HETEROGENEOUS UPTAKE OF HCL BY SULFURIC-ACID-SOLUTIONS

The uptake of HCl molecules by aqueous sulfuric acid droplets was meas ured in the temperature range 230-264 K at 39, 49, 54, 59, and 69 wt % acid and as a function of time (2-15 ms). These experiments utilized a droplet train apparatus in which a stream of monodisperse droplets ( 120-250 mu m in diameter) is passed through a low-pressure flow contai ning HCl(g), The droplet area is changed in a step-wise fashion, while the HCl(g) density is continuously monitored by infrared absorption. The uptake coefficient is obtained from the measured change in the HCl density. The product of HD-l(1/2) (H*, solubility; D-l, liquid phase diffusion coefficient) and the mass accommodation coefficient cu of t he species as a function of temperature and sulfuric acid concentratio n were obtained from the uptake coefficient. The good agreement of mea sured and modeled HD-l(1/2) values validates current formulations of HCl reactivity in stratospheric aerosols, While the solubility of HCl decreases steeply with sulfuric acid concentration because increasing acidity reduces the dissociation of HCl into H+ and Cl- in solution, t he mass accommodation coefficient is independent of acid concentration in the region studied. As with previously studied species, alpha is i nversely proportional to temperature increasing from similar to 0.06 a t 294 K to near unity at similar to 230 K. The mass accommodation coef ficient is well expressed in terms of an observed Gibbs free energy as alpha/(1 - alpha) = exp (-Delta G(obs)/RT), suggesting that the clust ering model for the accommodation process is applicable in this case a s well. The mass accommodation measurements are well fitted by the par ameters Delta H-obs = -13.8 +/- 0.9 kcal mol(-1) and Delta S-obs = -52 .2 +/- 0.3 cal mol(-1) K-1. Under stratospheric conditions a for HCl i s unity. Implications of the HCl uptake studies for atmospheric chemis try are examined.