Heterogeneous uptake of HCl by sulfuric acid solutions

The uptake of HCl molecules by aqueous sulfuric acid droplets was measured 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 μm in diameter) is passed through a low‐pressure flow containing 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 H * D l 1/2 ( H * , solubility; D l , liquid phase diffusion coefficient) and the mass accommodation coefficient a of the species as a function of temperature and sulfuric acid concentration were obtained from the uptake coefficient. The good agreement of measured and modeled H * D 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, the mass accommodation coefficient is independent of acid concentration in the region studied. As with previously studied species, α is inversely proportional to temperature increasing from ∼0.06 at 294 K to near unity at ∼230 K. The mass accommodation coefficient is well expressed in terms of an observed Gibbs free energy as α/(1 − α) = exp (−Δ G obs / RT ), suggesting that the clustering model for the accommodation process is applicable in this case as well. The mass accommodation measurements are well fitted by the parameters Δ H obs = −13.8±0.9 kcal mol −1 and Δ S obs = −52.2±0.3 cal mol −1 K −1 . Under stratospheric conditions α for HCl is unity. Implications of the HCl uptake studies for atmospheric chemistry are examined.