2.2.1. PM Emissions

Particulate matter concentration was measured with an Engine Exhaust Particle Sizer (EEPS 3090 analyzer, Shoreview, MN, USA). EEPS enables performance of the particle size distribution for discrete range of particle diameters, from 5.6 nm to 560 nm. The exhaust gases are directed to the device via dilution system and temperature maintenance system. The pre-filter collects particles larger than 1 μm in diameter that are outside the measuring range devices [37,38].

During the tests, the number concentration of particles in the exhaust gas was measured by EEPS, and the particulate matter sample was collected separately on the filter. The filter used in test was a Glass Fiber Filter 37 mm. For the diesel engine, the EEPS probe was behind the diesel particulate filter, while the Glass Fiber Filter was in front of diesel particulate filter. The filter was placed in a separate pipe in which the pump drew in the exhaust gases from the exhaust system. For the petrol engine, the EEPS probe and the filter were in the exhaust of the motor scooter, and the filter also was between the exhaust system and the pump, which drew the exhaust gases in to pass them through the filter.

#### 2.2.2. Fourier Transform Infrared Spectroscopy (FTIR)

FTIR spectra of PM adsorbed on the filter were collected using Vertex 70, produced by Bruker, Billerica, MA, USA. The PM adsorbed on filter without any treatment were analyzed by use of the Attenuated total reflection (ATR) technique. The PIKE MIRacle™ accessory with diamond crystal was used. The measurements were performed at a resolution of 0.5 cm−<sup>1</sup> in the wavenumber range 4000–400 cm<sup>−</sup>1.
