A REANALYSIS OF CARBONYL SULFIDE AS A SOURCE OF STRATOSPHERIC BACKGROUND SULFUR AEROSOL

This study investigates the importance of carbonyl sulfide (OCS) in th e formation of stratospheric background sulfur aerosol. Specific quest ions examined include the loss rate in the stratosphere, the net flux into the stratosphere, and the contribution of OCS to the stratospheri c background sulfur aerosol. From an analysis of current atmospheric m easurements of OCS, the total amount of OCS in the atmosphere is evalu ated 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 st rength of 1.2 Tg yr(-1), the global atmospheric lifetime of OCS is est imated to be 4.3 years. Using a one-dimensional photochemical model, t he stratospheric photochemical lifetime of OCS is estimated to be 10 y ears, more than 2 times longer than its global atmospheric lifetime. T hese results suggest that most of the OCS transported into the stratos phere returns to the troposphere where it is mainly taken up by surfac e vegetation. The production of stratospheric background sulfur aeroso l from OCS oxidation is calculated to be 3.0 x 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. Poss ible 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 wit h a substantial yet unidentified systematic error.