Not sure I understand this. Alumin(i)um, chemically combined, actually is about 8% of the earth's crust, but "micrograms/L" (microgram/liter) is not a dimensionless ratio like ppm or ppb. A gram is a unit of mass while a liter is a unit of volume now equal to 0.001 m3. (It used to be defined as the volume of 1 kg of water at 4C and standard pressure, which is almost exactly but not quite the same.)
Sea level air has a density of roughly 1 g/L or 1 kg/m3 (actually 1.2) while liquid water is about 1 kg/L or 1000 kg/m3, almost 1000 times as dense. So I suppose micrograms/L is equivalent to parts per billion when talking about water, and it is roughly equivalent to parts per million when talking about sea level air. That uses the American (not old British) definition of billion, i.e., 109 or giga.
Was it Churchill who said that Americans and the British are two peoples separated by a common language? A few English words do get us in trouble when we don't realize they mean different things on opposite sides of the Atlantic, the classic example being the verb table. (Never mind, Chew, I see you're another Yank -- in both the US and non-US meanings of the word.)