Molecular-level constraints on springtime urban new particle formation in Helsinki using Multi-Pressure Chemical Ionization Mass Spectrometry

Secondary organic aerosols (SOA) and their precursor vapors comprise a major fraction of atmospheric particulate matter, yet the molecular pathways linking precursor oxidation, new particle formation (NPF), condensation, and particle growth remain insufficiently constrained in complex urban environments. We will investigate these processes during a spring 2026 field measurements campaign in Helsinki, Finland, deploying a new Orbion 120 platform with multi-pressure chemical ionization to obtain high time-resolution, molecular-level measurements of key nucleation- and growth-relevant species.The instrument will operate with two complementary reagent-ionization schemes: isotopically labelled nitrate chemical ionization to target highly oxygenated organic molecules (HOMs) and strong acids, and uronium chemical ionization to target atmospheric bases and high-proton-affinity species that can stabilize acidic clusters and influence early particle growth. This dual-chemistry approach is designed to resolve co-variations between strong acids/HOMs and basic species under rapidly evolving springtime urban conditions, and to probe transformation processes (e.g., functionalization/aging and brown-carbon-relevant chemistry) alongside gas-to-particle partitioning prior to and during NPF events.We will describe the field setup and operating strategy (inlet configuration, switching scheme, and overall analytical method design) and report the first campaign-derived observations, focusing on event-non-event contrasts and molecular fingerprints associated with NPF in spring-time Helsinki. These measurements aim to provide new ambient constraints on the coupled roles of oxidized organics/strong acids and atmospheric bases in urban springtime particle formation, supporting improved mechanistic understanding and model representation of SOA and NPF.