The decreasing NOX concentrations at urban measurement stations in Germany are in agreement with the reduction of NOX emissions from vehicular traffic. However, the measured NO2 concentrations are stagnating nationwide. In 2010, at more than the half of the urban measurement stations in Germany, annual mean values for NO2 exceeded the new Europe-wide limit value of 40 μg/m3 (20 ppbv) NO2. Similar findings are reported from many other member states of the European Union.
The observed trend of the airborne NO2 concentrations has different reasons. Firstly, the NO2/NOx emission ratio has increased significantly during the last two decades. Furthermore, secondary NO2, caused by the titration reactions of NO with ozone (O3) and peroxy radicals (RO2), is responsible for the major fraction (approximately 70%) of the measured NO2. However, secondary NO2 shows a highly nonlinear dependency on NOx and thus, is decreasing much more slowly than expected from the decreasing NOx levels. Based on the results from the present study, the increased NO2/NOX emission ratio can only explain a minor fraction of the observed high airborne NO2 concentration in the city center.
A further reduction of primary NO2 emissions, due to improved exhaust gas treatment, will not have a strong influence on urban NO2 levels, and a further significant reduction of the NOX emissions, in particular from vehicular traffic, is necessary in order to meet the annual mean limit value for NO2 of about 20 ppb in the future.
Keywords:Nitrogen dioxide; Air quality; Traffic emissions
Fine particulate matter and nitrogen dioxide (NO2) are the key problems for increasing air quality in Europe. Whereas particulate matter and the exceedance of PM limiting values have attracted considerable public attention during the last couple of years, the NO2 problem is a relatively new one, which became mature through the introduction of new European limiting values in January 2010.
The reduction of nitrogen oxide (NOX = NO + NO2) emissions has been historically one of the key objectives for improving air quality in Europe. NOX emissions have started to decrease considerably since the mid eighties of the last century in many European areas.
In Germany, nitrogen oxide emissions are still largely caused by road traffic. Nationwide, the share of road transport in the total NOX emissions is currently around 60% . Since the mid-80s, a significant decrease in NOX emissions from road transport has been reported from Germany. According to emission calculations of the German Federal Environmental Agency, the decrease in total NOX emissions between 1990 and 2005 is approximately 60% . Figure 1 shows an example of NO and NO2 concentrations measured on major roads in the German Federal State of North Rhine-Westphalia (NRW) for the period 1990 to 2008 .
Figure 1. The NO and NO2concentrations. NO and NO2 concentrations measured on major roads in the German Federal State of North Rhine-Westphalia for the period 1990 to 2008.
One can see a significant decline in the annual averages of NOX pollution from about 80 to about 40 ppbv. This decrease is in agreement with the calculated NOX emission trends. In contrast, the NO2 concentrations in NRW stagnate in the same period at about 23 ppbv. This trend has been also observed nationwide  and in other European countries [4-7].
Nitrogen dioxide is already a problem for many cities due to its toxicity and key role in the formation of tropospheric ozone . In Germany in 2010, still more than half of the major roads were well above the current annual limit of 40 μg/m3 and approximately 20 ppbv, respectively . Because of the problems of many EU member states in complying with the new NO2 annual concentration limit, the European Commission introduced time extensions to meet limits until 1 January 2015. Since early 2004, the Physical Chemistry Laboratory of the Faculty for Mathematics and Natural Sciences of the University of Wuppertal, in cooperation with the State Office for Nature, Environment and Consumer Protection (LANUV) of the German federal state of North Rhine-Westphalia performed extended pollution measurements at two monitoring stations in Wuppertal and Hagen in order to clarify the reason for the almost stagnant NO2 pollution.
Results and discussion
Generally, in combustion processes, such as in engines of motor vehicles, NO2 and NO are primarily formed and emitted directly. The directly emitted NO, NO (direct), is converted in the atmosphere, partly by O3 or peroxy (RO2) radicals into NO2, which is called in the following NO2 (indirect):
In the presence of sunlight, NO2 is partially photolysed back to NO:
Assuming that the RO2 photochemistry is negligible at the polluted urban kerbside stations investigated and that the background O3 concentration is constant, the level of oxidants (OX) is given by :
The reason for the observed reverse trend of the NOX and NO2 concentrations is twofold and is an example for the complexity of the atmosphere. Firstly, the NO2/NOX emission ratio of road traffic has increased during the last two decades. In addition, the secondary NO2 formed by reaction (1) has not decreased significantly caused by the nonlinear dependency of the Leigthon equilibrium, reactions (1) and (2), on the NOX level.
To determine the NO2/NOX emission ratio, OX is plotted as a function of NOX. After linear regression, one can obtain from the slope of the straight line the NO2/NOX emission ratio and from the intercept the O3 background concentration . The primarily emitted NO2 is obtained by multiplying the NO2/NOX emission ratio with the measured NOX concentration. The difference between the measured NO2 (total) and the resulting primarily emitted NO2 is then equivalent to the NO2 (indirect), which is formed through the titration reaction of NO with ozone.
