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Online chemical characterization of atmospheric particles is often challenged by thermal decomposition, fragmentation, wall losses, ionization selectivity, and rapid changes in particle concentration and composition. To resolve these current limitations, we developed the Wall-Free Particle Evaporator (WALL-E) coupled to a chemical ionization mass spectrometer - model Vocus B4 (Bansal et al., 2025). WALL-E enables continuous evaporation of particles to detectable vapors using flash evaporation utilizing a mixture of heated sheath flow as well as a compact thermal desorption region, aiming to preserve fast atmospheric variability while reducing artefacts and decomposition linked to surface interactions by minimizing residence time (Gao et al., 2025). In this work, we present the first ambient field deployment of the WALL-E - Vocus B4 chemical ionization mass spectrometer equipped with an Aim reactor (Riva et al., 2024). Field measurements were conducted from mid-September to mid-October 2025 at the AIRPARIF background supersite named Paris 1er – Les Halles in France.Figure 1: Temporal evolution of the some of the trace gasesThe campaign provides a real-world test of WALL-E performance under highly variable urban conditions. The resulting particle-phase molecular time series captures short-timescale variability alongside sustained background changes. To further identify the main aerosol sources, we applied matrix factorization to the WALL-E–Vocus B4 dataset to resolve distinct composition modes with characteristic temporal signatures. A key outcome is the prominent role of cooking-related emissions, which emerge as a robust factor with clear diurnal structure (enhanced during meal-time periods) and diagnostic molecular features in the particle-phase spectra. The analysis also separates recurring daily patterns from more persistent background/regional influences. Overall, this work provides a compact, interpretable description of urban particle-phase variability in central Paris based directly on online molecular composition.This work was supported by the CLOUD-DOC project (Grant Agreement No. 101073026) under HORIZON-MSCA-2021-DN-01. This work was also supported by the European Research Council (ERC) under the European Union’s Horizon Europe research and innovation program through the Starting Grant CHANEL (Grant Agreement No. 101076276).Bansal, P., et al. “Comprehensive airborne molecular contamination monitoring with single-digit parts-per-trillion sensitivity.” Journal of Micro/Nanopatterning, Materials, and Metrology 24(4), 044003 (30 December 2025). https://doi.org/10.1117/1.JMM.24.4.044003.Gao, L., Zgheib, I., Stergiou, E., Carstens, C., Sari Doré, F., Dupanloup, M., Bourgain, F., Perrier, S., and Riva, M.: Characterization of the newly designed wall-free particle evaporator (WALL-E) for online measurements of atmospheric particles, Atmos. Meas. Tech., 18, 5087–5101, https://doi.org/10.5194/amt-18-5087-2025, 2025.Riva, M., Pospisilova, V., Frege, C., Perrier, S., Bansal, P., Jorga, S., Sturm, P., Thornton, J. A., Rohner, U., and Lopez-Hilfiker, F.: Evaluation of a reduced-pressure chemical ion reactor utilizing adduct ionization for the detection of gaseous organic and inorganic species, Atmos. Meas. Tech., 17, 5887–5901, https://doi.org/10.5194/ , amt-17-5887-2024, 2024.