In the year 2000, the Guinness Book of World Records called ethyl mercaptan,C₂H₆S , the smelliest substance known. The average person can detect its presence in air at levels as low as 9 x 10^-4 μmol/m³. Express the limit of detectability of ethyl mercaptan in parts per billion (ppb). (Note: 1 ppb C₂H₆S means there is 1 g C₂H₆S per billion grams of air.) The density of air is approximately 1.2 g/L at room temperature.