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Study finds air quality models significantly underestimate traffic as source of NOx in Europe

Traffic contributes more to NOx emissions in Europe than previously thought, according to a new study by a team at the University of Innsbruck. Using urban eddy covariance measurements, the researchers found that traffic-related NOx emissions in current operational air quality models can be significantly underestimated by up to a factor of 4 across countries with a sizeable fraction of diesel-powered cars in their fleet. An open-access paper on their work appears in Scientific Reports.

Large metropolitan areas throughout Europe consistently breach maximum permissible values of NOx; furthermore, this phenomenon appears to be spreading, with many smaller scale cities and towns—including their surrounding rural areas—seeing frequent NO2 concentration violations, the researchers noted.

It has been a challenge to determine how much each polluter contributes to the emission output. Until now emission levels were mainly calculated by collecting emission data at laboratory testing facilities and subsequently extrapolating them in models. However, the amount of pollutant emissions that vehicles emit on a daily basis depends on numerous factors, for example on individual driving behavior.

The recent diesel scandal showed, for example, that measurements at engine test stands based on the New European Driving Cycle (NEDC) or similar emission testing procedures can be highly uncertain for predicting actual environmental impacts. A large number of new studies have recently been published suggesting that emission levels from test stands have to be adjusted upwards.

Environmental protection and health agencies base their air pollution management on atmospheric models that rely on these experimental data from test facilities. While there have been some doubts about nitrogen oxide emissions for some time, scientists lacked the technology to measure the actual amount of emitted pollutants in a specific area and to determine their overall source strength.

The team of physicists headed by Thomas Karl from the Institute of Atmospheric and Cryospheric Sciences at the University of Innsbruck has expanded on earlier work that has used urban flux measurement for energy and CO2 by simultaneously measuring NOx, selected tracer NMVOCs, CO and CO2. The result is a well-constrained flux dataset that allows testing the understanding of prominent NOx emission sources.

We continuously measure the concentration of carbon dioxide, nitrogen oxide and volatile organic compounds at our urban observatory in Innsbruck. We record 36,000 data points per hour.

—Thomas Karl

Using statistical methods, the scientists infer emissions from these data within a radius of about one kilometer of the measurement location. The analysis of the data of a 3-month measurement campaign showed two main sources for nitrogen oxide concentrations in the Innsbruck air: traffic and residential combustion, with traffic accounting for more than 80% of the nitrogen oxide emissions in the surroundings of the test station at the University. The majority of the emissions is caused by diesel cars.

Generally, we observe 50–70% higher NOx emissions relative to CO2 from road traffic than what is calculated with the most recent traffic emission models. In these detailed bottom-up models, mobile source emissions are treated for different engine sizes and fuels, that, in the past, relied on standardized protocols obtained in test facilities, but were recently updated based on a number of RDE tests. Exhaust from modern gasoline powered engines, despite higher ignition temperatures than those powered by diesel, can be effectively treated for NO along with NMVOC (and CO) using a three way catalytic (TWC) converter. TWC treatment can lead to a 10 fold reduction of NO. This has been hard to achieve for diesel powered cars, which nowadays mostly rely on selective catalytic reduction due to the high air to fuel ratio during combustion.

Generally, different combustion and exhaust treatment characteristics result in significantly higher NOx/CO2 emission ratios for diesel powered cars than for gasoline. The modelled fleet average contribution suggests that at least 90% of urban NOx emissions should originate from diesel driven vehicles at the present location. 85% is modelled to be emitted by the passenger car fleet based on the COPERT model and TRACCS database.

The current Austrian passenger car fleet comprises about 50% diesel cars and the percentage across Europe grew at a substantially faster rate compared to the US. Based on our measurements the current average Austrian car fleet emits about 36 times more NOx per CO2 molecule compared to the US TIER II emission standard and a factor of 8–10 more than Euro 6 emission standards. Factoring in differences in fuel economy between the European and US car fleet, this would equate to about an order of magnitude more NOx emissions per travelled distance compared to newly introduced emission standards.

… The presented flux measurements indicate that traffic-related NOx emissions in current operational air quality models can be significantly underestimated by up to a factor of 4 across countries exhibiting a sizeable fraction of diesel powered cars in their fleet. As diesel fuels (including bio-diesel) could account for 70% of the growth in transportation fuels by 2040, with significant demand in Asian markets according to industry projections, a better understanding of the uncertainty in associated changes of NOx fluxes and ozone chemistry will therefore be important for future environmental impact studies.

—Karl et al.

The researchers said that their work demonstrated that parallel flux measurements of a wide range of chemical species can be used to benchmark urban emission sources, complement traditional approaches and significantly improve uncertainties inherent to bottom-up scaling in atmospheric chemistry models.


  • T. Karl, M. Graus, M. Striednig, C. Lamprecht, A. Hammerle, G. Wohlfahrt, A. Held, L. von der Heyden, M.J. Deventer, A. Krismer, C. Haun, R. Feichter, J. Lee (2017) “Urban eddy covariance measurements reveal significant missing NOx emissions in Central Europe,” Scientific Reports 7, 2536 doi: 10.1038/s41598-017-02699-9


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