Fig. 4. (a) Median diurnal mixing ratio variation of methyl tertiary butyl ether (MTBE), isopentane, and n-pentane above their regional background levels (see legend), together with the median wind direction pattern for 17–20 June 1999 (thick line). (b) Median diurnal change of the isopentane to MTBE ratio for that time, including the background ratio (dotted line).