Surface solvation of Martian salt analogues at low relative humidities.

Xiangrui Kong,Andrey Shavorskiy,Jun Li,Jan B C Pettersson,Ruth Signorell,Sen Wang,Suyun Zhu,Wanyu Liu,Pablo Corral Arroyo

doi:10.1039/d1ea00092f

Abstract

Salt aerosols play important roles in many processes related to atmospheric chemistry and the climate systems on both Earth and Mars. Complicated and still poorly understood processes occur on the salt surfaces when interacting with water vapor. In this study, ambient pressure X-ray photoelectron spectroscopy (APXPS) is used to characterize the surface chemical environment of Martian salt analogues originating from saline lakes and playas, as well as their responses to varying relative humidities. Generally, APXPS shows similar ionic compositions to those observed by ion chromatography (IC). However, XPS is a surface-sensitive method while IC is bulk-sensitive and differences are observed for species that preferentially partition to the surface or the bulk. Element-selective surface enhancement of Cl− is observed, likely caused by the presence of SO42−. In addition, Mg2+ is concentrated on the surface while Na+ is relatively depleted in the surface layer. Hence, the cations (Na+ and Mg2+) and the anions (Cl− and SO42−) show competitive correlations. At elevated relative humidity (RH), no major spectral changes were observed in the XPS results, except for the growth of an oxygen component originating from condensed H2O. Near-edge X-ray absorption fine structure (NEXAFS) measurements show that the magnesium and sodium spectra are sensitive to the presence of water, and the results imply that the surface is fully solvated already at RH = 5%. The surface solvation is also fully reversible as the RH is reduced. No major differences are observed between sample types and sample locations, indicating that the salts originated from saline lakes commonly have solvated surfaces under the environmental conditions on Earth.

Highlights

IntroductionSalt aerosols play important roles in many processes related to atmospheric chemistry and the climate system,[1] especially as active components in aerosol and cloud formations[2,3] due to
Salt aerosols play important roles in many processes related to atmospheric chemistry and the climate system,[1] especially as active components in aerosol and cloud formations[2,3] due toSalt particles are associated with haze formation in more populated regions
In a recent study on Qaidam salts, it was found that some saline lake brines contain relatively high contents of Mg2+ and SO42À but the naturally precipitated salts lack magnesium sulfates, which indicates that different mechanisms have in uenced the salt compositions on Earth and Mars.[9]

Summary

Introduction

Salt aerosols play important roles in many processes related to atmospheric chemistry and the climate system,[1] especially as active components in aerosol and cloud formations[2,3] due to. Salt particles are associated with haze formation in more populated regions. The formation of secondary inorganic aerosols (SIAs) in Shanghai is promoted by salt aerosols a er long-distance transportation from East Asian desert regions.[17] The formed haze is more harmful to human health compared to the primarily emitted dust and salt aerosols.[20] during transportation the salt–dust mixture can undergo heterogeneous reactions and transform into materials with higher hygroscopicity.[21,22] It has been reported that the Cl-

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Conclusion