Influence of Meltwater Percolation on Preservation of Organic Aerosol Tracers in Glacier Archives.

Ice cores are valuable archives for reconstructing past atmospheric aerosol composition. However, their interpretability is increasingly challenged by climate change, which accelerates melting and may lead to the vertical relocation or loss within the firn or ice matrix of organic and inorganic impurities. Using a hybrid target/nontarget screening approach, we present a comprehensive assessment of organic compound preservation from a firn core collected at the Grand Combin (Switzerland) covering the period 2008-2020. A total of 248 different molecules were identified, mainly secondary organic aerosol tracers composed of carbon, hydrogen, and oxygen. The core stratigraphy for organics appears relatively undisturbed down to a depth of 9.60 m. Below this depth, corresponding to the year 2015, the intensities of 166 compounds decreased to near-blank values, suggesting relocation or removal by meltwater. These compounds were mainly monoterpene and isoprene oxidation products, exhibiting statistically significant lower molecular weight, fewer carbon atoms, and higher polarity compared to the others, supporting the idea that smaller, more oxidized molecules are more prone to meltwater-driven relocation. As an increasing number of glaciers are affected by melt or rain events, this study provides an experimental benchmark for improving the reliability of organic aerosol reconstructions from ice cores influenced by melting.