Diesel exhaust particles: On-road and laboratory studies

Abstract

Traffic is one of the most important sources of particulate pollution and its role is emphasized especially in roadside and urban environments. Because the fine particles of the ambient air have an impact on human health and environment, the interest in the formation and the characteristics of traffic related particles has been increased. Also, the limitations for particle emissions have become stricter causing a continuing need to improve vehicle technology. Both to estimate the effects of traffic related particles, on human health and on atmospheric environment, and to minimize the emissions, the studies concerning the particle characteristics and the formation mechanisms are needed. Within the vehicle fleet, diesel vehicles make a relatively high contribution to particle emissions. For diesel vehicles, the submicron exhaust particles can typically be divided into a nucleation mode and an accumulation mode. In addition to particle size, particles in the nucleation mode and the accumulation differ from each other by structure, composition and formation mechanisms. This thesis is based on experimental studies of traffic related particles in roadside environment and on the exhaust particle measurements conducted with individual diesel vehicles and with a diesel engine. With the individual vehicles, both on-road and laboratory experiments were made. While the on-road chasing measurements provide information on particle formation and characteristics in real-world conditions, the measurements on chassis dynamometers and at an engine test bench were conducted in order to clarify the correlation between the laboratory measurements and the realworld emission and to study the particle formation and the particle characteristics in well-defined conditions. The focus of this thesis is in the nucleation mode particles. In roadside environment, the particle number concentration depends on traffic rate and it is dominated by the nucleation mode particles. In winter conditions, the particle concentration is higher because the conditions seem to favour the formation of the nucleation mode particles. The formation of the diesel exhaust nucleation mode particles can be divided into different paths. If the nucleation mode particles are observed without an exhaust after-treatment, the particle formation is based on the existence of non-volatile core particles in raw exhaust and on the particle growth by hydrocarbon compounds during the exhaust dilution and cooling process. When a diesel oxidation catalyst is used, the formation of the nucleation mode particles can be based on the formation of the non-volatile core particles and, further, the particle growth by hydrocarbon or sulphur compounds depending on the engine load. However, in the case of the oxidation catalyst, also the sulphur driven nucleation during the dilution and cooling process is possible. In this case, the nucleation mode particles seem to be volatile and the particle formation is affected by exhaust dilution conditions and driving history. When a diesel particle filter is used, the formation of the nucleation mode particles occurs during the dilution and cooling process and the formation seems to be sulphur driven. The particle characteristics and trends in the particle formation seem to be similar in the real-world conditions and in the laboratory measurements. This indicate that, using appropriate exhaust sampling and dilution methods and parameters, the laboratory measurements can be used in the studies concerning both the nucleation mode particles and the accumulation mode particles of diesel exhaust.