Express a mercury concentration of 9.6 ppm in parts per billion.

1. ppm scales the mass ratio by 10^6 and ppb scales it by 10^9. 2. One ppm therefore equals 10^3 ppb. 3. Multiply the ppm figure by 1000: 9.6 x 1000. 4. The concentration is 9600 ppb. 5. Since ppb is the smaller unit, the number must get larger, which rules out any answer below 9600. _Source: OpenStax Chemistry (CC BY 4.0), Ch 3 "Composition of Substances and Solutions", section 3.4 Other Units for Solution Concentrations_