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Traffic is a major source of atmospheric aerosol particles

A recent study by TUT researchers indicates that traffic is a significant source of the smallest atmospheric aerosol particles, i.e. nanoclusters.

The findings help to understand the origin and formation of atmospheric aerosol particles in urban areas and create new possibilities to achieve the goal of better air quality.

Traffic-originated particle emissions affect the air quality in cities and are therefore linked to human health.   The research was conducted under the supervision of the Tampere University of Technology’s Aerosol Physics research group, and it was the first one to connect the urban area’s nanocluster aerosol concentrations to traffic.  The research was published in the highly respected PNAS publication series.

The research focused on nanoclusters, i.e. the smallest aerosol particles. Especially in connection to urban air, many issues related to atmospheric nanoclusters have been unclear. These issues include, for example, the concentrations and sources of nanoclusters. The quantities and properties of nanoclusters are estimated to affect, for example, the formation of clouds. Depending on their composition, nanoclusters may also have health effects.

"Earlier there wasn’t any information available on the effects of traffic to the smallest particles in outdoor air. According to our research, as much as over half of the particles present in a roadside environment might be nanoclusters," says the research leader, research manager Topi Rönkkö from the aerosol physics unit of TUT.

The researchers also determined the emission factors of nanoclusters in several different traffic environments. The emission factors mean the number of nanoclusters formed in relation to one kilogram of fuel spent in road traffic.

"I believe that emission factors could benefit especially the modelling research on air quality and climate," Rönkkö says.

Driving through Europe to make measurements

The researchers carried out measurements in two locations in Helsinki: By the roadside of Ring I and in a street canyon. In addition, measurements were carried out on the road while driving from Northern Spain to Tampere, Finland. Stationary measurements were carried out in the Helsinki Region Environmental Services Authority’s measuring stations, and the measurements on  road were carried out by a research vehicle, a so-called mobile laboratory. In all of the measurements, the most important equipment was an instrument (PSM, Particle size magnifier) made by a Finnish company called Airmodus Ltd.

"The mobile laboratory and the diverse measuring equipment it contains enable the study of nanoclusters in different environments and even on the road," says TUT professor Miikka Dal Maso, who carried out the measurements on the road with three other researchers.

Even though the average nanocluster emissions, i.e. the quantity of nanoclusters emitted by traffic in relation to one kilogram of fuel consumed, were usually on the same level in these measurements, the concentration of nanoclusters in the air varied. According to the research, this is caused by, for example, the wind conditions and the openness of the environment. The highest average nanocluster concentrations were measured in tunnels, but the concentration levels temporarily reached high levels also in open environments.

"It might be advisable to take notice of the findings in, for example, the design of traffic environments," Topi Rönkkö states.

Vehicle technology affects the level of nanocluster emissions

The researchers supplemented the measurements made in the traffic environments with experiments carried out in the engine laboratory. The findings supported the outcome of the measurements carried out in a traffic environment: nanoclusters form an essential part of particle emissions from engines, but their quantity is also dependent on, for example, the driving situation. According to the researchers, it is also probable that the techniques used in a car, such as the fuel type and the exhaust gas cleaning system, all have an effect in nanocluster emissions. This is also supported by observations made in the roadside: nanoclusters were not present in the exhaust gas of every car that drove past the measuring station.

"It is likely that, for example, the high content of sulphuric acid in the exhaust gas advances the formation of nanoclusters, and therefore increases the vehicles’ nanocluster emission level. According to earlier research, the sulphuric acid content of the exhaust gas depends at least on the engine load, the temperature of the catalyst and the sulphur content of the fuel and lubricant," says Rönkkö.

The research is available online at the PNAS website http://www.pnas.org/content/early/2017/06/26/1700830114. Research was carried out in close cooperation with TUT, the Finnish Meteorological Institute, Helsinki Region Environmental Services Authority HSY, Metropolia University of Applied Sciences and Turku University of Applied Sciences. The research has been funded by Tekes, the Academy of Finland, CLEEN, Dinex Ecocat Ltd, Neste Corporation, AGCO Power, AB Nanol Technologies Ltd and the Strategic Research Council.

Further information: research manager Topi Rönkkö, Aerosol Physics laboratory, TUT. Tel. +358 40 198 1019, topi.ronkko@tut.fi

News submitted by: Mika Puonti
Keywords: science and research, physics, pnas, aerosol physics