Development and Application of Oxidation Flow Reactors in Engine Exhaust Studies

Combustion engines emit pollutants, including aerosol particles that affect air quality and climate. Some of the gaseous pollutants create secondary aerosol mass, that is, they react with oxidants in the atmosphere and form new particle mass in a time frame of hours to days after the initial emission. To decrease the atmospheric impacts of vehicle emissions, one should address the secondary aerosol formation potential because the primary particle emissions of modern vehicles are already low. However, the measurement of secondary aerosol formation potential is not as straightforward as that of the primary emissions.

This thesis focuses on oxidation flow reactors (OFR) that can be used to measure the secondary aerosol formation potential from combustion engines. The first part of the thesis concerns the OFR development, characterization and methodology of OFR measurements and data analysis in engine exhaust studies. The second part utilizes the established methodology to study the effect of driving conditions, exhaust after-treatment and fuel type on the secondary aerosol formation potential.

Characterization of an OFR is an essential step in OFR usage since the OFR characteristics affect both experimental planning and post-experimental data analysis. Characterization of the OFR response to changing gas concentrations is important especially in transient driving cycles, where the measured secondary aerosol must be aligned with changes in exhaust conditions.

The experiments in this thesis showed that it is possible to decrease the secondary organic aerosol (SOA) formation with a fuel change or by increasing exhaust after-treatment. The most notable effect of driving condition on SOA formation was related to engine and after-treatment temperature, but also differences in secondary aerosol formation between e.g. accelerations and steady speed driving were observed.