Cascade impactors: Cascade impactors size and collect particles through inertial impaction onto a series of stages. A filter is placed onto each stage to collect the PM. We used a 5-stage SKC Inc. Sioutas impactor with Whatman and Zefluor PTFE filters (25 mm diameter on stages 1-4 and 37 mm diameter on stage 5, 0.2 µm pore size). Filters were acid-washed following the same procedure as described for filter packs. The Sioutas impactor resolves 5 size fractions between >2.5 µm and >0.25 µm at a flow rate of 9 l min-1. Airflow is created by an external pump with inbuilt battery (SKC Leland Legacy). The pumps had been calibrated prior to both campaigns by the manufacturer, with reported accuracy in the flow rate of 5%..