Vehicle exhaust measurement

Emissions from individual car traffic, particularly diesel cars play an important role in urban air quality, such as concentration of soot aerosol particles in urban environments. Vehicle emissions consist of primary emitted particles (mainly soot) and secondary produced ultrafine particles. A mixture of both is usually measured at traffic dominated locations.

The goal of this project was the development of a measurement system to study aerosol transformation processes within the first meter behind the tailpipe under ‘real-world’ dilution conditions and to understand the dilution and aerosol dynamics within and around the exhaust plume using simulations. The advantage of this method compared to former experiments is that primary emitted as well as secondary formed particles could be measured. One major goal of the project was to understand the formation of ultrafine particles (10 – 20 nm in diameter). This is impossible by laboratory measurements because dilution process is more or less instantaneous.

Measurements have been performed in the exhaust of a diesel and a petrol passenger car for constant speed driving on a highway as well as for variable speeds in the city. The main focus was set on the diesel car measurements because these emissions are suspected to have major effects on human health.

Figure 1: Time series of car and engine speed (a) as well as particle number size distributions measured behind a diesel passenger car.

Number size distributions from the diesel car show always a clear soot mode for constant speed driving. The expected formation of high number concentrations of ultrafine particles within the first meter behind the tailpipe was found only under high engine load conditions. Higher engine revolutions by selecting a lower gear led to high number concentrations of particles < 20 nm. These driving conditions could be comparable with acceleration periods or driving on roads with ascending slope, thus maybe the ultrafine particles are mainly produced during such periods which happen frequently in urban traffic.

Additionally, coupled CFD and aerosol dynamics simulations have been conducted for H₂SO₄-H₂O and soot particles based on the measurements within the diesel vehicle exhaust plume for constant speed driving. The simulation confirmed the occurrence of nucleation during certain conditions like high speed and high engine revolutions. Also other characteristics like temperature, trace gas concentrations have been reproduced within a good agreement. The simulated growth of H₂SO₄-H₂O nucleation particles was insufficiently low compared with measurements. The possible role of low and semi volatile organic components on the growth processes was roughly estimated. Simulations for simplified H₂SO₄-H₂O-octane-gasoil aerosol resulted in faster and sufficient growth of nucleation particles.