Non-target screening of organic aerosol tracers applied to a high-alpine firn core

Ice cores are unique natural archives that provide important information about the past evolution of the Earth’s atmosphere. Whereas the inorganic atmospheric aerosol fraction is well characterized, the organic composition is less understood. The organic aerosol burden is consistently underestimated in the current state-of-the-art models, thus highlighting major gaps in our understanding of the pathways by which organic aerosols accumulate and evolve in the atmosphere. So far, organic aerosols in ice cores have been primarily reported as either bulk (e.g., water insoluble or dissolved organic carbon) or specific parameters (e.g., biomass burning tracers).
To provide a more comprehensive characterization of the organic fraction, we applied a non-target screening approach optimised for determining oxidation products of volatile organic compounds to a firn core collected on the Corbassière glacier (Grand Combin, Swiss Alps), in 2020, covering the period 2008-2020. In comparison with a firn core drilled two years earlier (2018), we observe a drastic disturbance of seasonal trends for certain species, such as major ions at depths corresponding to the annual layers from 2008 to 2016, induced by meltwater percolation.

As organic tracers are present in low concentrations in the firn core, we performed solid phase extraction. The organic tracers were analysed with high-resolution mass spectrometry based on Orbitrap technology coupled with liquid chromatography. This technique makes it possible to study a wide range of individual compounds at low concentration and to identify them with MS/MS fragmentation. We can attribute molecular formulas to detected compounds by comparing the MS/MS spectra with spectral libraries (e.g., mzCloud) or reference standards. With this approach we will present a unique record of molecular composition of organic aerosol in the Corbassière firn core.
Furthermore, this firn core presents a unique opportunity to examine the effect of melting on the organic tracers. We found that specific burning tracers (e.g., vanillic acid, vanillin and syringaldehyde) are less affected than other biomass tracers (e.g., pinic acid) by meltwater percolation. In general, we observe a decrease in concentration of the organic tracers in the same firn core section where we also observe a decrease in major ion concentrations.