Stratospheric aerosol growth and HNO₃ gas phase depletion from coupled HNO₃ and water uptake by liquid particles

Abstract

Aqueous sulphuric acid droplets, which constitute the background stratospheric aerosol, strongly absorb HNO 3 and HCl under cold conditions. A thermodynamic model is used to predict partitioning of HNO 3 , HCl and H 2 O between gas and aerosol phases, and show that a 50‐fold increase in aerosol volume, observed in the Arctic stratosphere as temperature approached the frost point (188.9 K), can be explained in terms of uptake of HNO 3 and H 2 O by liquid aerosols. Calculated degrees of saturation of the droplets with respect to solid hydrates, taking into account the reduction in vapour phase HNO 3 , suggest that the droplets remain liquid to the frost point. Near this temperature, they can yield larger aerosol volumes than would have been the case for solid NAT (HNO 3 •3H 2 O) particles. The depletion of gas phase HNO 3 into enhanced volumes of liquid aerosols resulting from volcanic eruptions may hamper NAT formation.