Heterogeneous Chemistry of Mercuric Chloride on Inorganic Salt Surfaces

Gaseous oxidized mercury (GOM) is a major chemical form responsible for deposition of atmospheric mercury, but its interaction with environmental surfaces is not well understood. To address this knowledge gap, we investigated the uptake of gaseous HgCl₂, used as a GOM surrogate, by several inorganic salts representative of marine and urban aerosols. The process was studied in a fast flow reactor coupled to an ion drift-chemical ionization mass spectrometer, where gaseous HgCl₂ was quantitatively detected as HgCl₂·NO₃^-. Uptake curves showed a common behavior, where upon exposure of the salt surface to HgCl₂, the gas-phase concentration of the latter dropped rapidly and then recovered gradually. None of the salts produced a full recovery of HgCl₂, indicating the presence of an irreversible chemical reaction in addition to reversible adsorption, and all salts showed reactive behavior consistent with the presence of surface sites of a high and a low reactivity. On the basis of the decrease in the uptake coefficient with increasing concentration of gaseous HgCl₂, we conclude that the interaction follows the Langmuir-Hinshelwood mechanism. The reactivity of a deactivated salt surface after uptake could be partially restored by cycling through an elevated relative humidity at atmospheric pressure. The overall surface reactivity decreased in the series Na₂SO₄ > NaCl > (NH₄)₂SO₄ > NH₄NO₃. The uptake on NH₄NO₃ was nearly fully reversible, with low values of the initial (0.4 × 10^-2) and steady-state (3.3 × 10^-4) uptake coefficients, whereas Na₂SO₄ was significantly more reactive (3.1 × 10^-2 and 1.7 × 10^-3). Depending on the aerosol loading, the lifetimes of gaseous HgCl₂ on dry urban and marine particles (as pure (NH₄)₂SO₄ and NaCl, respectively) were estimated to range from half an hour to about a day.