10Be/7Be tracer of atmospheric transport and stratosphere‐troposphere exchange

Abstract

The 10Be/7Be ratio is a sensitive tracer of atmospheric transport and stratosphere‐troposphere exchange (STE). Data from five NASA aircraft field missions (PEM: West A and B, Tropics A; SONEX; and SUCCESS) have been assembled to produce the largest data set of 10Be, 7Be, and their ratio collected to date (>300 samples). Ratios near 0.60 are indicative of tropospheric air with little stratospheric influence, while higher ratios are found in stratospheric air. Samples from the lower stratosphere were all collected within 2.5 km of the tropopause and had ratios >1.27. Of these lower stratosphere samples only 16% had ratios in excess of 3.0, suggesting that higher ratio air resides away from the tropopause. Seasonality observed in the 10Be/7Be ratios results from the downwelling of air with elevated ratios from higher in the stratosphere in the spring and summer (midlatitudes) and from the decay of 7Be during descent in the winter polar vortex (high latitudes). Our results illustrate the complexity of STE and some of the mechanisms through which it occurs, including tropopause folding, mixing associated with subtropical jets, and the effect of synoptic systems such as hurricanes and northeasters. The 10Be/7Be ratio provides important information beyond that which can be derived from studies that rely on chemical mixing ratios alone.