Nitrogen dioxide (NO2) pollution, which largely stems from fossil fuel combustion, can have serious impacts on human health, including decreased lung function, increased asthma attacks, inflamed airways, and more. Diesel fuel is an acute source of NO2 pollution, particularly in Europe, where diesel fuel is more common. Scientists from the Barcelona Supercomputing Center (BSC), however, have used their supercomputing resources to discover that Barcelona’s vehicle-driven air pollution has been severely underestimated.

Despite the European standards for diesel emissions, most diesel light-duty vehicles (e.g. personal cars) fail to meet those standards in real-world conditions, leading to widespread failure to meet air quality limits – mostly in densely populated areas with lots of car traffic. Prior to the new study, the general nature of this effect had been well-studied. However, these studies were far from perfect: first, these researchers said, they omitted NO2 estimates in favor of more well-understood hazardous pollutants like particulate matter; and second, the studies had been conducted entirely at medium- or large-scale resolutions.

To zero in on more granular – and hopefully, more accurate – answers, the researchers used their in-house MareNostrum4 supercomputer. MareNostrum4 consists of 3,456 nodes equipped, each with dual Intel Xeon Skylake CPUs, as well as an aggregate 384.75 TB of memory. All in all, MareNostrum4 delivers around 6.5 Linpack petaflops, placing it 42nd on the latest Top500 list of the world’s most powerful publicly ranked supercomputers.

Using MareNostrum4, the researchers compared two scenarios for air pollution in Barcelona across three methods: a model with a four-kilometer resolution, a model with a one-kilometer resolution and a street-scale model. The modeling made use of CALIOPE-Urban, a new high-resolution hybrid model developed at BSC.

“Exposure to nitrogen dioxide, which is a potentially harmful pollutant for our health, is not homogeneous across city streets,” explained Jaime Benavides, a researcher in BSC’s Atmospheric Composition group and creator of CALIOPE-Urban. “For this reason, using higher resolution models, that is, on a street scale, helps us to better understand the real impact of the exposure of citizens to this dangerous pollutant.”

Based on the results, the researchers concluded that “the mesoscale model strongly underestimates the impact of diesel [light-duty vehicle] excess … emissions upon NO2 pollution” by 38 to 48 percent in absolute terms. These emissions, they found, were associated with around 20 percent of the city’s overall NO2 levels. Further, they showed that the higher-resolution model was able to capture the gradients in air pollution between denser and more open areas. 

In terms of public health impact, they concluded that around 90.2 percent of citizens in Barcelona are exposed to unhealthy NO2 levels under a business-as-usual scenario, but that if the city’s vehicles actually adhered vehicle emission standards, that percent would decrease to 76.6 – still above air quality limits.

“We have extrapolated the results and we have verified that, even if there had never been excess emissions from diesel cars and vans, Barcelona would continue to exceed the legal limits for NO2 air pollution at measuring stations exposed to traffic,” Benavides said. “This seems to indicate that, in addition to moving around in cleaner cars, it is necessary to drastically reduce traffic in general in order to breathe healthier air.”

The study, which was published in Environmental Research Letters, is available to read here.