Stratospheric and tropospheric components of ⁷Be in surface air

Abstract

7 Be and 90 Sr concentrations were measured simultaneously in samples from NASA's Global Atmospheric Sampling Program. The samples were collected from January 1978 through June 1979 at 9 to 12 km and between 11° and 60°N. 7 Be concentrations ranged from 150 to 6900 fCi/m 3 , while 90 Sr correspondingly varied from 10 to 320 fCi/m 3 . The average 7 Be concentration in the samples collected in the lower stratosphere was 4500 fCi/m 3 . In the upper troposphere, 7 Be concentrations were generally less than 1000 and often only a few hundred fCi/m 3 . In situ production of 7 Be in the upper troposphere at ∼11 km was estimated at 340 fCi/m 3 . Since 90 Sr is only present in the atmosphere because of nuclear testing, except immediately after such tests, 90 Sr in the troposphere is due only to the subsidence of stratospheric air. Since atmospheric processes are not expected to alter 7 Be/ 90 Sr ratios during transport, surface 90 Sr and stratospheric 7 Be/ 90 Sr ratios can be used to estimate 7 Be transported from the stratosphere. Our analyses show that on an annual basis the stratosphere contributed ∼25% of the observed 7 Be concentration. During spring and summer months, the stratospheric component can approach 40%, while on a daily basis this can be even larger.