In measurements of NO, NO2, and O3 at busy roads in the morning during the so-called’rush hour’, usually the O3 background concentration is almost constant, the RO2 chemistry in the atmosphere is negligible, and the variation in the measured NOX is largest. Accordingly, measurement data from this time interval provide the best linear correlation expected between OX and NOX.
Figure 2 shows as an example of the NO, NO2, and O3 pollution of the 1 min mean values measured at the LANUV station in Wuppertal Friedrich-Engels-Allee on 5 April 2006. One can easily determine the typical’ rush hour peak’ for NO and NO2 between 5:00 and 8:00 h in the morning. The pronounced anti-correlation between NO and O3 is caused by the rapid formation of secondary NO2 (indirect) through the titration reaction of NO by ozone. For the following interpretation of the OX and NOX correlation, only the data measured between 5:00 and 8:00 h have been used.
Figure 2. The NO, NO2,and O3mixing ratios. NO, NO2 and O3 mixing ratios (5 min averaged values) at the LANUV monitoring station Wuppertal, Friedrich-Engels-Allee on 05 April 2006.
Figure 3 exhibits the correlation between OX and NOX in the period 5:00 to 8:00 h at the LANUV site in Wuppertal on 5 April 2006 as an example. From the linear regression a NO2/NOX emission ratio of (0.13 ± 0.01) and a background ozone volume mixing ratio of (32 ± 1) ppbv was obtained.
Figure 3. Plot of NOxvs. OX (1 min averaged values) for the time 5:00 to 8:00 h at the LANUV monitoring station Wuppertal, Friedrich-Engels-Allee on April 05, 2006 as an example.
For 2006, at the monitoring station in Wuppertal, an annual average NO2/NOX emission ratio of (0.12 ± 0.01) and a background ozone mixing ratio of (33 ± 1) ppbv has been obtained. These values are comparable with the annual mean of (0.11 ± 0.01) for the NO2/NOX emission ratio and (31 ± 1) ppbv for the ozone background at the site in Hagen. For the period 2004 to 2009, at the monitoring station in Wuppertal, an average NO2/NOx emission ratio of (0.13 ± 0.02) and a background ozone mixing ratio of (35 ± 2) ppbv has been obtained.
For comparison, in a traffic tunnel study of the car fleet in Wuppertal in 1997 , a much smaller NO2/NOX emission ratio of (0.04 ± 0.01) has been reported. The observed increase of the NO2/NOX emission ratio for road traffic and the resulting higher NO2 emission is, thus, one reason for the observed stagnation of NO2 pollution [11-14]. The increasing NO2/NOX emission ratio is caused by the rising share of diesel cars to the fleet of motor vehicles and new motor vehicle emission control systems. [15,16]
Modern diesel cars are nowadays fitted with the so-called oxidation catalysts, which significantly increase the NO2 emission through the oxidation of NO with excess oxygen in the exhaust. Furthermore, modern diesel cars are more and more equipped with the so-called continuously regenerating particulate filters (CRT) for the deposition of soot particles in combination with an oxidation catalyst upstream of the filter. The NO2, which is formed in the oxidation catalyst, is then used in the downstream particulate filter as an oxidant to burn the trapped soot particles. These conventional filter systems operate with a large excess of NO2 so that they work even under adverse operating conditions properly. However, it is worth mentioning that new regulated CRT filter systems with the so-called NOxOPT technique  reduce the excess NO2 by 75% and, thus also, the direct NO2 emissions.
Figures 4 and 5 show an example of the 30 min mean values of primary and secondary NO2 at the LANUV station at the Graf-von-Galen-Ring in Hagen in July 2006. It is apparent that the NO2 (indirect) already exceeded most of the time the new limit value for NO2 (EU-2010, 40 μg/m3 to approximately 20 ppbv). In contrast, the proportion of primary NO2 is significantly smaller.
Figure 4. The plot of the primarily emitted NO2. Plot of NO2 (direct) (30 min mean values), LANUV monitoring station, Graf-von-Galen-Ring, Hagen in July 2006.
Figure 5. The plot of the secondary NO2. Plot of the secondary NO2 (indirect) (30 min mean values) LANUV monitoring station, Graf-von-Galen-Ring, Hagen in July 2006.
For the monitoring station in Hagen, an annual average for the year 2006 of (35 ± 17) ppbv NO2 was obtained, which is well above the new limit value. The NO2 fraction, which is formed through the titration of NO with ozone contributes (73 ± 12%) or (24 ± 4) ppbv to the total measured NO2, whereas the direct emitted NO2 contributes (27 ± 12%) or (11 ± 5) ppbv, respectively. These values are comparable with the mean for the period 2004 to 2009 of (68 ± 6%) for indirect emitted NO2 and (32 ± 6%) for direct emitted NO2 at the site in Wuppertal. These results are consistent also with a study performed by the Institute for Energy and Environmental Research Heidelberg GmbH , which was commissioned by the Ministry of Environment of the German Federal State of Baden Wuerttemberg. Consequently, even a drastic reduction of the directly emitted NO2 and the NO2/NOX emission ratio, respectively, for example through improved emission control systems  would not reduce NO2 concentrations below the limiting value, since it is determined to a larger extend by secondary NO2 formation.
