Street-level air quality in European towns and cities is running significantly above EU limit values in any densely populated areas. Most problematic is the rise of harmful NO2 at street level – largely an unintended consequence of the past 15 years of regulations focusing closely on CO2 and diesel particulate emissions. Air quality experts from Ricardo Energy & Environment explain to Tony Lewin how the excess NOx arises and how new models in the pipeline promise to make tomorrow’s diesels clean on all counts
There can be little doubt that Europe has a problem with nitrogen dioxide (NO2) emissions. This is a pollutant that can directly harm human health and, disconcertingly, monitoring has revealed that in many urban areas concentrations of NO2, especially at street level where it is most harmful to young and old, remain high at many locations.
Overall, Europe’s steadily tightening vehicle exhaust emissions standards have been highly successful in controlling vehicle pollution over the past four decades. Average emissions of CO2 and greenhouse gases from new vehicles have more than halved since CO2 rules were first proposed in the late 1990s; hydrocarbons and carbon monoxide from modern engines are now at microscopic levels, and particulate matter – soot – has been all but eliminated from diesel exhausts.
But there is one area where this otherwise successful strategy has come unstuck – NOx, or oxides of nitrogen. Dr David Carslaw is Ricardo Energy & Environment knowledge leader for air quality and scientific development and also a specialist on urban air pollution at the University of York, yet even from his high-level viewpoint in the air quality debate he confesses surprise at the way the issue of NOx has been allowed to develop.
“If you had asked me 15 years ago whether we would be as concerned about air pollution as we are now, I would have said no. Fifteen years ago we expected emissions legislation to reduce total NOx from vehicles substantially, but what has happened is that those reductions haven’t occurred.”
Unintended consequences
“It’s almost an unintended consequence of trying to do something else,” continues Carslaw, “trying to control other emissions in diesel engines, such as carbon monoxide, hydrocarbons and, in particular, the emission of particles.”
The way that has been done, he explains, is through the introduction of the oxidation catalyst and the particle filter. “It is ironic that NO2 is actually really useful in aftertreatment, burning off the soot and hydrocarbons captured in the particle filter. So it is a good thing to have around as it lowers the temperature at which you can burn off the particles; it’s very effective, and reduces the particles by over 90 percent.
“The unfortunate side-effect of that,” he continues, “is that we have increased the amount of NO2 in diesel exhaust, and that is the important part of the NOx-NO2 story and the whole urban air pollution story.”
When it comes to NOx emissions, vehicles loom disproportionately large in the overall picture. Not only are the exhaust gases emitted at ground level, where they have maximum impact, but they also occur largely in urban areas where there are more people in close proximity and where they do most damage to public health.
Unusually, however, NOx arising from motor vehicle use is not a particular issue in the US and Japan, but a largely European problem – with the finger pointing directly at diesel. Europe has a very much higher proportion of diesel vehicles than other regions: roughly half of new passenger cars are diesels, as well as virtually all light and heavy commercial vehicles. And as Carslaw’s colleague Dr Beth Conlan, business area manager for air quality modelling, points out, Europe has also taken enthusiastically to urban traffic control measures. Many towns and cities are restricting the use of private cars in central areas to leave only buses and taxis – invariably diesel powered – to populate the streets. This, she says, compounds the problem as ‘canyons’ of higher NO2 concentration form in streets which are not wide enough to allow natural dispersion.
Why the focus on NOx?
In their early enthusiasm to control CO2 and noxious emissions such as particulates (PM) and hydrocarbons (HC), the European rule-makers focused on NOx as the regulatory metric while NO2 emissions were not specifically controlled: it was only with the publication of a paper on urban air quality by Carslaw in 2005 that it was revealed that NO2 emissions were actually rising and that many locations were exceeding the legal thresholds.
The issue has become doubly concerning as more health studies reveal the extent of the damage done to human health by NO2. In the UK right now, says Conlan, some 5 percent of overall deaths are due to air pollution, with 29,000 attributable to PM2.5 particulates and 23,500 to NO2 each year.
