Mercury is a neurotoxin that impacts human health and the health of ecosystems globally. Mercury exists in the atmosphere in three major forms: elemental mercury, gaseous oxidized mercury, and particulate-bound mercury. To date, gaseous-phase mercury chemistry has been extensively studied. However, the heterogeneous and surface chemistry of mercury, which is associated with chemical reactions taking place at atmospherically relevant surfaces such as aerosols, air-water interfaces, particulate-bound mercury, etc., has been much less frequently investigated. This is mainly due to its complexity and experimental difficulties in making accurate measurements, for example, due to the wall effect and surface interferences. In this study, we created a micron-sized, single-particle reactor (SPR) freely levitated in air using optical trapping of a single particulate-bound mercury particle that was formed by mixing Hg(II) halides (HgX2, X2=Cl2, Br2 or ClBr) with single-walled carbon nanotubes (SWCNT). The experimental results show that the HgX2 adsorbed on the surface of a SWCNT particle is in the solid state. We observed chemical reactions on the surface of SPRs (HgX2 contained single SWCNT particles) in the O3 environment under the radiation of 532-nm light, based on changes in the single-particle Raman spectra. We propose a reaction mechanism for the observed reactions. This is the first study of reactions of solid-state Hg(II) halides on the surface of a single, micron-sized particle that is optically trapped. This SPR technique may serve as a new reaction platform for the study of surface and heterogeneous chemistry of mercury and aerosol particles in the simulated atmospheric state.

Optical-trapping, single-particle reactor for the study of surface and heterogeneous chemistry of mercury

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