Temporal variability of ultratrace biogenic secondary organic aerosol markers in the oldest ice from Beyond EPICA

Oxidation products of biogenic volatile organic compounds, such as monoterpenes and isoprene, are widely used to investigate variability in biogenic emissions and atmospheric transformation processes. Quantifying such tracers in ice-core matrices remains challenging because concentrations are ultralow and results can be affected by matrix effects and contamination. Here, we developed a targeted ultratrace LC–MS³ method using a SCIEX QTRAP 5500+ to enhance sensitivity and selectivity for five established SOA markers: cis-pinonic acid, pinic acid, keto-pinic acid, 3-methyl-1,2,3-butanetricarboxylic acid (3-MBTCA), and 2-methylerythritol (2-ME). Method performance was evaluated using procedural blanks and spike-recovery experiments, yielding compound-specific reporting limits of 0.01–0.05 ppt (limits of detection, LOD) and 0.1–0.25 ppt (limits of quantification, LOQ); instrument repeatability based on batch quality-control injections was 5–8% RSD.The method was applied to meltwater fractions from the oldest section (>700,000 years ago) of the Beyond EPICA ice-core collected sequentially within each core section, resulting in 878 analysed fractions from 183 sections spanning 2399.0–2581.8 m (≈0.66 to ≥1.47 Ma BP, modelled). Concentrations are reported as ppt in meltwater following direct analysis (no preconcentration). Pinic acid was detected above the LOD in 87% of analysed fractions and quantified above the LOQ in 62%, with concentrations ranging from 0.87 to 5.83 ppt (mean 2.19 ppt). 3-MBTCA was detected in 70% of fractions and quantified in 66%, with concentrations of 0.103–0.612 ppt (mean 0.196 ppt). In contrast, cis-pinonic acid and 2-ME were below the LOQ, 0.1 ppt, while keto-pinic acid was not detected in the analysed ice-core samples.These first measurements, placed within a preliminary age framework, demonstrate quantification of biogenic SOA tracers in Beyond EPICA ice-core at ultratrace levels. Ongoing work will integrate these data with co-measured glaciochemical tracers to evaluate transport, deposition and post-depositional effects, and to assess the potential of these compounds as proxies for past biogenic emissions and atmospheric oxidative processing.