Exchange Reactions Alter Molecular Speciation of Gaseous Oxidized Mercury

The knowledge of speciation of gaseous oxidized mercury (GOM) is crucial for understanding the atmospheric mercury chemistry and global cycle. Because of the low atmospheric abundance of GOM, its chemical analysis requires preconcentration and often involves the use of collection substrates, such as KCl, various adsorbents, or membranes. GOM molecules adsorbed on substrates can exchange ligands with the substrate material, each other, or other coadsorbed atmospheric chemicals, altering the composition of GOM and ultimately leading to speciation biases. Here, we investigated exchange reactions involving GOM surrogates HgBr2, HgCl2, and Hg(NO3)2 and different forms of Cl- (as KCl, NH4Cl, and HCl). The reactions were studied in aqueous solutions and on surfaces, and the products were analyzed in the gas phase (by ion drift-chemical ionization mass spectrometry), in solution (by electrospray ionization-mass spectrometry), and on surface (by Raman microscopy). In all cases, we observed a rapid formation of exchange products HgBrCl and HgCl2, which readily volatilized not only upon heating, but also often at room temperature, depending on substrate adsorptivity. We propose that a similar exchange may occur both on atmospheric aerosols and during analysis, where the original GOM species (e.g., BrHgONO and BrHgOOH) would react on surfaces/particles with each other and atmospherically abundant trace species to form other mercury(II) chemicals. For example, a readily volatilizable HgCl2 can be produced through the exchange with chloride.