Fig. 6. Dependence of the methyl tertiary butyl ether (MTBE) to toluene ratio on MTBE abundance for fall 1998 and summer 1999. The horizontal gray bar depicts the estimated emission ratio from the Caldecott tunnel experiments (Kirchstetter et al., 1999), the dotted line is a calculated, background-corrected ratio at Blodgett Forest assuming source emission ratio (ER_source) = 0.5, [OH] = 1.1 x 10⁷ molecules cm^-3, and t = 9 h.