A Thermoanalytical Approach to Speciation of Atmospheric Strong Acids

Abstract

A thermoanalytic device for the selective determination of atmospheric SO4, a thermo-denuder, was described. The acid droplets were thermally decomposed in a heated glass tube and the small fragments formed collected at walls of the tube by diffusion-controlled deposition. High sampling efficiency was achieved when inner wall of the glass tube was coated with NaCl as a trapping agent. Sulfate and bisulfate salt particles passed through the tube unaffected and were collected on a back-up filter. After the sampling, the tube was rinsed with water and dissolved coating analyzed for sulfate using isotope dilution analysis. Applying a nitrate-specific method to analysis of the extract, the sum of nitric acid and ammonium nitrate could also be determined by the thermo-denuder technique. Using Na36Cl as a wall coating, a direct signal for total airborne strong acidity was obtained. This was possible because a stoichiometric amount of H36Cl was evolved in the reaction between Na36Cl and a strong acid could be collected and measured.