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TECHNICAL REPORTS
Atmospheric Pollutants and Trace Gases

Atmospheric Methyl Tertiary Butyl Ether (MTBE) at a Rural Mountain Site in California

University of California at Berkeley, Dep. of Environmental Science, Policy, and Management (ESPM), Ecosystem Science Division, 151 Hilgard Hall, Berkeley, CA 94720-3110

* Corresponding author (gws{at}nature.Berkeley.edu)

Received for publication August 15, 2001. Methyl tertiary butyl ether (MTBE) was measured in air samples collected at hourly intervals near Blodgett Forest Research Station on the western slope of the Sierra Nevada, California, in July 1997, October 1998, and June through September 1999. Mixing ratios ranged from below the detection limit (< approximately 0.01 ppbv) to 0.5 ppbv, but were generally less than 0.3 ppbv. At these mixing ratios partitioning of MTBE into surface waters would lead to MTBE concentrations of less than 0.2 µg L^-1. As expected, MTBE mixing ratios were highly correlated with other anthropogenically emitted hydrocarbons. Based on the observed diurnal cycle of MTBE and its ratio to 2-methyl-butane (isopentane), we estimated the average regional daytime oxidant concentration to be (9 to 13) x 10⁶ OH radicals per cubic centimeter, consistent with our earlier estimates for this region. Furthermore, MTBE ratios to toluene, another ubiquitous anthropogenic hydrocarbon, were generally consistent with regional transport and dilution, as well as atmospheric oxidation. Exceptions, pertaining to MTBE mixing ratios below or close to the detection limit, were associated with the influence of marine air masses that did not experience anthropogenic hydrocarbon input from California. With all these constraints in place, evidence for an additional atmospheric loss process, such as nonreversible deposition of MTBE, could not be established, and we conclude that any deposition is slow compared with removal from the atmosphere by the OH radical.

Abbreviations: MTBE, methyl tertiary butyl ether • ppbv, parts per billion by volume (1 ppbv MTBE = 3.61 µg m^-3)