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No. 80: Effect of clean-air zones for air pollution with particulate matter

Jensen, S.S. & Ketzel, M. 2014. Effekt af ren-luftzoner for luftforurening med sodpartikler. Aarhus Universitet, DCE – Nationalt Center for Miljø og Energi, 32 s. -Videnskabelig rapport fra DCE - Nationalt Center for Miljø og Energi nr. 80. http://dce2.au.dk/pub/SR80.pdf

Summary

Background and Aims

EC (Elemental Carbon) is an indicator of combustion particles e.g. from vehicle tailpipe exhaust, and popular speaking EC is similar to soot particles, as they have a high content of carbon.

There is growing interest in EC from a health point of view since the World Health Organization (WHO) in 2012 upgraded their classification of diesel exhaust from probably carcinogenic to carcinogenic. About 2/3 of diesel exhaust consists of soot particles, while it is less than ¼ of gasoline exhaust.

Newer studies also suggest that carbon particles are more harmful to health than non-carbon particles (Rohr & Wyzga 2012;Hoek et al. 2013). Carbon particles are emitted primarily from e.g. road traffic and wood burning stoves, while the non-carbon particles as e.g. is the inorganic secondary particles that are emitted as gases and by chemical processes in the atmosphere are transformed into particles.

PM2.5 is particles less than 2.5 micrometres in diameter and is normally used as an indicator for health effects and assessment of external costs of health effects. There is a well-established link between PM2.5 and health effects. This is based on studies of the correlation between population exposure to PM2.5 typically from measuring stations located at urban background sites, and observed health effects. No distinction is made between the different particle sizes or chemical components of PM2.5.

However, there are strong indications that PM2.5 as indicator for assessment of the effects of certain abatement measures underestimate health effect e.g. in the case of reduction of particle exhaust (Janssen et al. 2011; WHO (2012)). A more targeted indicator is needed that reflects exhaust particles. Based on a review of a wide range of studies Janssen et al. (2011) show that the expected gains for life expectancy is 4-9 times greater per 1 µg/m3 for EC in relation to PM2.5.

A number of studies have also shown that distance/proximity to a major road increases the risk of a range of health effects (Hoek et al. 2002). This indicates that local combustion sources such as traffic have a relatively greater importance than PM2.5 as a health indicator.

The above considerations are reasons to focus on EC as an additional health indicator in the assessment of the impact of low emission zones, since this abatement measure reduces particulate emissions.

The aim of the project is to estimate and visualize the effect on EC air pollution of a low emission zone in Copenhagen/Frederiksberg based on 3 different scenarios.

Scenarios

The current low emission zone only prohibits diesel-powered buses and trucks above 3.5 tonnes that are up to and including the Euro 3 emission standard to enter the low emission zone. However, heavy-duty vehicles with particle filters are allowed to enter the zone. The geographic extent of the zone is almost similar to the extent of the two municipalities Copenhagen and Frederiksberg (Jensen et al., 2011).

The proposed low emission zone will stringent the existing low emission zone in Copenhagen by prohibiting older passenger cars and vans.

An impact assessment for EC and a visualization of the impacts have been carried out building on a previously conducted air quality assessment for low emission zones in Copenhagen (Jensen et al. 2012; 2013a, b). The following scenarios are included: Base scenario (2015 and 2017), Berlin scenario for passenger cars and vans (2015 and 2017), and Berlin scenario for passenger cars and vans including emission requirements for all Euro 4 diesel vehicles (2017).

The Berlin scenario is identical to the requirements of the low emission zone in Berlin where diesel passenger cars and vans up to and including Euro 3 and petrol passenger cars and vans up to and including Euro 0 are not allowed in the zone.

The Berlin scenario including Euro 4 also implies that all Euro 4 diesel vehicles (cars, vans, trucks and buses) are not allowed in the zone.

It is assumed that vehicles that are not allowed in the zone are replaced by newer vehicles or retrofitted with particle filter, open filters for passenger cars and vans and closed filters for heavy-duty vehicles.

The Study

The street air quality model OSPM with necessary input data for emissions, traffic, street geometry, urban background concentrations and meteorology is used to calculate annual mean concentrations of EC for a large number of streets in Copenhagen.

