Hydration of Nitric Acid and Its Collection in the Atmosphere by Diffusion Denuders

Abstract

The collection and determination of gas-phase nitric acid by use of denuder tubes are dependent either on the direct measurement of nitric acid collected In a denuder or on the analysis of total inorganic nitrate with and without prior removal of gas-phase nitrate by a denuder. In both cases, the amount of nitric acid predicted to be collected by the denuder Is dependent on the diffusion coefficient used to describe the collection of nitric acid by the denuder. It has been assumed in past studies that the species present in the atmosphere and collected by the denuder is HNO3(g). Both field and laboratory studies with tungstic acid and nylon denuders show that the species present and collected is actually HNO3-xH20(g), where the value of x is dependent on the atmospheric water concentration. Thus, the diffusion coefficient is also dependent on the atmospheric water concentration. Gas-phase ammonia is also shown to exist as the monohydrate at atmospheric water concentrations. In addition, the collection efficiency of tungstlc acid diffusion denuders for hydrated nitric acid is affected by the atmospheric water concentration. Both the hydration of nitric acid and the apparent hydration of the denuder surface must be taken into account if a tungstlc acid denuder is used to collect gas phase nitric acid. © 1985, American Chemical Society. All rights reserved.