Combustion-generated particles detection with a multi-wavelength UV photoelectric charging setup

Introduction
Vehicles powered by direct injection engines, both diesel and gasoline, are considered a primary source of ambient particle related pollution. Particles generated by internal combustion engines mainly consist of black carbon, metals, inorganic oxides, and volatile species that among other compounds contain the poly-aromatic hydrocarbons, PAHs. From a public health perspective, some species of PAHs are connected with adverse health effects and have been listed by the International Association for Research on Cancer as definitely carcinogenic.
In this study, we present a new aerosol detection instrument, the multi-wavelength UV photoelectric charger (UV-PEC) that detects on-line the PAH content of ultrafine particles providing the advantage to investigate their innate properties in real-time and without the interference of a substrate (filter etc.) that typically ex-situ techniques use. The principle of operation is based on the photoelectric effect; when an aerosol is irradiated with UV-light of energy below the ionization threshold of the carrier gas molecules but above the photoelectric threshold of the particles, photoelectrons may be emitted from the particles which acquire a positive charge (Burtscher et al., 1982). It is well-established that PAHs generate a high photoelectric yield compared to other particulate components in ambient air, permitting the selective charge of particles that originate from incomplete combustion (Burtscher, 1992).

Methods
UV-PEC consists of an electrostatic precipitator that removes the naturally charged particles, a UV light source, and an ionization chamber. The UV light beam is produced by a deuterium lamp with tunable wavelength (206, 214, 228, 239, and 250 nm) modified by using bandpass filters placed at a filter wheel. Downstream the UV-PEC an electrometer (Palas, Charme) measures the charged particles number concentration, Cch, while a CPC (TSI, 3756) measures the total number conce-ntration, Ctot. The ratio Cch/Ctot defines the charging efficiency, ηCE. In parallel, an SMPS that operates without a neutralizer, measures the size-dependent charging efficiency which can be directly related to the particle’s active surface area (Keller et al., 2001).

UV-PEC was employed for the detection of aerosol particles generated by a CAST generator and a diesel engine. The diesel engine was a single cylinder, four-stroke, 5kW, air-cooled direct injection engine that operated at 27% of its maximum load. Figure 1 plots the charging efficiency of diesel-generated particles using a low sulphur diesel against the light wavelength. Measurements were performed either downstream a catalytic stripper (T=400oC) or without any particle conditioning.
Conclusions
The developed UV-PEC setup exhibited the ability to detect PAH molecules attached on ultrafine combustion-generated particles surface. Such an instrument may be employed for the characterization of modern engine exhaust particles especially nucleation mode particles that their nature is still under discussion by the scientific community.