In this study, Shreya Dey, Brian Caulfield and Bidisha Ghosh explore the relationship between emission and concentration levels in different parts of Dublin city.

Air pollution is the single largest environmental health risk in Europe at this moment¹ and reducing pollution levels is a prime concern. Transportation is a significant source of emissions of these noxious pollutants: ~25% of total PM_2.5 and ~50% of total NOx levels in Europe.

Diesel engines tend to emit more compared to other fuel types and since the ‘dieselgate’ scandal it has been found that these vehicles are violating the Euro emissions standards with the help of defeat devices^{2, 3}.

However, it is not the emissions but the high levels of pollutant concentration which has a harmful effect on human health. High vehicular emissions may or may not lead to higher gas concentrations in the atmosphere.

The relationship between emission and concentration is complex and concentration levels are linked with many other factors linked with wind profile and building geometry.

In this study, we explore the relationship between emission and concentration levels in different parts of Dublin city. The emissions and concentrations were modelled in all the major streets within the canal cordon in Dublin city. Emissions and concentrations were modelled in 1,743 street segments covering 155 urban streets located within the canal cordon.

Emissions modelling 

COPERT Street Level version 2.4 was used in this study to calculate emissions on the road segments. The methodology of COPERT Street Level is based on COPERT which is used to prepare official national emission inventory and its subsequent applications in 22 European countries⁴, but it is structured to work alongside a traffic analysis tool. It can calculate emissions of regulated, non-regulated pollutants and greenhouse gases (CO, CO₂, PM, NOx and VOC).

The basic input data include hourly traffic volume, the average speed of traffic on each road link, and length of each segment. Co-ordinates of each road also can be provided by the user to visualise the emissions on GIS maps. Traffic volume can further be disaggregated into fuel type, vehicle size and Euro class for each vehicle type.

The emission levels modelled for 2015 and projected for 2030 considering business as usual are shown in Figure 1.

Figure 1. Modelled NO_x emission levels on streets of Dublin in 2015 and 2030

Concentration modelling

The Operational Street Pollution Model (OSPM), which is a dispersion model, was used to assess the environmental impact of vehicular emissions by modelling pollutant concentrations⁵.

OSPM calculates the concentration of exhaust gases based on a combination of a Gaussian plume model for direct contribution and box model for recirculation of the pollutants. OSPM models turbulence in the street considering the effect of traffic as well as the effect of wind.

The wind profile (speed and direction) is a key factor in dictating the concentration levels. OSPM is a parameterised semi-empirical model and needs substantial numbers of input data including street configuration, hourly meteorological data (temperature, global radiation, wind speed, and wind direction), background pollutant levels, traffic volume and emission factors.

The concentration levels modelled for 2015 and projected for 2030 considering business as usual are shown in Figure 2.

Figure 2. Modelled NO_x concentration levels on streets of Dublin in 2015 and 2030

Overall, it can be expected that the concentration and emission levels will continue to increase without significant policy intervention focusing on reduction of motor vehicles and/or change to alternative fuel vehicles.

The modelled roads were ranked from high to low based on the quantity of NOx emissions and concentrations on the street. Top 10 roads with the highest NOx emissions and concentrations were chosen and listed in the following table (Table 1).

As some roads are longer than others, it is more appropriate to compare the per unit length emissions with concentrations.

The results showed that out of about 150 modelled streets, only six streets correspond to the similar high or low levels of emissions and concentrations.

To investigate the relationship further, streets were chosen that have (a) higher emissions (HE) and higher concentration (HC); (b) higher emissions but lower concentrations (LC); (c) lower emissions (LE) but higher concentrations; (d) lower emissions and lower concentration.

The four streets that fulfil these conditions were picked, namely, Amiens Street (HE-HC), Usher’s Quay (HE-LC), D’Olier Street (LE-HC), and Wilton Terrace (LE-LC) (Figure 3).

(a): Amiens Street (AADT> 40,000veh/day, 7.3m height)

(b): Usher’s Quay (AADT 35000-40,000veh/day, 0-8.3m height)

(c): D’Olier Street (AADT 20000-25,000veh/day, 11.9m height)

(d): Wilton Terrace (AADT 10000-15,000veh/day, 0m height)

Figure 3. Four locations to illustrate the complex relationship between emission and concentration in Dublin City

Amiens Street has high AADT leading to high levels of NOx emission, 0.035 tonnes/metre. Moreover, the average height of buildings on both sides of roads is 7.3 metres.

Therefore, it can be said that pollutants are trapped at the street level for a while before they disperse. In the case of Usher’s Quay, though the AADT and resulting emission levels are high, concentrations are not proportionally high. 

Ushers Quay has buildings on only one side as shown in Figure 3. D’Olier Street has an AADT between 20,001-25,000, which is lower than Usher’s Quay, and therefore has lower NOx emission levels.

However, the annual average daily NOx concentration is higher (59 µg/m³) than that in Usher’s Quay (51 µg/m³). In addition, D’Olier Street has bus stops and pedestrian crossings, leading to higher pollutant concentrations compared to Usher’s Quay.

For Wilton Terrace, where both the emissions and concentration levels are low, it can be observed that the AADT is in the range of 10,001-15,000 and that is why the emissions are relatively lower, at 0.004 tonnes per metre.

Also, it has a significantly low level of NOx concentration of 43 µg/m³.  Wilton Terrace does not have buildings on either side, therefore the wind vortex is not formed, and the concentration level is low. 

Building heights had a significant impact on emission dispersion and thereby on the street level concentration – even in Dublin – which generally has low building heights compared to many other European capitals.

Although the emission levels were found to be about 45% and 100% higher than the respective Euro standard emission levels in 2015 and 2030 with no additional measures taken than the existing ones, NO₂ concentrations were found to meet WHO/EU safe guideline values in most of the modelling locations.

References 

1.) EEA. 2016. Air quality in Europe – 2016 report. European Environment Agency, EEA Report No 28/2016. 

2.) Dey, S., Caulfield, B., and Ghosh, B. (2018a). The potential health, financial and environmental impacts of dieselgate in Ireland. Transportation Planning and Technology, Vol. 41:1, pp. 17-36, DOI: 10.1080/03081060.2018.1402743

3.) Dey, S., Caulfield, B., and Ghosh, B. (2018b). Potential health and economic benefits of banning diesel traffic in Dublin, Ireland. Journal of Transport & Health, Vol. 10, pp. 156-166, DOI: 10.1016/j.jth.2018.04.006

4.) Kioutsioukis, I., Tarantola, S., Saltelli, A., and Gatelli, D. (2004). Uncertainty and global sensitivity analysis of road transport emission estimates. Atmospheric Environment, Vol. 38. pp. 6609–6620. DOI: 10.1016/j.atmosenv.2004.08.006

5.) Berkowicz, R. 2000. OSPM—a parameterised street pollution model. Environmental Monitoring and Assessment, 65, 323–331.

Authors: Shreya Dey, Brian Caulfield and Bidisha Ghosh