Improving local air quality in cities by reducing nitrogen dioxide pollution from road traffic

Trucks and buses play a major role in our lives, transporting goods and thousands of people to cities every day. But these vehicles, although in a much smaller number than the car generates a significant amount of air pollutants. The daily NO2 concentrations measured by a traffic monitoring station over a period of two years are used to identify the temporal variation of NO2 pollution as a result of measures to ban the circulation of trucks that do not meet the EURO 6 standard on Stresemannstrase Street in Hamburg. The data shows a decrease in NO2 concentration due to the measure taken so that in January 2017 the maximum daily NO2 concentration was 86 µg/m3 compared to 63 µg/m3 in 2019. There was also a difference between the daily minimum concentrations during the same period, being approximately 28 µg/m3 in 2017 and 10 µg/m3 in 2019. The daily NO2 observations show a significant decrease in concentration since May 2018 when the non-EURO 6 trucks were banned. The largest decrease in daily concentrations was recorded in March 2019 compared with levels in March 2018, with a lower concentration for 28 days. A different situation was observed in October 2018, when compared to October 2017, showed an increase in concentration for 23 days.


Introduction
The urban air in many European cities has become inadequate for breathing due to the high levels of pollution caused mainly by vehicles, especially by diesel cars. Urban citizens are forced to breathe this highly polluted air despite EU legislation that limit ambient airpollution levels, thus leading to hundreds of thousands of premature deaths every year at European level [1].
Vehicles are the main source of pollution because of their ubiquity and the proximity of gas emitted to people. Some progress has been made to reduce particulate emissions by introducing Euro 6 limits for diesel cars, but NOx remains a key issue, especially from diesel engines. NOx convert in the air to toxic nitrogen dioxide and finally into nitrate aerosol particles and ozone. A stringent problem is the particulate and pollutant emission from older diesel engines and from diesel vehicles with particle filters that have been damaged or removed illegally.
Breathing air with a high concentration of NO 2 it may irritate the airways of the human respiratory system and even for short exposures may worsen respiratory diseases, especially asthma. Longer exposure to high concentrations of NO 2 can contribute to the development of asthma and the most exposed to the effects of this pollutant are children and the elderly.
Local concentrations of NO 2 are largely determined by road traffic emissions from passenger cars, light trucks and heavy duty vehicles [2,7,9].
The Euro standards for exhaust emission gases are important tools in Europe to reduce NOx emissions from road traffic in order to reduce NO 2 concentration levels below the limit value and to improve air quality in cities and along highways.
The German government has adopted an ambitious air quality program aimed to ensure that the air quality limit is accomplished throughout Germany over time.
Low emission zones can be an effective way to reduce NOx emissions in cities. An increasing number of large cities implement low emission zones, which ban circulation either for all vehicles or just for heavy goods vehicles only. Low emission zones must be large in size, meet severe emission standards (Euro 6 or better) and be strictly enforced [3,4].
The ban on circulating certain types of more polluting vehicles in low emission zones could lead to the relocation of pollution sources instead of reducing them, so they do not necessarily lead to a good result.
Low emission zones are areas -usually in cities with different restrictions on the operation of more polluting vehicles, usually older (vehicles with higher emissions cannot enter into the area or have to pay a higher tax if they enter into the area with low emissions) [5,6,8].
Cities and governments have adopted low emission zones as a measure to reduce air pollution to comply with EU air quality standards. Low emission zones are often considered the most effective measure that cities can take to improve air quality and the number of these zones has increased steadily and they have been implemented in many EU countries [10,11].
Low emission zones requirements are not harmonized in the EU, they are rather governed by different regulations at local and / or national level.
Typical requirements of low emission zones includes: prohibition of higher emission vehicles and of vehicles below a certain EURO emission standard. Some low emission zones have different standards for petrol and diesel cars to take into account different levels of NO 2 emissions.
On 27 February 2018, a decision of the highest court of the federal administration in Germany confirmed that the German cities could introduce restrictions for diesel cars with immediate effect and clarifies that the right of citizens to breathe clean air takes precedence over the rights of private car owners to drive polluting vehicles. As more cities introduce restrictions for polluting diesel cars, the air pollution problem could become more severe for Central and Eastern European countries due to second-hand cars imported from Western Europe.

