Study of heterogeneous chemistry and photochemistry of single sea-spray aerosols containing Hg(ii) in air using optical trapping – Raman spectroscopy†

Sea-spray aerosols (SSAs) contribute to atmospheric loading, bringing toxic compounds like mercury (Hg) to the atmosphere, affecting the climate and human health. Despite their importance, the investigation into surface modification, including heterogeneous chemical and photochemical reactions of SSAs, is limited. In this work, we studied the heterogeneous chemistry and photochemistry of a single suspended SSA particle and a SSA containing Hg(II) in a reactive environment using optical trapping – Raman spectroscopy. The experiments are focused on the study of hygroscopicity, heterogeneous chemical reaction with ozone (O₃), photochemical reaction with UVC radiation of an optically suspended single SSA particle, and photo-reduction of Hg(II) in SSAs under UVC radiation. Results show different Raman signal responses of a single SSA particle when it is optically trapped in air under varying relative humidity conditions as the aerosol particle uptakes and loses liquid water from the surrounding environment. The state and size of the aerosol are determined through the on-time images and different single-particle Raman spectral features. Results also show that the formation of chlorate (ClO₃⁻) is a reaction product of the heterogeneous reaction between the SSA particle and O₃. The photochemical reaction products, as the SSA particle suspended in air under UVC radiation, are ClO₃⁻ and perchlorate (ClO₄⁻). Further, we observed that these reactions occur only on the surface of the SSA particle. Based on the results, we hypothesize that Hg(II) can be photo-reduced to Hg(I) in SSAs through UVC radiation, and the amount of Hg(I) in SSAs is minor and balanced between the photo-oxidation and photo-reduction reactions.