Heterogeneous uptake of HCl by sulfuric acid solutions

Abstract

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*Dl1/2 (H*, solubility; Dl, 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*Dl1/2 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 (−ΔGobs/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 ΔHobs = −13.8±0.9 kcal mol−1 and ΔSobs = −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.