Sampling and experimental strategy overview. Particles were sampled with four 13 stage low pressure Dekati impactors running in parallel for 22 to 98 h according to samples (Step 1). For chemical analysis (Step A), gravimetry, ions and carbon content were determined on each stage filter. For biological analysis, filters were gathered to constitute 4 PM-size fractions (PM0.03–0.17, PM0.17–1, PM1–2.5, PM2.5–10). They were briefly sonicated directly in 600 μL culture medium (Step 2). HBECs were exposed for 24 hrs to the different PM-size fractions (Step 3) either at the same volume of particle suspension (isovolume exposure) or at the same concentration of particle suspension (isomass exposure). After exposure, GM-CSF release was measured in the culture medium and cell viability was assessed using a propidium iodide (PI) assay (Step 4). Specific short samplings (20 min) were performed to collect particles on specific supports for transmission electron microscopy (TEM) observations (step B).