Odor threshold studies performed with gasoline and gasoline combined with MTBE, ETBE (ethyl-tert-butyl ether) and TAME, with cover letter dated 02/16/95
The Clean Air Act Amendments of 1990 require that gasoline sold In areas of nonattainment for carbon monoxide or ozone contain specified amOullts of fuel oxygenates. Fuel oxygenates Include, methyl-tertiary-butyl ether (MTBE), ethyltertiary-butyl ether (ElBE). and tertiary-amyi-methyl ether (TAME). These oxygenated compounds Increase the oxygen content of fuels. producing a more complete combustion, resuhlng In a reduction In carbon monoxide emissions. Oxygenated compounds ~uch as MTBE have been previously added to gasoUne to enhance octane ratings. More recently. larger amounts of oxygenates, MTBE, In particular. have been added to fuels to meet Clean Air Act Amendment requirements. This study examines the effect of oxygenate addition on the odor of gasoline blends.
Three blends of gasoline (summer. winter and a wcomposite") were evaluated for their odor detection and recognition thresholds In air. These gasolines were also combined wilh the gasoDne oxygenates MTBE. ETBE N TAME to e.,a/uate the effect of the oxygenates on the gasollnes' odor detection and recognition thresholds. Additionally, commercial" grade MTBE (97% pure. obtained from ARCO Chemical Co.) was evaluated for its odor detection and recognition thresholds In air and water as well as Its taste threshold In water. The detection threshold is defined as the minimum concentration at which 50 percent of a given population can differentiate between a sample containing the odorant and a sample of odor free air. The recognition thre5hold value Is defined as the minimum concentration at which 50 percent of a given population can recognize or Identify the odorant. These evaluations were conducted at TRC Environm9ntal Corporation's (TRC's) Odor Laboratory In Windsor. Connecticut.
The average detection and recognition threshold values for commercial grade MTBE were determined to be 0.053 and 0.125 parts-par-million (ppm), respectively. The average detection and recognition threshold values for this MTBE In water were determined to be 0.045 and 0.055 ppm, respectively. In general, compounds with odor thresholds balow'1 ppm are considered highly odorous. The panelists descriptions of MTBE's odor Included alcohol, chemical, ather and butane. Finally, the average taste detection threshold value for this oxygenate was determined to be 0.039 ppm. The panelists found the taste of MTSE to be highly objectionable. The average detection and recognition threshold values for the headspace vapor of the three gasoline blends are as follows: summer blend· 0.576 and 0.802 ppm, respectively; winter blend· 0.479 and 1.121 ppm, respectively; and -compositeblend· 0.474 and 0.765 ppm, respectively. In generl1l, the panelists described all three blends as smelling like gasoline.
The average detection and recognition threshold values for the headspace vapor of the gasoline-oxygenate mixtures are as follows: summer blend + 3% MTBE (97% purity) • 0.5 and 0.696 ppm, respectively; summer blend + 11% MTBE (97% purity) 0.275 and 0.710 ppm, respectively; summer blend + 15% MTBE (97% purity) - 0.264 and 0.68e ppm, respectively; summer blend +15% MTBE (99% purity) ·0.113 and 0.358 ppm, respectively. The odors associated with these mixtures Included organic volatile, gasoline, ether, car exhaust, sweet gasonne and gasoline with ather. The summer blend of gasoline was also mixed with 15% ETBE (99% purity) and also with 15% TAME (94% purity). The average detection and recognition threshold values for these mixtures are 0.064 and 0.139 ppm (summer blend + ElBE) and 0.114 and 0.207 ppm (summer blend + TAME). The odors the pane6sts associated with these mixtures Included ether, gasoline, chemical with gasoline, cleaning fluid and natural gas.
The winter and composite gasolines were each mixed with 15% MTBE (97% purity), respectively. The average detection and recognition threshold values for these mixtures were 0.219 and 0.398 ppm (winter blend MTBE) and 0.085 and 0.185 ppm (composite blend + MTBE), respectively. The odor of the winter gasoline - MTBE mixture was associated with gasoline, chemical and ether by the panelists. The odor of the "composite" gasoline - MTBE mixture was associated with gasoline. gasoline with ether. and permanent marker by the panalists