The kinetics and rate-limiting mechanism of reactive uptake of nitric acid into submicron, deliquescent sodium chloride aerosols are determined. The reaction was performed in a flow tube with a real-time single-particle mass spectrometer connected to the outlet to determine the chloride-to-nitrate ratio of the transformed particles. Pseudo-first-order conditions were maintained by keeping the nitric acid pressure constant, as measured with a chemiluminescence analyzer. Experiments were performed with 100-220 nm diameter droplets at ca. 80% relative humidity and nitric acid concentrations between 60 and 380 ppb. Rate constants and uptake coefficients were determined from time-dependent changes in the chloride concentration of individual particles that traversed the reactor. The initial reactive uptake coefficient for 100 nm droplets was found to be 4.9 +/- 2.7 x 10(-3) and, to a first approximation, independent of the nitric acid concentration. The uptake coefficient also was found to increase linearly with increasing droplet size over the size range studied. These dependencies suggest that transport phenomena (diffusion of HNO3(g) to the droplet surface, accommodation of HNO3 into the droplet, diffusion of HNO3(aq) and HCl(aq) inside the droplet, and transport of HCl across the droplet surface into the gas phase) do not limit the reaction rate. Instead, reactive uptake of HNO3(g) is limited by formation of molecular HCl in the aqueous phase-a thermodynamically controlled process. The results are consistent with related measurements that show uniform chloride/nitrate/sodium mole ratios in submicron-size aerosols, a large uptake coefficient for aerosols in the low micron-size range, and decreasing chloride-to-nitrate ratios (because of transport limitations) in supermicron-size aerosols.