Why are some pollutants measured in both parts per million and parts per billion?
The measurement scale is usually determined by the expected concentrations, the standard for a particular pollutant and the type of monitor that is being used. In most cases where data is being collected to determine compliance with a standard or to monitor the impact of a large source, a standard monitor will be used and data will be collected in the parts per million scale. If data is being collected in a rural location for background concentrations or for research work a trace level monitor may be used and the data will be collected in parts per billion. In those cases the monitors are usually more sensitive and designed to work at very low concentrations. Data reported as parts per million can be converted to parts per billion by moving the decimal point three places to the right.
