Traffic emissions of elemental carbon (EC) and organic carbon (OC) and their contribution to PM2.5 and PM10 urban background concentrations

Urban increment of carbon in PM caused by traffic

The contribution of elemental carbon (EC) and organic carbon (OC) to the urban background concentrations of PM₁₀ and PM_2.5 was studied by means of measurements and models, for the Rotterdam-Rijnmond area. In the Netherlands, carbonaceous compounds, on average, contribute 20 to 30% to PM₁₀. It is important to have knowledge on anthropogenic sources that determine the composition and urban background concentrations of especially PM_2.5 , as the new EU Air Quality Directive has set a target for reducing urban background concentrations of PM_2.5 with 15 or 20% by 2020, compared to 2010 levels.

Increment in urban EC and OC mainly from urban traffic emissions

Reduction in EC and OC emissions may be important for reducing urban background concentrations of PM_2.5 . In addition, EC and OC are believed to be health-relevant fractions of PM. This study was intended to improve the knowledge on:

• the increment in urban EC and OC concentrations
• EC and OC emission factors for urban traffic

Contribution of EC and OC emissions from road traffic to urban background concentrations of PM_2.5 and PM₁₀ was about 0.5 µg/m³. The average urban background concentrations of PM_2.5 and PM₁₀, in the Netherlands, were in the range of 15 to 20, and 20 to 30 µg/m³, respectively. It was concluded that the potential for reducing urban background concentrations of PM_2.5 and PM₁₀ through a reduction in exhaust emissions from road traffic, would be less than 5%. The study indicated that, in the Rotterdam-Rijnmond area, sources other than road traffic, such as industry, refineries, aviation and shipping, did not contribute significantly to urban background concentrations of EC and OC.

Other findings could be summarised as follows

EC concentration

The average EC concentration outside the city, measured over the period of about one year was 2.1 ± 1.2 µg C/m³. The average urban increment was 0.3 ± 0.2 µg C/m³, and the average concentration at heavy-traffic inner-city roads was 4 ± 2.2µg C/m³.

OC concentration

The average OC concentration outside the city, measured over the period of about one year was 1.6 ± 0.5 µg C/m³. The average urban increment was 0.1 ± 0.2 µg C/m³ and thus not significant, and the average OC concentration at heavy-traffic inner-city roads was 1.9 ± 0.8 µg C/m³. These results illustrate that OC concentrations at urban and traffic locations were mainly determined by regional OC background concentrations.

Emission factors for urban traffic

The EC emission factor for urban traffic was established at 10 mg C/km, which is at the low end of the range of values of 8 to 20 mg C/km, presented in the literature. For OC, no emission factor was established.

EC as PM indicator for traffic

Model results for Rotterdam showed that, for EC concentrations, the contrast between locations near heavy road traffic and background locations was much higher than for PM_2.5 and PM₁₀. This confirmed that EC is a sensitive indicator for the dispersion of traffic-related PM emissions.

Black Smoke (BS) as proxy for EC trend

From BS measurements at an urban background location in Rotterdam and a location outside the city was concluded that EC concentrations have been decreasing significantly, with 5% per year, since 2001.

EC and OC measurements still very uncertain

It is recommended to further standardise sampling and analytical monitoring procedures, and to perform more research on EC as an indicator for traffic-related particulate matter

More information

This study was carried out by TNO, in cooperation with the ECN, the RIVM and the PBL, as part of the Netherlands Research Program on Particulate Matter (BOP).