A thermoanalytical approach to speciation of atmospheric strong acids

A thermoanalytic device for the selective determination of atmospheric sulfuric acid, called a thermo-denuder, is described. The acid droplets are thermally decomposed in a heated glass tube and the small fragments formed are collected at the walls of the tube by diffusion-controlled deposition. High sampling efficiency is achieved when the inner wall of the glass tube is coated with sodium chloride as a trapping agent. Sulfate and bisulfate salt particles pass through the tube unaffected and can be collected on a back-up filter. After the sampling the tube is rinsed with water and the dissolved coating analyzed for sulfate using isotope dilution analysis. Applying a nitrate-specific method to the analysis of the extract, the sum of nitric acid and ammonium nitrate can also be determined by the thermo-denuder techniaue. Using labelled sodium chloride (Na ³⁶ CI) as a wall coating, a direct signal for total air-borne strong acidity can be obtained. This is possible because a stoichiometric amount of H ³⁶ C1 is evolved in the reaction between Na ³⁶ Cl and a strong acid and can easily be collected and measured.