“It is only now [following a reorganisation of responsibilities] that directors of public health within local authorities are beginning to think of air quality,” she says.
In the early 1990s, notes Carslaw, the focus had been very much on particulate pollution, and the measures were successful: “It is very easy with the benefit of hindsight to say that we should have done something different, but the health evidence at that time was pointing much more towards particulates than NO2.
“It is understandable why we got into this situation,” he continues, “but it also illustrates that it is important to tie up the engineering developments with what actually happens in the atmosphere – and this is where I think Ricardo is uniquely placed in terms of understanding that impact.”
In particular, strong links have been forged between the air quality team based at Harwell and the automotive emissions and aftertreatment chemists led by Jon Andersson at Shoreham. This has led to a unique understanding of the complex science of both disciplines, providing insights that have not previously been available.
Between 2000 and 2013, reports the European Environment Agency, “the fraction of the urban population exposed to NO2 concentrations in excess of the EU limit value and the identical WHO guideline value gradually decreased to around 10 percent, with a minimum of 8 percent in 2012. At 27 percent, the highest proportion of the urban population exposed occurred in 2003.”
The EU limit values for NO2 exposure relate to annual NO2 concentrations above 40 μg/m3; there is an additional hourly limit of 200 μg/m3, with only 18 exceedances permitted per year. One of the worst examples is Oxford Street in London – a thoroughfare closed to traffic other than public service vehicles. This is one of the most heavily polluted corridors in the world and with daytime NO2 concentration spikes as high as 642 μg/m3, well in breach of statutory European limit values. In 2015 its annual mean NO2 concentration was more than triple the limit value, with the location counting 1335 exceedances of the 200 μg/m3 hourly threshold during the year. Last year the street had breached its annual allowance for hourly violations by January 4th. Data is unavailable for this year as the monitoring station was malfunctioning, but Putney High Street, another London pollution black spot, exceeded the annual limits on January 8th 2016.
Why is progress so slow?
Given the frequency of exceedances of the 2010 concentration limits in so many urban locations across Europe, street-level NO2 is currently the focus of intensive research. “Most member states are struggling to achieve those limits because we haven’t seen the reduction in emissions that had been expected. They’re not necessarily worse since 2010, but they’re not getting better fast enough,” explains Carslaw.
There are two principal reasons: the rapid growth of diesel passenger car sales, especially in the UK, and the earlier generations of emissions control technologies which use NO2 to tackle other pollutants, without afterwards eliminating the excess. “Those two things together produce the perfect storm,” he says.
Aside from the growth in diesel numbers, the key upset is in real-world emissions, especially in diesels built to Euro 4 and Euro 5 standards between 2005 and 2014. These vehicles had been expected to have lower NOx outputs than their Euro 3 predecessors but, critically, most failed to deliver in everyday driving and, with many millions still running strongly on Europe’s roads, it will be a long time before they are phased out of the emissions equation. It was only with the introduction of Euro 6 in 2014 that specific technologies for the control of NOx began to be required.
The key thing now, says Carslaw, is that exhaust aftertreatment systems must actually work under urban driving conditions – regardless of what they do in lab-based test cycles. There is a general feeling within the industry that the Euro 6c update, entering into force in 2017, will deliver genuine and reliable reductions in NOx emissions, and the extra dimension of legally-mandated real driving emissions (RDE) levels provides much greater confidence that actual exhaust emissions values will closely match the predicted values fed into the air quality models.
Is there a quick fix?
From outright bans on diesel to totally traffic-free zones, there is no shortage of suggestions for quick solutions to the NO2 crisis. But the awkward reality, says Beth Conlan, is that any truly effective measures tend to be politically unacceptable and are discounted, and obvious policies such as incentives to scrap earlier diesel models can have unforeseen consequences.
