Continuous measurement and speciation of sulfur-containing aerosols by flame photometry

Abstract

A sulfur-specific flame photometer has been used for the real time measurement of sulfur-containing aerosols. Specificity for the aerosols was achieved with a diffusion tube stripper which removed sulfur-containing gases from the air stream by diffusion to an adsorbing wall but transmitted particles to the flame photometer. The sensitivity of the flame photometer to (NH 4) 2SO 4 and NH 4HSO 4 aerosols was identical, but a reduced response was found for H 2SO 4 aerosol. This occurred because of the high temperature (145°C) of the flame photometer burner block which caused the evaporation and loss of H 2SO 4 to the wall before reaching the flame. The addition of NH 3 to the sample air just upstream of the burner converted H 2SO 4 to (NH 4) 2SO 4 or NH 4HSO 4 and produced the expected increase in the flame photometer response. Heating the aerosol upstream of the diffusion stripper converted the sulfates to sulfur-containing gases over a temperature range characteristic of the aerosol being sampled. These gases were removed in the stripper, thereby decreasing the flame photometer output. The normalized response to aqueous H 2SO 4 aerosol decreased from unity at 50°C to 0.04 at 110°C and for (NH 4) 2SO 4 and NH 4HSO 4 aerosols from unity at 115°C to zero at 190°C. When the aerosol contained both H 2SO 4 and (NH 4) 2SO 4, the resultant thermogram was a function of both the (NH 4) 2SO 4/H 2SO 4 ratio and the manner in which the two components were mixed in the particles comprising the aerosol (i.e. homogeneously with constant (NH 4) 2SO 4/H 2SO 4 ratios or heterogeneously with varying ratios).