The nature of nitrate at the ice surface studied by XPS and NEXAFS

Abstract

Trace contaminants such as strong acids have been suggested to affect the thickness of the quasi-liquid layer at the ice/air interface, which is at the heart of heterogeneous chemical reactions between snowpacks or cirrus clouds and the surrounding air. We used X-ray photoelectron spectroscopy (XPS) and electron yield near edge X-ray absorption fine structure (NEXAFS) spectroscopy at the Advanced Light Source (ALS) to probe the ice surface in the presence of HNO₃ formed from the heterogeneous hydrolysis of NO₂ at 230 K. We studied the nature of the adsorbed species at the ice/vapor interfaces as well as the effect of HNO₃ on the hydrogen bonding environment at the ice surface. The NEXAFS spectrum of ice with adsorbed HNO₃ can be represented as linear combination of the clean ice and nitrate solution spectrum, thus indicating that in the presence of HNO₃ the ice surface consists of a mixture of clean ice and nitrate ions that are coordinated as in a concentrated solution at the same temperature but higher HNO₃ pressures.