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

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 (NH₄)₂SO₄ and NH₄HSO₄ aerosols was identical, but a reduced response was found for H₂SO₄ aerosol. This occurred because of the high temperature (145°C) of the flame photometer burner block which caused the evaporation and loss of H₂SO₄ to the wall before reaching the flame. The addition of NH₃ to the sample air just upstream of the burner converted H₂SO₄ to (NH₄)₂SO₄ or NH₄HSO₄ 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 H₂SO₄ aerosol decreased from unity at 50°C to 0.04 at 110°C and for (NH₄)₂SO₄ and NH₄HSO₄ aerosols from unity at 115°C to zero at 190°C. When the aerosol contained both H₂SO₄ and (NH₄)₂SO₄, the resultant thermogram was a function of both the (NH₄)₂SO₄/H₂SO₄ ratio and the manner in which the two components were mixed in the particles comprising the aerosol (i.e. homogeneously with constant (NH₄)₂SO₄/H₂SO₄ ratios or heterogeneously with varying ratios).