Seasonal distribution of lowest OCS upper limit values from each position 11 measurement sequence, as a function of latitude (panel a) and altitude (panel b). Crosses denote upper limits of below 1σ significance, circles denote detections of between 1σ and 2σ, and squares denote detections of between 2σ and 3σ. Brown shaded regions indicate major dust storm events, as in Fig. 5. No positive detections of OCS above 3σ were found in the data.