MTBE mixing ratios at a remote site in the Sierra-Nevada mountains, California: Implications for OH concentration in an urban plume
Gabrielle Dreyfus, Gunnar Schade, and Allen Goldstein
Mixing ratios of MTBE (methyl-tert-butyl-ether) were measured at a remote forest site in the Sierra-Nevada mountains, California (38o 53’ 42.9" N, 120o 37’ 57.9" W, 1315 m elevation), in June 1999, along with other C2-C10 hydrocarbons. MTBE has been used in California gasoline year round since 1996, this study, however, represents some of the first atmospheric MTBE measurements outside the main, mostly urban source areas. Median mixing ratios for MTBE, toluene, and pentane were highly correlated, suggesting a common source, and varied from 47 ppt, 27 ppt, and 19 ppt in the morning to 103 ppt, 59 ppt, and 44 ppt, respectively, in the late afternoon. Compared to MTBE concentrations in the Sacramento valley, this represents a reduction of approximately 90%. The diurnal pattern of mixing ratios is driven by daytime westerly winds transporting these compounds up-slope to the measurement site from an urban plume originating around Sacramento (~80 km up-wind), and nighttime winds in the opposite direction. The ratio of these species was used to make a preliminary estimate of the hydroxyl radical concentrations in the pollution plume. This calculation assumed a source toluene to MTBE ratio of 1.7, the pollution originated near Sacramento, and that loss was due entirely to reaction with OH. The resulting OH concentrations ranged from 3x106 to 1.5x107 molecules cm-3. A back-trajectory analysis of the polluted air mass should refine the assumption about the transport processes, potentially improving this estimate.
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