogenic beryllium‐7 (7Be) in the lower troposphere is described based on the analysis of long‐term (1973‐1979) ground level 7Be measurements. 7Be is used as a tracer of stratospheric air to suggest that the annual mean outflow of stratospheric air in the northern hemisphere can be 20 to 30% higher than in the southern hemisphere. This asymmetry is maximum during the northern hemisphere spring and reaches its peak in April. Over North America, the 7Be concentrations show a high positive correlation (linear coefficients of 0.8 to 0.9) with the occurrence of tropospheric low‐pressure troughs over several latitude belts, thus supporting the postulate that tropopause folding events, associated with low‐pressure troughs, are the dominant stratospheric/tropospheric (S/T) exchange mechanism. It is demonstrated that S/T air exchange shows a distinct seasonal variation that is highly dependent on the latitude. At midlatitudes (40‐50°N) over North America, the S/T air exchange is maximum in summer; at low altitudes (20‐30°N), it is maximum in winter and early spring. The study suggests a mechanistic model that can be used to characterize S/T air exchange at any geographical location using routinely available meteorological information.