A reanalysis of carbonyl sulfide as a source of stratospheric background sulfur aerosol

This study investigates the importance of carbonyl sulfide (OCS) in the formation of stratospheric background sulfur aerosol. Specific questions examined include the loss rate in the stratosphere, the net flux into the stratosphere, and the contribution of OCS to the stratospheric background sulfur aerosol. From an analysis of current atmospheric measurements of OCS, the total amount of OCS in the atmosphere is evaluated to be 5.2 Tg. Of this total, 4.63 Tg is in the troposphere and 0.57 Tg in the stratosphere. Based on the estimated global OCS source strength of 1.2 Tg yr −1 , the global atmospheric lifetime of OCS is estimated to be 4.3 years. Using a one‐dimensional photochemical model, the stratospheric photochemical lifetime of OCS is estimated to be 10 years, more than 2 times longer than its global atmospheric lifetime. These results suggest that most of the OCS transported into the stratosphere returns to the troposphere where it is mainly taken up by surface vegetation. The production of stratospheric background sulfur aerosol from OCS oxidation is calculated to be 3.0×10 10 gS yr −1 , 2 to 5 times smaller than the most recent estimates of the amount of sulfur required to maintain the stratospheric background aerosol level. Possible explanations for this difference include (1) an overevaluation of the nonvolcanic background aerosol burden; (2) an underevaluation of the lifetime of stratospheric background aerosol; (3) the presence of other sulfur sources such as high‐altitude aircraft emissions; or (4) that the stratospheric OCS database used in our analysis is flawed with a substantial yet unidentified systematic error.