Towards harmonized measurements of condensable vapors: insights from the intercomparison of six chemical ionization mass spectrometers at a boreal forest site

Atmospheric new particle formation is driven by condensable vapors such as sulfuric acid and highly oxygenated organic molecules (HOMs). Measuring these gases is challenging because they are present at trace concentrations, they are easily lost through condensation onto surfaces, and they include a large number of chemically diverse species. Chemical Ionization Mass Spectrometry (CIMS) has been extensively used for their detection; however, results obtained in different studies are not always directly comparable. This limitation reflects differences in instrument designs, operating configurations, and reagent ion schemes employed. To investigate these factors, the Aerosol, Clouds and Trace Gases Research Infrastructure (ACTRIS) organized its first CIMS field intercomparison campaign (CI-FI1) during summer 2024 at a Finnish boreal forest site, the SMEAR II (Station for Measuring Forest Ecosystem-Atmosphere Relations) station. Six instruments employing different inlet designs, mass analyzers, and reagent ions were operated using their routine configurations and calibrated according to standard procedures. For sulfuric acid measured in nitrate mode, the conventional sulfuric acid calibration enabled moderately good agreement among instruments, although larger discrepancies were observed at the lower concentrations, typically observed during nighttime. When targeting higher-mass compounds, such as monoterpene-derived HOM monomers (m/z 240–390) and dimers (m/z 480–630), sulfuric acid calibration alone was insufficient to ensure measurement intercomparability and taking into consideration mass-dependent transmission differences proved important to achieve consistent results. Notably, good agreement was observed also for selected compounds measured in bromide mode by different instruments. Overall, the results demonstrate that comparable field measurements of condensable vapors by different CIMS instruments are achievable when all relevant calibration and correction factors are carefully considered. The results further highlight that similarities in instrument behavior are more closely associated with the inlet design and instrument operating conditions than with the reagent ion choice.