High-temperature solid particle emission measurements in the sub-23nm mobility size range with the Advanced Half-Mini DMA

Current particle number related vehicle emission legislation is limited down to 23 nm. The rationale behind the 23 nm cut-off size is based on the avoidance of significant uncertainties created during sampling and measuring sub-23 nm solid particles. Difficulties in introducing a robust measurement protocol in this particle size range led regulation authorities to ignore their contribution to ambient pollution. However, the sub-23 nm particles draw high attention since current gasoline engines emit almost equal fraction of sub- and above- 23 nm particles, while diesel engines with after-treatment devices may also emit such ultrafine particles under certain conditions (i.e. regeneration of particulate filters) (Giechaskiel and Martini, 2014).

Number concentration measurement certification methods rely exclusively on a protocol defined from the Particle Measurement Programme (PMP) working group. Accordingly, the raw exhaust is driven to a full flow dilution tunnel and subsequently to a Volatile Particle Remover (VPR) to remove possible volatile and semi-volatile material that condenses on solid particles or creates a separate nucleation mode. The VPR consists of a hot and a cold dilution stage while an evaporation tube or a catalytic stripper (CS) are inserted between these two dilution stages. One of the problems identified with the standard sampling procedures is the potential formation of particles in the sub-23 nm region, the so-called artefacts, as well as the high particle losses due to diffusion.

To overcome the above-mentioned uncertainties we investigate a novel approach for sub-23 nm measurements that necessitates a single hot dilution stage and a high-temperature size classification measurement with the Advanced Half-Mini Differential Mobility Analyzer (HM-DMA). The Advanced HM-DMA system is a supercritical DMA, with a 2 cm working section, able to classify aerosol particles in the mobility size range 4 – 30 nm with high resolution and fast spectrum acquisition (de la Mora, 2017). Due to recent modifications on the semiconducting glass tube in the path from the inner electrode to the grounded outlet, it can accommodate a hot sample flow up to 200 °C (Amo et al., 2017).     

 Advanced HM-DMA was initially tested against solid sub-23nm NaCl and soot particles generated by an Electrospray Aerosol Generator (TSI, 3482) and a Combustion Aerosol Standard (CAST, Matter Engineering) burner, respectively. CAST-generated particles were catalytically pre-treated to ensure their solid nature. Hot and cold aerosol flow measurements were in good agreement showing that the HM-DMA resolution is not influenced in the hot operation mode.

In order to test the Advanced HM-DMA performance with diesel exhaust, a solid particle nucleation mode was generated with a single cylinder, four-stroke, air cooled and direct injection diesel engine by using high-sulfur diesel with addition of lubricant oil. Two different experimental setups were used; a single 10-fold hot dilution stage followed by the Advanced HM-DMA in hot operation mode and a PMP-compliant setup with a hot dilution stage, a CS, and a cold dilution stage followed again by the Advanced HM-DMA for reference measurements. The excellent agreement between the two measurements confirms the reliability of the Advanced HM-DMA hot operation mode and indicates the possibility of using a simple setup for solid particles measurement.