Unveiling organic aerosol sources through combined offline and online measurements at two French urban sites

Carbonaceous aerosols play a key role in urban air quality. However, their sources, particularly those contributing to secondary organic aerosol (OA), remain difficult to disentangle using conventional measurements. This study investigates the sources of carbonaceous aerosol at two French urban sites, in Lyon and Talence, by combining online (Aerosol Chemical Speciation Monitor (ACSM)) and offline (daily filter‑derived organic molecular markers) observations using the Multi‑Time‑Resolution Positive Matrix Factorization approach (MTR‑PMF). ACSM-PMF analysis yields a dominant fraction of oxygenated OA (OOA ~ 70%) at both sites, but without clearly resolving its secondary origins. The MTR-PMF method improves the identification of OOA sources and strengthens the chemical interpretability of primary OA source contributions. Using MTR‑PMF, six factors were identified at each site: traffic, biomass burning, biogenic secondary OA, anthropogenic secondary OA, low‑volatility OOA, and either cooking-influenced OA (in Lyon; 15%) or marine secondary OA (in Talence, 5%). The behavior of secondary sources exhibited strong spatial variability. Overall, this study highlights the added value of integrating multi‑timescale aerosol observations to disentangle urban OA sources, providing here new insights into the origins of OOA in urban environments and offering improved constraints on OA source contributions.