Once sampling was finished, each high volume sampler filter was folded in half face to face and placed in a plastic bag. Samples were then wrapped with aluminium foil and stored in a freezer before analysis. The mass of particles collected by each high volume sampler was obtained using a gravimetric method whereby the filters were conditioned for 48 h before and after sampling in a chamber having a relative humidity of 50% controlled by a salt solution (Young, 1967). The mass difference was achieved using a 5 figure electronic balance. After weighing a piece (45×45 mm2) was cut from each filter and extracted in 20 ml ultra-pure water (resistivity >18 M Ω cm) for 20 min with ultrasonication.