Non-target screening analysis on ice and snow samples: a new opportunity to enhance our understanding on past and present atmospheric aerosol composition.

Organic aerosols make up to 70-90% of the total aerosol mass, yet ice-core studies have predominantly focused on a limited set of compounds or bulk fractions altogether. Previous investigations have centered on biomass burning tracers, marine phytoplankton oxidation products, low molecular weight carboxylic acids and persistent organic pollutants, leaving a large majority of molecules unidentified. Advances in high-resolution mass spectrometry (HRMS) have recently enabled the exploration of a wider chemical space through the development of non-target screening (NTS) workflows.

In this work, we present three applications of a novel NTS method. Designed to detect secondary organic aerosol compounds in ice-core and snow samples, the method has contributed to a more comprehensive characterization of past molecular aerosol composition and has supported the development of new molecular proxies. Initially, the method was applied to the Belukha ice core (Siberian Altai, 4072 m. a.s.l.) between 1830 and 1980 CE, providing the first NTS ice-core record that embraces both the pre-industrial and industrial periods. More than 400 compounds were identified, and a clear anthropogenic fingerprint was recognized over the industrial period. Subsequently, the ice core samples from Colle Gnifetti (Switzerland, 4500 m. a.s.l.) covering the period from 1750 to 2000 CE were analyzed. Here, a smaller number of molecules was detected (≈200), consistent with the lower concentrations of dissolved organic carbon observed at this site. In both cores, most of the molecules are composed of carbon (C), hydrogen (H) and oxygen (O) and are associated with atmospheric oxidation of monoterpenes and isoprenes (e.g., succinic acid, pinic acid, azelaic acid). The industrial onset was characterized by an increase in nitrogen and sulfur containing compounds, likely due to the atmospheric reactions with anthropogenic NO_x and  SO₂. The higher occurrence of compounds with higher O/C ratios during the industrial period observed at both locations, suggests an increase in the atmosphere oxidative capacity. Lastly, the method was applied to 56 snow samples collected in springtime close to Ny-Ålesund (Svalbard Archipelago) and covering both pre- and phytoplankton bloom periods. Together with marine observations of algal bloom, the NTS results suggest promising evidence towards new ice-core marine productivity proxies for long-term reconstructions.