Kinetics of the heterogeneous reaction of nitric acid with mineral dust particles: an aerosol flowtube study.

The heterogeneous reaction of HNO3 with mineral dust aerosol (Arizona Test Dust) was studied in an aerosol flow tube at atmospherically relevant conditions (298 K, approximately 1 atm, 6-60% RH) and using radioactively labelled HNO3. The uptake of nitric acid was found to depend on HNO3 and H2O concentrations in the gas phase. A reaction mechanism is suggested to describe the heterogeneous interaction, involving Langmuir type adsorption and surface reaction. This mechanism is incorporated in a flux based aerosol kinetic model framework that is able to reproduce the observations within the range of experimental conditions. The experiments show that the reactive surface sites of the relatively calcium poor, but silicate rich dust surface are efficiently depleted at higher HNO3 concentrations in the gas phase or longer exposure times. A set of kinetic parameters is extracted from the data, which can be used to calculate uptake coefficients as function of time, concentration and humidity for use in atmospheric chemistry models to improve especially the representation of the effects of relative humidity on dust aging and to allow following dust aging with time.