Free tropospheric reservoir of natural sulfate

¹⁰Be is used as a spike of the natural background atmospheric aerosol to calculate the global flux of sulfur (F_S) into the free troposphere. The sulfate and¹⁰Be concentrations determined in polar snow are compared. On the basis of an annual¹⁰Be production rate of 1.21 10⁶ at.cm^-2, a very low figure of 2.9 Tg S a^-1 is calculated for F_S, which suggests that most of the sulfur emitted at ground level remains in the boundary layer. The role of OCS in the upper tropospheric sulfur budget is reviewed. It is also shown that cataclysmic volcanic eruptions may disturb considerably for 1–2 years this vast background tropospheric sulfur reservoir.