In air quality reporting, what units are commonly used for ozone concentration and what is the primary 8-hour standard?

• A. Parts per trillion; 7 ppb.
• B. Parts per billion; 70 ppb.
• C. Parts per million; 0.07 ppm.
• D. Parts per cubic meter; 0.07 g/m3.

Answer: (B)

Ozone concentration in air quality reports is typically expressed as a mixing ratio in parts per billion. The primary eight-hour standard used by U.S. air quality programs is 70 ppb, which is the same as 0.070 ppm. This means that the eight-hour average concentration should stay below 70 ppb to meet the standard. The other options use less common or inappropriate units for ambient ozone reporting, or mix the same value in a different unit (0.07 ppm is equivalent to 70 ppb, but the standard is usually cited as 70 ppb).