Pre baked 90 mm quartz fiber filters

23-h filter samples were collected from 08:00 to 07:00 the following day. PM_2.5 samples were collected onto pre-baked 90 mm quartz fiber filters (Pall Life Sciences) using a medium volume sampler (URG Corp.) equipped with a sharp-cut cyclone to select PM <2.5 μm at a flow rate of 90 L min⁻¹. The sampler was mounted on to a wooden platform with the inlet 2.5 m above ground level with a flow rate of 90 L min⁻¹ Size-resolved PM samples were collected over 23 h by three 5-stage Sioutas Cascade impactors (SKC Inc.) that were installed on the wooden platform 2 m above ground level and collected particles with aerodynamic diameters >2.5, 1.0–2.5, 0.50–1.0, 0.25–0.50, and <0.25 μm (50% cutoff diameters). Samples were collected at a flow rate 9 L min⁻¹ onto 25 mm Teflon filters (SKC Inc.) for chemical and biological analyses. A third impactor operating at a flow rate 8.5 L min⁻¹ collected particles onto 25 mm polycarbonate filters (Isopore, Fisher Scientific, pore size 0.1 μm) for scanning electron microscopy (SEM) analysis. Pre-baked quartz fiber filters (37 mm, Pall Life Sciences) were used as after-filters in all three impactors. Teflon and quartz filters were stored in a freezer at −20°C while polycarbonate filters were stored in a desiccator (relative humidity <20%) at ambient temperature.