The Hybrid Single-Particle Lagrangian Integrated Trajectory model (HYSPLIT), developed by NOAA’s Air Resources Laboratory, is one of the most widely used models for atmospheric trajectory and dispersion calculations. We present the model’s historical evolution over the last 30 years from simple hand-drawn back trajectories to very sophisticated computations of transport, mixing, chemical transformation, and deposition of pollutants and hazardous materials. We highlight recent applications of the HYSPLIT modeling system, including the simulation of atmospheric tracer release experiments, radionuclides, smoke originated from wild fires, volcanic ash, mercury, and wind-blown dust.

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An electrodynamic balance apparatus has been used to suspend single, 20-#tm-diameter, charged droplets of aqueous salt solutions in a humid environment. From measurements of the humidity surrounding the droplet and the relative mass of the particle in its wet and dry states, the dependence of water activity on solute concentration was determined for aqueous solutions of the following salts at 20 °C: NaCl, NaBr, KC1, KBr, NH4C1, Na2S04, (NH4)2S04, CaCl2, MnCl2, and FeCl3. This technique allowed the thermodynamics of highly supersaturated solutions to be studied, and for most of the salts, the measurements were made to higher concentrations than have previously been reported at comparable temperatures. The dependence of the solute activity coefficient on concentration was calculated for each of the salts, and the features of this dependence are discussed in relationship to ionic hydration and association. Several semiempirical electrolyte solution models were tested against the data, and it was found that salt-specific model parameters estimated from low-concentration data could not be reliably used to predict the solution behavior at high concentrations. However, with estimated parameters based on the full range of the data, the models were able to represent the experimental data to within the uncertainty in the measurements.

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The nucleation of crystals from aqueous solution has been investigated for several common inorganic salts alone and in mixtures. Single, charged solution droplets approximately 20 pm in diameter were suspended in an electrodynamic balance and continuously weighed. The solute concentration in the droplet was changed by adjusting the relative humidity of the air surrounding the particle. Nucleation theory was used to estimate the surface excess free energy and critical nucleus size from the measured supersaturation at which nucleation occurred.