Total sulfur aerosol concentration with an electrostatically pulsed flame photometric detector system

This paper describes a new system for continuous measurement of ambient aerosol sulfur concentrations. The system consists of a pulsed electrostatic precipitator followed by a total sulfur flame photometric detector (FDP). With this system the output signal of the FPD is composed of a DC signal related to the gas phase sulfur concentration and an AC signal related to aerosol sulfur concentration. The AC signal is separated from the DC signal by a phase and frequency selective lock-in amplifier. Using a standard Meloy SA-285 FPD and operating at a chopping frequency of 0.2 Hz the system has a sensitivity of about 0.2 μg m^-3 (as sulfur) for ammonium sulfate at a signal to peak-to-peak noise ratio of one and has a system response time of about 1.5 min. Some interference from SO₂ exists but should be minimized by modification of the precipitator and of the linearizer electronics of the FPD. A thermal speciation system has been developed for use in conjunction with the system to yield information on the chemical composition of the aerosol sulfur measured. Preliminary field evaluations of the instrument were performed at the Los Angeles Roadway Study. The instrument performed well and aerosol total sulfur concentrations measured with it were in good agreement with measurements by other methods.