Data
Starting from 31st May 2018, the circulation of diesel lorries that did not comply with the Euro 6 standard on Stresemannstrasse street in Hamburg was banned ( Figure  1).
For our study, we obtained data on NO 2 concentrations collected at the traffic monitoring station located on Stresemannstrasse street in Hamburg (the sampling height of the monitoring station: 4 m above ground). The monitoring station is operated by the Hamburg Monitoring System for Air Quality, Germany. Our objective of this study was to evaluate the air quality improvement by applying the measure to ban the circulation of diesel trucks [12,13].
In our analysis, we compared the NO 2 concentrations measured prior to the implementation of the diesel car ban, with the NO 2 levels measured after the measures came into force. For the analysis, the measurements in the period prior to the application of the prohibition measure from 1 January 2017 to 31 May 2018 were used. The data from the second analysis period from 1 June 2018 to 30 March 2019 covered the period when the trucks transit ban was effective.   In May, the days in which the limit value of NO 2 concentrations was exceeded was 28 in 2017 and 22 in 2018 respectively. In April, there was an increase in NO 2 concentrations in 2018 compared to 2017 for 16 days, while in May the exceedance was registered for 13 days. The results of the analysis performed for different time periods (June to December) are shown in Figure 4. The average daily variation of the NO 2 concentration shows a decreasing trend during the period of restriction application compared to the time before restriction introduction. The greatest value of the NO 2 2017.
From Figure 4 it can be seen that in the months before the restriction, the days in which the limit value were exceeded are higher, except October when the trend was reversed and the exceedances were higher during the period of restriction application (2018) and have recorded higher concentrations than in the same period of 2017.
The results show that the daily average concentration of NO 2 has a downward trend after application of traffic restriction on diesel cars, registering higher concentrations than the limit value for 10 days in January 2019 to 18 days in September 2018.
In the period before restriction introduction, the limit value of NO 2 concentrations was exceeded for 11 days in December 2017 and for 28 days in March and May 2017.
Lower values of NO 2 concentrations in December may be due to lower traffic, especially during the holidays (the Christmas) when it is observed a greater reduction in the NO 2 concentrations than the usual values. During the nine months before restrictions apply to diesel lorries (June 2017 -March 2018), 66% of days exceeded the NO2 concentration limit. After the restriction application has been a decrease of 12% of days in which there were breaches of the limit on concentrations of NO 2 . Due to the measures implemented, the daily average concentration of NO 2 decreased, the strongest effect was in March when except two days all the daily average values were lower than in the pre-ban period and at the same time there were exceedances of the limit value of the NO 2 concentration for only 10 days as shown in Figure 2.

Conclusion
The results presented in this paper clearly confirm the beneficial effect of the measure taken on Stresemannstrasse Street in Hamburg. However, the limit value for NO 2 concentration is still exceeded. In order to comply with the minimum level, important measures need to be taken further and interpretation of past air quality trends can generate relevant results for planning beneficial actions.
Since the ban applies only for lorries and not to all diesel vehicles, in a future analysis, we must study the influence of the background concentrations due to ban on all diesel cars that do not comply with Euro 6 standard applied on nearby Max-Brauer-Allee and the possible relocation of these cars on the Stresemannstrasse street. When the ban does not apply to a large area, the expected effects could be minimal due to influence of the background concentrations that could increase due to increasing traffic on adjacent streets.
The analysis showed that in some days, even after the introduction of the traffic restrictions, there were exceedances of concentration compared with the previous period this may have been due to the influence of atmospheric factors (solar radiation, relative humidity, wind speed) on the lifetime of NO 2 .