News 13.5.2024

Vehicle exhaust emissions can be reduced through aftertreatment technologies and fuel composition

Finnish Meteorological Institute participated in a study that investigated exhaust emissions from vehicles complying with the Euro 6 emission standard. Particle filters proved effective in reducing particle emissions, but fuel choice also plays an important role.
Photo: Juha Tuomi / Rodeo

The study found that particle number concentrations in gasoline cars without particle filters were over 200 times higher compared to those equipped with filters. Diesel car particle emissions were low when the filter was in use, except during filter regeneration, when particle concentrations temporarily increased. 

The use of aromatic compound-free fuel emerged as an effective method to reduce vehicle exhaust emissions. Transitioning to aromatic-free fuel led to decreased particle number and mass concentrations. However, the impact of fuel aromatics on nitrogen oxide concentrations was less significant. This suggests that altering fuel composition may be a faster way to influence vehicle exhaust emissions than modifying aftertreatment systems. 

The study also found that aromatic compounds both in diesel fuel and gasoline affect the toxicity of exhaust emissions. Particularly, the toxicological response of cells increased in gasoline cars using aromatic fuel. Additionally, driving temperature influenced toxicity; measurements conducted at colder temperatures produced a greater inflammatory response in cells compared to measurements at warmer temperatures. Exhaust toxicity also increased during diesel particulate filter regeneration compared to normal driving conditions. 

Nanoparticles and secondary aerosols also require emission limits 

The study measured particles across a wide size range, from 1.2 nanometers to 1 µm. Although the Euro 6 emission standard restricts solid particles larger than 23 nanometers, this research demonstrated that smaller particles should also be included in emission standards. Notably, emissions contained a significant number of particles sized between 1.2 and 10 nanometers. The proposed size limit for particles in the upcoming Euro 7 standard is 10 nanometers, but even smaller particle emissions should be restricted in the future. 

Secondary particle emissions have no established limits in emission standards. In this study, secondary particle emissions could be over 500 times greater than fresh particle emissions. The highest secondary particle emissions occurred in gasoline cars without particle filters, using regular gasoline containing aromatic compounds. However, relative to fresh particle emissions, the most significant secondary particle emission occurred in gasoline cars equipped with filters, as their fresh particle emissions were very low. The study also observed that aromatic-free alkylate gasoline produced significantly fewer secondary particles than aromatic gasoline. Although strict emission limits have been set for vehicles, emission standards still do not cover all compounds or exhaust characteristics harmful to human health and the environment. 

Road traffic significantly impacts air quality, especially in urban areas. Vehicle exhaust emissions can be reduced in various ways. In addition to driving behavior and conditions, emissions can be minimized through engine technology or exhaust aftertreatment devices. The most common aftertreatment devices include three-way and oxidation catalysts, selective catalytic reduction (SCR), as well as gasoline and diesel particulate filters. Furthermore, fuel properties and composition also influence exhaust emissions. 

This study was conducted as part of the EU Horizon 2020 program’s “Transport derived Ultrafines and the Brain Effects (TUBE)” project. The study involved collaboration between Finnish Meteorological Institute, VTT Technical Research Centre of Finland, Tampere University, and the University of Eastern Finland. Vehicle emissions were measured in laboratory conditions using a dynamometer at two different temperatures: cold (-8–2 °C) and warm (22–24 °C). The measurements followed the Worldwide Harmonized Light Vehicles Test Cycle (WLTC) driving cycle. The vehicles used various fuels, including EN228 gasoline, alkylate gasoline, EN590 diesel fuel, renewable diesel fuel (hydrotreated vegetable oil, HVO), and compressed natural gas. The vehicles were equipped with three-way catalysts, particulate filters, and SCR systems. 

Additional information: 

Senior Scientist Sanna Saarikoski, Ilmatieteen laitos, 

Postdoctoral Researcher Henri Hakkarainen, University of Eastern Finland, 

The research articles are available from:

Saarikoski, S., Järvinen, A., Markkula, L., Aurela, M., Kuittinen, N., Hoivala, J., Barreira, L. M. F., Aakko-Saksa, P., Lepistö, T., Marjanen, P., Timonen, H., Hakkarainen, H., Jalava, P., Rönkkö, T. (2024). Towards zero pollution vehicles by advanced fuels and exhaust aftertreatment technologies, Environ. Pollut., 347,

Hakkarainen, H., Järvinen, A., Lepistö, T., Kuittinen, N., Markkula, L., Ihantola, T., Yang, M., Martikainen, M., Mikkonen, S., Timonen, H., Aurela, A., Barreira, L., Ihalainen, M.,   Saarikoski, S., Aakko-Saksa, P. T., Rönkkö, T., Jalava, P. (2024). Effects of fuel composition and vehicle operating temperature on in vitro toxicity of exhaust emissions, Environmental Science: Atmospheres, 

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