Chemical transformations of Hg° during Arctic mercury depletion events sampled from the NASA DC-8

Abstract. Atmospheric Mercury Depletion Events (MDEs) in Arctic springtime were investigated utilizing a box model based on airborne measurements from the NASA DC-8 during the Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) field campaign. Measurements showed that MDEs occurred near the surface and always over the Arctic Ocean accompanied by concurrent ozone (O₃) depletion, enhancement in Br₂ mixing ratios, and decreases in ethyne and light weight alkanes. Backward trajectories indicated that most air masses inside the MDEs originated at low altitude over the ocean presumably generating a halogen-rich environment. We developed a box model which considered only gas phase reactions of mercury, halogen species, and O₃ chemistry. We conducted a series of sensitivity simulations to determine the factors that are of most importance to MDE formation. The box model results suggested that continuous enhancement of Br₂ mixing ratios, a high intensity of solar radiation, or a relatively high NO_x regime expedited Hg° depletion. These environments generated high concentrations of Br radical, and thus the model results indicated that the Br radical was very important for Hg° depletion. Utilizing different rate constants for reaction of Hg° + Br produced times to reach Hg° depletion ranging from 22 to 32 h.