Volatility of ultrafine and solid particles at a traffic site in Athens, Greece.

Atmospheric aerosols significantly impact the Earth’s climate and human health. The fraction of ultrafine particles (UFPs) present significant health risks because of their high surface area-to-mass ratio and their ability to penetrate deep into the lungs. Urban areas are hot spots for human exposure to UFPs, because they are strongly influenced by traffic exhaust emissions. Those emissions can be emitted directly as primary particles (soot, carbonaceous aggregates) or as so called secondary particles formed in the tailpipe after cooling of the exhaust gases or even in ambient air by further oxidation process induced by sunlight (volatile nucleation mode particles).

Once they are released into the atmosphere, soot particles undergo aging processes, during which they become internally or externally mixed with secondary organic and inorganic species, resulting in a core-shell structure. The solid core mainly consists of black carbon, while the shell is primarily composed of volatile and semi-volatile organic compounds. To identify the link between traffic exhaust emission in the tailpipe with “real-word” emission, also solid particle number (SPN) measurements have to be performed in ambient conditions. Thermodenuders, catalytic strippers, and Volatility Tandem Differential Mobility Analyzers (VTDMA) are state of the art techniques for SPN measurements. Thermodenuders and catalytic strippers are designed for the rapid removal of volatile components from solid particles, while VTDMA offers more detailed insights into the mixing state of size-selected aerosol particles.

This study aims to characterize UFPs at a traffic site in Athens by analyzing particle number concentrations, size distributions and volatility. Those measurements were compared to the Demokritos Athens suburban research station. Volatility measurements of size-selected particles (30 nm, 50 nm, 80 nm, and 120 nm) were performed using a custom-made VTDMA, operated at temperatures of 25°C, 110°C, 200°C, and 300°C. Data processing was carried out using the TDMAinv algorithm (Gysel et al., 2009). Additionally, a comparison of the VTDMA and catalytic stripping techniques was conducted under controlled conditions during lab experiment with miniCAST test aerosol at the PTB facility for exhaust emission tester.

Volatility was expressed in terms of aerosol particle number fraction (NFR) and volume fraction (VFR) remaining, shrink factor (SF) and mixing state. The aerosol particles at the traffic site appear to be closely linked to road traffic, as their solid particle number concentration peaks during traffic rush hours.  These particles are externally mixed and consist of volatile, semi-volatile, and refractory components. The NFR at 300°C was 8 % for nuclei mode particles, while for Aitken mode the  NFR was higher (>50%), indicating that a significant fraction of Aitken mode particles is composed of a solid particle fraction consist of black carbon particles. The SF was 70% for nuclei mode particles and around 50% for Aitken particle.

Gysel, M., McFiggans, G.B., Coe, H., Inversion of tandem differential mobility analyser (TDMA) measurements, J. Aerosol Sci., 40, 2009, https://doi.org/10.1016/j.jaerosci.2008.07.013, 2009.