“An accelerated move to Euro 6 would be good,” counsels Carslaw, “but you do need to be careful. Some newer vehicles produce more NOx and NO2 than older models. If you encourage Euro 5 at, say, the expense of Euro 1 and pre-Euro, you could paradoxically make the situation worse as far as NO2 is concerned. It’s only Euro 6 that truly changes things.”
But because NO2 pollution tends to be a localized phenomenon, it also lends itself to local actions such as traffic-light phasing, one-way systems to smooth out traffic flow and – perhaps less palatable politically – road pricing schemes to reduce morning and evening rush-hour spikes in traffic. Local encouragement of zero-emission EVs and plug-in hybrids could have a role to play once they have achieved sufficient market penetration, a clear case of new technology being exploited to solve the NO2 problems caused by the outgoing technology.
It is here, too, that new testing methods have a role to play in encouraging vehicles that perform cleanly in real-world driving as well as in laboratory tests. As we reported in our last issue (see Getting ‘real’ about emissions testing, RQ Q4/2015) the introduction of the WLTC emissions homologation test cycle together with Real Driving Emissions compliance requirements will apply to new vehicle type approvals from Euro 6c onwards from 2017, and to all new vehicles from 2019. RDE shifts the onus firmly back to where emissions matter most – in real conditions, on actual roads and, especially, in the all-important urban areas where NO2 pollution hits hardest.
Tougher annual re-testing of vehicles in service is another frequent suggestion, but evidence points to it being a very expensive and somewhat blunt instrument with which to catch the relatively few rogue vehicles that are defective and polluting. Instead, the Colorado model of roadside remote sensing could be highly effective. The system can analyse the exhaust plumes of large volumes of vehicles in real time, even in dense traffic, linking the readings with the licence plate database in the state vehicle records. Vehicles which pass by clean three times in a row are exempted from the official pollution checks, and the scheme has immense potential in aggregating the in-service emissions of all vehicle types, both old and new. And as proof of its effectiveness, data collected from this type of sensor in Switzerland and in Colorado helped scientists to identify the excess NOx emissions being produced by VW and Audi models with two-litre diesel engines.
Looking ahead
What really matters for the atmosphere and for health with the new RDE requirements is that we see a reduction in real world emissions, stress both Conlan and Carslaw: “I don’t buy the argument that cars will be allowed to emit more,” adds Carslaw. “If we see a 50 percent reduction in NOx in real world driving with the early introduction of Euro 6 – which I think is about right – that will be good progress, with greater reductions to come with Euro 6c. We shouldn’t lose sight of the fact that we should be focusing on changing what happens under real-world driving conditions.”
Further into the future, as emissions of the familiar regulated compounds begin to taper away, other pollutants could come into play. Perhaps controversially, one of these could be non-exhaust particulates, in particular the fine dust deposited on road surfaces from vehicle tyres, brake discs and friction materials. With the near universal fitment of particulate filters, this type of pollution is rising as a proportion of overall PM and could come to affect the heavy truck sector – as well as having the potentially paradoxical consequence of removing the zero-emission status from battery electric vehicles.
As for the frequently discussed question of the future for diesel passenger cars, Carslaw is unhesitating in his response: “Yes, there will definitely be diesel cars in 2030, and though I have a nagging doubt that diesel may never be as clean as gasoline for NOx, everything will have much lower emissions by then and it might not matter.”
Amid all today’s concerns about NO2 it is all too easy to forget the major successes in other areas of emissions reduction achieved through the many steps of European automotive legislation. Yet at the same time there is a lesson in the NO2 issue – that measures that are effective in one domain can have unintended consequences in other areas. And it is here that the multifaceted capabilities of the Ricardo group come into their own: “It’s joining up the engineering and the atmospheric science that really allows you to understand what’s going on,” says Carslaw, “and this forms the basis for making the most robust decisions to improve air quality.”
This article is an edited version of a feature 'Champions of our air' published in RQ magazine, Q1, 2016 - click on the magazine cover below to go to the magazine version:
