Nitrogen dioxide 6.1. European air-quality standards and World Health Organization guidelines for NO2

6. Nitrogen dioxide

6.1. European air-quality standards and World Health Organization guidelines for NO₂

The Ambient Air Quality Directive (EU, 2008) also defines an ‘alert’ threshold value of 400 μg/m³. When this threshold is exceeded over three consecutive hours in areas of at least 100 km² or in an entire air-quality management zone, authorities have to implement short-term action plans. These action plans may include measures in relation to motor-vehicle traffic, construction works, ships at berth and the use of industrial plants or products and domestic heating. The framework of these plans may also consider specific actions for the protection of sensitive population groups, including children, by reducing their exposure to high NO₂ levels.

The threshold values used in the human health objectives set by the Ambient Air Quality Directive (EU, 2008) are identical to the WHO AQG for NO₂, as shown in Table 6.1 (WHO, 2006a). The only difference is that WHO AQG does not allow any exceedance of the 1-hour limit value threshold.

6.2. Status and trends in concentrations

6.2.1. Exceedances of limit values for the protection of human health

No exceedances occurred at rural background stations. The highest concentrations, as well as 94 % of all exceedances, occurred at traffic stations. Never the less, concentrations above 55 μg/m³ were also measured at one urban background station in Serbia and another in the UK. Traffic is a major source of NO₂ and of NO, which reacts with O₃ to form NO₂. Traffic emissions are close to the ground, contributing relatively more to NO₂ ground concentrations, than, for example, high industrial stacks, emissions from which are diluted before reaching the ground. In traffic and urban areas with fresh inputs of NO, some of the O₃ present is therefore depleted during the oxidation of NO to NO₂.

Figure 6.1 shows the attainment of annual mean NO₂ values for 2014 for all EU Member States. Seventeen (²¹) of the 28 EU Member States recorded exceedances of the annual limit value (equal to the WHO AQG) at one or more stations. In addition, exceedances were also observed in Norway and Serbia. These findings demonstrate that NO₂ concentrations still need to be substantially reduced in large areas of Europe (focusing on traffic and urban locations) for the annual limit value to be met.

The hourly limit value threshold for NO₂ is less stringent. This limit value was exceeded in 2014 in 0.5 % of all the reporting stations, mostly at urban traffic stations except for two (urban and suburban) background stations. The exceedances occurred in seven Member States (²²):

2. Trends in ambient NO₂ concentrations

Table A1.6 (Annex 1) shows the calculated NO₂ annual mean trends by country and by station type over the last 15 years. All countries had an average decreasing trend at urban and suburban background and traffic stations. On the other hand, a few countries (Hungary, Ireland, Latvia, Lithuania, Luxembourg, Poland and Slovenia) registered increasing trends at rural background stations, but mostly non-significant.

Averaged over the past five years (2010-2014), 9 % of the stations (in consistent set) exceeded the NO₂ annual mean limit value. Assuming we can extrapolate the observed trend over the last 15 years to 2020, the fraction of stations in non-attainment would only be reduced to 7% in 2020. This shows the importance of new measures to be implemented in order to meet the goals stated in the Clean Air Policy Package for Europe (European Commission, 2013b) (see also Box 6.1).

6.2.3. Relationship of nitrogen oxides emissions and nitrogen dioxide concentrations

The average decrease in annual mean NO₂ concentrations measured over all stations in Europe is slower than the decrease in NO_x emissions (see chapter 4.2.3). The main reason for it may be attributed to the increase in the share of NO2 in the NO_x emissions from diesel vehicles (Grice et al., 2009; ETC/ACC, 2010b).

For certain pollutants, like NOx and CO₂, there is a wide gap between official emission measurements and the average real-world driving emissions. This gap has increased in the last years, counteracting the effect of more stringent emission regulations. The amount of fuel that cars use on the road, and hence also the CO₂ emissions, was in 2014 around 40 % higher than the official measurements (EEA, 2016d). The differences are even higher for NOx emissions, in particular for diesel vehicles.

As illustrated in Figure 6.2, real-world NO_X emissions from petrol cars in the EU have decreased significantly since 2000, in line with the increasingly stringent emission limits. In contrast, NO_X emissions from diesel cars have not improved much over the same period, meaning reductions have not been as large as foreseen in legislation. In addition, until Euro 5 diesel cars were permitted to emit three times more NO_X than petrol cars (EEA, 2015b).

The discrepancy between real-world and test cycle emissions has been known for several years (Rubino et al., 2007, 2009; Weiss et al., 2011a, 2011b, 2012; Vojtisek-Lom et al., 2009). The current EU test cycle (New European Drive Cycle) is not representative of real-world driving conditions, and the test procedure contains loopholes the manufacturers can exploit to get lower emission results. Furthermore, in some cases there is evidence that emissions are optimised for the test conditions only. These are for example software installed in vehicles to detect when they are being tested and deploy techniques to reduce emissions under testing. This was the case uncovered in September 2015 in many Volkswagen diesel vehicles (EPA, 2015).

Diesel vehicles accounted for about 52 % of new cars sold in the EU in 2014 (EEA, 2016e). The fact that NOx emissions from diesel cars are considerably higher than what was the intention in the Euro 5 and Euro 6 regulations for diesel vehicles, affects the Member States possibilities to comply with the limit and target values set by the Ambient Air Quality Directive for NO₂, O₃, PM₁₀ and PM_2.5 ambient air concentrations.

Improvements in engine technology have reduced exhaust emissions, but are generally insufficient to meet emission goals. On the other hand, existing additional exhaust after-treatment technologies (e.g. selective catalytic reduction and diesel particulate filters) are available and allow meeting the required emission standards, if correctly used.

Two initiatives are planned in Europe to improve consistency between the official and real-world driving vehicle emissions: change the official test procedure to one that is more representative of real-world emissions, as well as the introduction of a procedure for measuring the real driving emissions of vehicles on the road (EEA, 2016d). There is also a commission proposal for a Regulation on the approval and market surveillance of motor vehicles.