The debate on diesel vehicles focuses on nitrogen oxides – air pollutants that attack the mucous membranes and respiratory tract and increase the risk of cardiovascular diseases. In order to comply with the limits value set by the EU, cities could therefore impose driving bans on diesel cars in the future. This would reduce the concentration of nitrogen oxide in particularly congested zones. It is less obvious that this may have an undesirable side effect: the increased formation of ozone, which is also a health hazard.
High up in the stratosphere at an altitude of 15 to 30 kilometres, ozone protects against dangerous UV radiation. In the lower air layers, it causes, for example, headaches, coughing, watery eyes and breathing problems. These effects have been known for a long time: there are Europe-wide limits to protect the population. “Even 20 years ago, there were regular ozone alarms in Germany,” explains Franz Rohrer from the Jülich Institute of Energy and Climate Research (IEK-8). “Today they are rare. This is because air pollution control measures were implemented in the late 1980s. The industry uses better filters, cars have become cleaner. This ultimately has resulted in lower ozone levels.”
This could change again as a result of the diesel driving bans. “One of the reasons for the reduction of ozone levels in busy areas is the high nitrogen oxide levels,” explains Rohrer’s colleague Robert Wegener. Nitrogen oxides are one of the parameters in a complicated series of chemical reactions that lead to the formation and decomposition of ozone. Two other actors are hydrocarbons and hydroxyl radicals (OH radicals). The hydrocarbons come from exhaust gases, but are also emitted by plants; OH radicals are formed under the influence of sunlight.
Ozone is the product of a cascade of chemical processes beginning with the reaction of OH radicals to hydrocarbons. “The nitrogen oxides influence this reaction,” explains Wegener. “In low concentrations, they can accelerate the formation of ozone. If the amount of nitrogen oxides is too high, however, they react directly with the OH radical and the hydrocarbons miss out, so to speak. This suppresses the formation of ozone.”
The ratio of nitrogen oxides to hydrocarbons is decisive here. In simple terms, if both substances are present in a certain ratio, a particularly large amount of ozone is produced. “We can display this dependence for a definite quantity of hydrocarbons in a simple graph. It looks like a mountain, its peak marking the nitrogen oxide-hydrocarbon ratio that produces the most ozone. To the left and right of it – that is, with less and more nitrogen oxides – ozone formation decreases,” explains Wegener.
Taking both concentrations into account, nitrogen oxides and hydrocarbons, results in what resembles a topographic map: contour lines correspond to the ozone production at a certain quantity of hydrocarbons and nitrogen oxides.
In the mid-1990s, the concentrations of hydrocarbons and nitrogen oxides were high. A lot of ozone was produced: the production rate was in the red. The introduction of catalytic converters reduced hydrocarbon concentrations to one-fifteenth, while nitrogen oxide levels fell by only half. The ratio of nitrogen oxide to hydrocarbon increased, the OH radicals were increasingly removed from the atmosphere by the nitrogen oxides and thus less ozone was formed. Today we are in the blue area on our “hiking map” – the ozone production rate is significantly lower.
But what happens if driving bans reduce only nitrogen oxide emissions in future? “We would walk to the left on our map and thus backwards up the mountain again, so to speak – and more ozone would be produced again in busy areas,” according to Rohrer.
How can this be prevented? Appropriate technical measures would have to be developed for exhaust gas aftertreatment. In addition to the reduced nitrogen oxides, the amount of hydrocarbons could thus be further reduced. “This could be achieved by further improving the cold start behaviour of automotive catalytic converters, for example,” said Rohrer. Alternatively, Wegener knows, structural modifications can reduce locally high concentrations of all air pollutants: “Nitrogen oxides are very short-lived and thus hazardous to health only right where they occur. They stay in the street canyons of the big cities only for a few minutes before they spread further. If the air flowed faster through a street, they would not be a problem.” Some cities follow this path by removing, for example, individual houses in terraced housing estates. This creates a crosswind; the nitrogen oxides – and other pollutants – disperse. In modern Asian cities, such effects are included in the construction planning from the outset. This also contributes to less ozone.
However, the ratio of nitrogen oxides to hydrocarbon is not the only parameter in ozone formation. The process is extremely complex. This starts with the fact that there are thousands of different hydrocarbons with different lifetimes and chemical structures. Some are more involved in the formation of ozone, others not at all. The situation is similar with nitrogen oxides. The most important ones here are nitric oxide and nitrogen dioxide, the two of which have different effects on ozone formation.
In addition, external conditions such as climate zone, wind strength and direction, solar radiation as well as the time of day and night play an important role.
“In order to really assess how we can improve air quality in a particular place, we have to take regional characteristics into account. This requires sophisticated measurement data and reliable computer models,” says Rohrer. The Jülich climate researchers are therefore using their MobiLab measuring vehicle to collect data on the actual pollution levels in German cities. How exactly are nitrogen oxides and other pollutants distributed in the big cities? Where are the particularly heavily polluted areas? Who are the polluters? By answering these and similar questions, the researchers hope at some point to be able to reliably say where a driving ban makes sense and where it does not.
Near-ground ozone production (green circles, ppb/h - parts per billion per hour) depends on the ratio of nitrogen dioxide concentration (bottom axis) to hydrocarbon concentration (left axis) – see diagram above.
Over the past years, Germany’s overall ozone pollution has decreased, but with significant regional variations (comparative data for 1994 and 2014). While in the suburbs (green) and inner city areas (dark green) the very high levels dropped considerably, they have remained almost unchanged in rural areas (light green).
The extreme decline in inner cities is due to the introduction of catalytic converters for motor vehicles. As a result, the hydrocarbon concentration fell to one fifteenth of its previous level, while the nitrogen oxide levels were halved. The altered ratio of these substances means that much less ozone is now being generated in busy areas.
The air composition is different in rural regions and suburban areas. There, the hydrocarbons in the air come largely from vegetation and less from vehicle exhaust gases. The concentration has fallen less here, so ozone production has not decreased to such a degree.
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