In Figure 6, simple box-model calculations are shown, in which the photostationary state NO2 mixing ratio is calculated for a background ozone level of 40 ppbv and four different NO2/NOX emission ratios as a function of NOX. Only the NO2 photolysis, reaction (2) and the ozone titration by NO, reaction (1), were considered. The rate coefficient for reaction (1) was taken from Atkinson et al. , and for the photolysis frequency of NO2, a typical noontime summer value of J(NO2) = 8 × 10−3 s−1 was used. Since secondary NO2 is also formed in the atmosphere in the presence of hydrocarbons and sunlight through RO2 chemistry (see above) and since J(NO2) values (NO2 sink reaction) are often lower than used here, the actual NO2 concentrations may be even higher.
Figure 6. The photostationary mixing ratio of NO2. Photostationary mixing ratio of NO2 as a function of the NOX mixing ratio, shown for different NO2/NOX emission ratios. Ozone background 40 ppbv, NO2 photolysis rate 8 × 10-3 [s-1] representing typical conditions during summertime in Germany.
Figure 6 shows a highly nonlinear dependence of the steady state NO2 level with NOx which is explained by the linear reaction kinetics of the NO2 photolysis, sink reaction (2), and the second order kinetics of the NO2 source reaction (1). An important conclusion from this result is that a reduction of the NOx levels, e.g., by a factor of two will result in a much smaller reduction in the NO2 mixing ratios, as long as the NOx levels are significantly higher than the background O3 levels. Thus, it is apparent that a further significant reduction of NOX emissions is prerequisite to meet the current limit value for NO2, almost regardless of the NO2/NOX emission ratio. For example, the NOX annual average mixing ratio in Hagen for 2007 of 92 ppbv or for Wuppertal of 55 ppbv has to be reduced to ca. 35 ppbv. This result is also consistent with other studies from Germany suggesting a further reduction of NOX emissions of at least 50% in order to achieve the required reduction of the NO2 concentration in the atmosphere [11,12]. A similar conclusion has been drawn also for other European Countries [5,7]. Thus, the exceedance of NO2 limit values will remain a European problem within the next couple of years .
The observed decrease in NOX at monitoring stations located close to road traffic is in agreement with the reduction of NOX emissions from road transport. However, the NO2 concentrations at these stations remained either constant or even slightly increased not only in Germany but throughout Europe. In 2010, at more than half of the monitoring stations at main roads, the NO2 concentrations still exceeded the limit for the annual mean of 40 μg/m3 and approximately 20 ppbv, respectively.
The reason for the observed NO2 trend is twofold. Firstly, the NO2/NOx emission ratio has increased significantly during the last two decades. Furthermore, caused by the nonlinear dependency on the NOx level, secondary NO2 is decreasing much more slowly than expected from the decreasing NOx levels. A detailed analysis of the data at two monitoring stations in Germany confirmed that the NO2 concentrations are mostly determined by secondary NO2 formation.
A reduction of the primarily emitted NO2 (direct) due to improved emission control systems alone is not sufficient to reduce the NO2 concentrations significantly. Compliance with the NO2 annual limit of approximately 20 ppbv requires a further drastic reduction of NOX emissions in the near future. However, the exceedance of NO2 limit values will remain a European problem within the next couple of years.
Measurements of traffic-related NO, NO2, and ozone (O3) concentrations in the city of Wuppertal, Friedrich-Engels-Allee, and Hagen, Graf-von-Galen-Ring were carried out in the period 2004 to 2009 to determine the direct and indirect NO2 emission of vehicular traffic. Figure 7 shows the monitoring stations in Wuppertal and Hagen.
Figure 7. Monitoring stations close to main roads in Wuppertal (Friedrich-Engels-Allee) and Hagen (Graf-von-Galen-Ring).
Nitrogen oxides NO and NO2 were measured online with commercial NOX chemiluminescence analysers (Environnemental Sat AC 31 M with molybdenum converter) and O3 online with commercial O3 monitors (Environnemental Sat 41 M with UV absorption). The well-known positive interferences in the NO2 channel of the molybdenum converter NOx instruments by reactive nitrogen species (NOy) was recently demonstrated to play only a minor role at kerbside stations caused by the proximity to the major NOx source by vehicle emissions .
The authors declare that they have no competing interests.
PW provided the ideas and supervision and wrote the manuscript. RK and AN were primarily responsible with the support of JK for the experiments and data analyses. JK provided the box-model calculations. All contributed jointly and equally to the discussion of the manuscript. All authors read and approved the final manuscript.
The authors gratefully acknowledge the financial support of the Stadtwerke Wuppertal AG for operating the monitoring station at Wuppertal. The authors are indebted to Prof. Peter Bruckmann, State Office for Nature, Environment, and Consumer Protection (LANUV) of the German Federal State of North Rhine-Westphalia for his continuous interest and support.
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