Aqueous sulphuric acid droplets, which constitute the background stratospheric aerosol, strongly absorb HNO3 and HCl under cold conditions. A thermodynamic model is used to predict partitioning of HNO3, HCl, and H2O 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 HNO3 and H2O by liquid aerosols. Calculated degrees of saturation of the droplets with respect to solid hydrates, taking into account the reduction in vapor phase HNO3, 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 (HNO3⋅3H2O) particles. The depletion of gas phase HNO3 into enhanced volumes of liquid aerosols resulting from volcanic eruptions may hamper NAT formation. ¿ American Geophysical Union 1994