As part of quality assurance performance audits on three U.S. Environmental Protection Agency ambient air monitoring projects, the suppression in the total sulfur response due to carbon dioxide was measured on flame photometric detector (FPD) total sulfur analyzers. Each analyzer was challenged at the 50, 100, and 150 ppb SO2 level using air containing 0, 360, and 750 ppm CO2. Four different models of FPD total sulfur analyzers were evaluated. Six or more analyzers were evaluated for each model. Suppression in analyzer response due to CO2 was observed in all four models at all three SO2 levels. At each monitoring project, ambient air is scrubbed and used as the dilution air during a multipoint calibration. Simultaneous measurements of CO2 in the ambient air and CO2 in the scrubbed calibration air showed partial and variable removal of CO2 by the scrubbing system. At EPA monitoring Project No. 1, simultaneous measurements of CO2 in the ambient air and in the calibration air were performed at 23 monitoring stations. On an average for the total 23 stations, the CO2 in the calibration air was 328 ppm lower than that in the ambient air being sampled at the stations. Concern exists that some monitoring networks may be calibrated with calibration air containing low CO2 and then returned to service to analyze ambient air containing a significantly higher CO2 concentration. By proper treatment of the calibration air, the CO2 suppression on FPD total sulfur response may be minimized. An activated charcoal scrubber is recommended for proper treatment of the calibration air.