The Model 306 Ozone Calibration Source is a highly portable ozone transfer standard that may used to calibrate any ozone monitor or sensor. Ozone is generated at any chosen concentration in the range 0-1000 ppb in ozone-scrubbed ambient air at a volumetric flow rate of 3 L/min. No external air source is required. The instrument may be programmed to output a series of up to 10 ozone concentration steps with a chosen time interval. The Ozone Calibration Source is provided with a NIST-traceable calibration, and the menu allows the user to calibrate the output to their own reference instrument. This Ozone Cal Source, which qualifies as an EPA transfer standard, is particularly useful for calibrating networks of ozone monitors in the field.