The project has reused former data for roads, traffic and street geometry from the so-called ASSET project, where air quality calculations were carried out for all street segments in Copenhagen with more than 2,000 vehicles a day with OSPM, and the AirGIS system was used to generate input data for OSPM for traffic and street geometry based on GIS data with roads and traffic, and building footprints with building heights as well as calculation points (Jensen et al., 2009a).

As a new feature EC emission factors have been implemented in OSPM based on the COPERT emission model, where EC emission factors are described as a percentage of the particulate emission (PM2.5) for the different vehicle categories and Euro emission classes (Ntziachristos & Samaras, 2010).

Sufficient EC emission information lacks in order to model urban background concentrations of EC. Therefore, the annual mean of urban background concentration of EC has been estimated from analysis of EC measurements at Lille Valby (regional concentrations) and street measurements at H.C. Andersens Boulevard together with campaign measurements at H.C. Ørsted Institute (urban background). This single urban background value is used for all areas of Copenhagen although there are likely to be differences across Copenhagen.

Applied urban background concentrations are based on measurements and include the contribution from all sources, but it is only the EC contribution from traffic that has been modelled and visualized. Other combustion sources such as wood burning stoves could also contribute with EC, but there is a lack of knowledge to model the contribution.

Main Conclusions

Impacts on EC emissions of low emission zone

The impacts on particulate exhaust emissions of the different low emission zones have been calculated. The Berlin scenario reduces on average particulate exhaust emissions by 32% in 2015 with 34% reduction for passenger cars and 80% reduction for vans. The percentage reduction is smallest for passenger cars, since there are many gasoline passenger cars, which are not affected, while vans are almost entirely diesel vehicles.

The Berlin scenario including requirements for Euro 4 reduces on average particulate exhaust emission by 59% in 2017 with 69% reduction for passenger cars and 94% reduction for vans.

The same percentage reduction can be expected for EC as EC is a fraction of particulate exhaust emissions.

Impacts on EC concentrations of low emission zone

EC street concentrations are up to 1.5 µg/m3 in 2015 in the base scenario, which is 3.6 times as much as urban background concentrations. This illustrates that the EC concentration in busy streets are dominated by EC vehicle emissions in the street, and reductions will lead to significant reductions in EC concentrations.

We produced a series of maps which show the EC concentrations before and after each of the scenarios. Clear reductions in EC concentrations are visualized.

The maximum percentage reduction in EC concentrations, which can be found in a streets in Copenhagen is 41% and achieved in the Berlin scenario including requirements for Euro 4.

From the maps it is visually clear that the percentage reduction is slightly larger for the Berlin scenario in 2015 than in 2017 since a postponement of the low emission zone requirements will reduce the effect at the implementation year due to the continuous replacement of the vehicle fleet.

Comparison between EC measurements and model results

Comparison of EC measurements and calculations with OSPM for the street contribution at H.C. Andersens Boulevard in Copenhagen in 2012 showed that the model underestimates. The model gives about 40% of EC measurements and about 80% of the PM2.5 measurements for the street contribution. The street contribution is the difference between street concentrations and urban background concentrations and is an indicator of the emission in the street.

Since it is the first time that EC emission factors have been built into OSPM, and it has not been possible within the project's framework to carry out a more detailed analyses of the correlation between calculations and measurements, calculations are presented, as they are, and no attempts have been done for modifications e.g. calibration of model results to measurements.

Project results

Visualization of EC concentrations

Figure 1 shows how large the modelled EC street concentration is in relation to urban background concentration in 2015, which illustrates how much traffic contributes to street concentrations. 100% indicates that street concentrations are twice as large as the urban background concentrations. Street concentrations are up to 3.6 times larger than urban background concentrations.

Impacts on EC concentration of low emission zone

The Berlin scenario reduces EC street concentrations on average 12% in 2015 and 10% in 2017, while the Berlin scenario including Euro 4 reduces EC street concentrations on average 20%. Other statistical parameters are also given.  For example, min and max values vary depending on the traffic flow and vehicle distribution, and thus in relation to urban background levels. Minimum indicates a situation with a very small street contribution due to low traffic flow whereby the urban background concentration dominates.

Reductions in EC street concentrations will be slightly larger than specified above as urban background concentration is assumed to be constant before and after implementation of the low emission zone (0.42 µg/m3). However, the low emission zone will also reduce urban background concentrations thereby lowering street concentrations.